Flex manual

Flex manual
Lexical Analysis with Flex
Edition 2.5.37, 22 July 2012
Vern Paxson, Will Estes and John Millaway
The flex manual is placed under the same licensing conditions as the rest of flex:
c 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2012 The Flex Project.
Copyright c 1990, 1997 The Regents of the University of California. All rights reserved.
Copyright This code is derived from software contributed to Berkeley by Vern Paxson.
The United States Government has rights in this work pursuant to contract no. DEAC03-76SF00098 between the United States Department of Energy and the University of
California.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of
conditions and the following disclaimer in the documentation and/or other materials
provided with the distribution.
Neither the name of the University nor the names of its contributors may be used to endorse
or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED “AS IS” AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
i
Table of Contents
1
Copyright . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2
Reporting Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4
Some Simple Examples . . . . . . . . . . . . . . . . . . . . . . . . . 4
5
Format of the Input File . . . . . . . . . . . . . . . . . . . . . . . 6
5.1
5.2
5.3
5.4
Format of the Definitions Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Format of the Rules Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Format of the User Code Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comments in the Input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
7
7
7
6
Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7
How the Input Is Matched . . . . . . . . . . . . . . . . . . . 14
8
Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9
The Generated Scanner . . . . . . . . . . . . . . . . . . . . . . . 19
10
Start Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
11
Multiple Input Buffers . . . . . . . . . . . . . . . . . . . . . . . 27
12
End-of-File Rules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
13
Miscellaneous Macros . . . . . . . . . . . . . . . . . . . . . . . . 32
14
Values Available To the User . . . . . . . . . . . . . . . 33
15
Interfacing with Yacc . . . . . . . . . . . . . . . . . . . . . . . . 34
ii
16
Scanner Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
16.1
16.2
16.3
16.4
16.5
16.6
Options for Specifying Filenames . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Options Affecting Scanner Behavior . . . . . . . . . . . . . . . . . . . . . . . . . .
Code-Level And API Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Options for Scanner Speed and Size . . . . . . . . . . . . . . . . . . . . . . . . . .
Debugging Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Miscellaneous Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
36
39
41
43
44
17
Performance Considerations. . . . . . . . . . . . . . . . . 45
18
Generating C++ Scanners . . . . . . . . . . . . . . . . . . 50
19
Reentrant C Scanners . . . . . . . . . . . . . . . . . . . . . . . 54
19.1 Uses for Reentrant Scanners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.2 An Overview of the Reentrant API . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.3 Reentrant Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.4 The Reentrant API in Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.4.1 Declaring a Scanner As Reentrant . . . . . . . . . . . . . . . . . . . . . . .
19.4.2 The Extra Argument . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.4.3 Global Variables Replaced By Macros . . . . . . . . . . . . . . . . . . .
19.4.4 Init and Destroy Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.4.5 Accessing Variables with Reentrant Scanners . . . . . . . . . . . .
19.4.6 Extra Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.4.7 About yyscan t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19.5 Functions and Macros Available in Reentrant C Scanners . . . . .
54
55
55
56
56
56
56
56
57
58
59
59
20
Incompatibilities with Lex and Posix . . . . . . . 61
21
Memory Management . . . . . . . . . . . . . . . . . . . . . . . 64
21.1
21.2
21.3
22
The Default Memory Management . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Overriding The Default Memory Management . . . . . . . . . . . . . . . . 65
A Note About yytext And Memory . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Serialized Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
22.1
22.2
22.3
Creating Serialized Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Loading and Unloading Serialized Tables . . . . . . . . . . . . . . . . . . . . . 67
Tables File Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
23
Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
24
Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
25
Additional Reading . . . . . . . . . . . . . . . . . . . . . . . . . . 75
iii
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
When was flex born? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
How do I expand backslash-escape sequences in C-style quoted strings?
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Why do flex scanners call fileno if it is not ANSI compatible? . . . . . . . 76
Does flex support recursive pattern definitions? . . . . . . . . . . . . . . . . . . . . . 76
How do I skip huge chunks of input (tens of megabytes) while using
flex? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Flex is not matching my patterns in the same order that I defined them.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
My actions are executing out of order or sometimes not at all. . . . . . . 77
How can I have multiple input sources feed into the same scanner at the
same time? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Can I build nested parsers that work with the same input file? . . . . . . 78
How can I match text only at the end of a file? . . . . . . . . . . . . . . . . . . . . . 78
How can I make REJECT cascade across start condition boundaries?
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Why can’t I use fast or full tables with interactive mode? . . . . . . . . . . . 79
How much faster is -F or -f than -C? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
If I have a simple grammar can’t I just parse it with flex? . . . . . . . . . . . 79
Why doesn’t yyrestart() set the start state back to INITIAL? . . . . . . . 79
How can I match C-style comments?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
The ’.’ isn’t working the way I expected. . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Can I get the flex manual in another format? . . . . . . . . . . . . . . . . . . . . . . . 80
Does there exist a "faster" NDFA->DFA algorithm? . . . . . . . . . . . . . . . . 80
How does flex compile the DFA so quickly? . . . . . . . . . . . . . . . . . . . . . . . . . 80
How can I use more than 8192 rules? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
How do I abandon a file in the middle of a scan and switch to a new
file? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
How do I execute code only during initialization (only before the first
scan)? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
How do I execute code at termination? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Where else can I find help? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Can I include comments in the "rules" section of the file? . . . . . . . . . . . 82
I get an error about undefined yywrap(). . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
How can I change the matching pattern at run time? . . . . . . . . . . . . . . . 82
How can I expand macros in the input? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
How can I build a two-pass scanner? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
How do I match any string not matched in the preceding rules? . . . . . 83
I am trying to port code from AT&T lex that uses yysptr and yysbuf.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Is there a way to make flex treat NULL like a regular character? . . . . 83
Whenever flex can not match the input it says "flex scanner jammed".
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Why doesn’t flex have non-greedy operators like perl does? . . . . . . . . . 84
Memory leak - 16386 bytes allocated by malloc. . . . . . . . . . . . . . . . . . . . . . 84
How do I track the byte offset for lseek()? . . . . . . . . . . . . . . . . . . . . . . . . . . 85
How do I use my own I/O classes in a C++ scanner? . . . . . . . . . . . . . . . 85
iv
How do I skip as many chars as possible? . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
deleteme00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Are certain equivalent patterns faster than others? . . . . . . . . . . . . . . . . . . 86
Is backing up a big deal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Can I fake multi-byte character support? . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
deleteme01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Can you discuss some flex internals? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
unput() messes up yy at bol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
The | operator is not doing what I want . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Why can’t flex understand this variable trailing context pattern? . . . . 92
The ^ operator isn’t working . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Trailing context is getting confused with trailing optional patterns . . 93
Is flex GNU or not? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
ERASEME53 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
I need to scan if-then-else blocks and while loops . . . . . . . . . . . . . . . . . . . 95
ERASEME55 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
ERASEME56 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
ERASEME57 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Is there a repository for flex scanners? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
How can I conditionally compile or preprocess my flex input file?. . . . 97
Where can I find grammars for lex and yacc? . . . . . . . . . . . . . . . . . . . . . . . 97
I get an end-of-buffer message for each character scanned. . . . . . . . . . . . 98
unnamed-faq-62 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
unnamed-faq-63 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
unnamed-faq-64 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
unnamed-faq-65 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
unnamed-faq-66 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
unnamed-faq-67 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
unnamed-faq-68 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
unnamed-faq-69 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
unnamed-faq-70 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
unnamed-faq-71 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
unnamed-faq-72 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
unnamed-faq-73 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
unnamed-faq-74 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
unnamed-faq-75 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
unnamed-faq-76 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
unnamed-faq-77 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
unnamed-faq-78 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
unnamed-faq-79 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
unnamed-faq-80 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
unnamed-faq-81 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
unnamed-faq-82 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
unnamed-faq-83 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
unnamed-faq-84 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
unnamed-faq-85 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
unnamed-faq-86 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
unnamed-faq-87 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
v
unnamed-faq-88 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-90 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-91 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-92 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-93 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-94 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-95 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-96 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-97 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-98 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-99 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
unnamed-faq-101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What is the difference between YYLEX PARAM and YY DECL? . .
Why do I get "conflicting types for yylex" error? . . . . . . . . . . . . . . . . . .
How do I access the values set in a Flex action from within a Bison
action? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A
112
113
113
115
115
115
116
117
117
118
118
119
119
120
120
120
Appendices . . . . . . . . . . . . . . . . . . . . . . 121
A.1 Makefiles and Flex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2 C Scanners with Bison Parsers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3 M4 Dependency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4 Common Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.1 Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.2 Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.3 Quoted Constructs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4 Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
121
122
124
124
124
125
125
125
Indices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Concept
Index of
Index of
Index of
Index of
Index of
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Functions and Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scanner Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
127
131
131
132
132
132
Chapter 1: Copyright
1
1 Copyright
The flex manual is placed under the same licensing conditions as the rest of flex:
c 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2012 The Flex Project.
Copyright c 1990, 1997 The Regents of the University of California. All rights reserved.
Copyright This code is derived from software contributed to Berkeley by Vern Paxson.
The United States Government has rights in this work pursuant to contract no. DEAC03-76SF00098 between the United States Department of Energy and the University of
California.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice, this list of
conditions and the following disclaimer in the documentation and/or other materials
provided with the distribution.
Neither the name of the University nor the names of its contributors may be used to endorse
or promote products derived from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED “AS IS” AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
Chapter 2: Reporting Bugs
2
2 Reporting Bugs
If you find a bug in flex, please report it using the SourceForge Bug Tracking facilities
which can be found on flex’s SourceForge Page.
Chapter 3: Introduction
3
3 Introduction
flex is a tool for generating scanners. A scanner is a program which recognizes lexical
patterns in text. The flex program reads the given input files, or its standard input if
no file names are given, for a description of a scanner to generate. The description is in
the form of pairs of regular expressions and C code, called rules. flex generates as output
a C source file, ‘lex.yy.c’ by default, which defines a routine yylex(). This file can be
compiled and linked with the flex runtime library to produce an executable. When the
executable is run, it analyzes its input for occurrences of the regular expressions. Whenever
it finds one, it executes the corresponding C code.
Chapter 4: Some Simple Examples
4
4 Some Simple Examples
First some simple examples to get the flavor of how one uses flex.
The following flex input specifies a scanner which, when it encounters the string
‘username’ will replace it with the user’s login name:
%%
username
printf( "%s", getlogin() );
By default, any text not matched by a flex scanner is copied to the output, so the net
effect of this scanner is to copy its input file to its output with each occurrence of ‘username’
expanded. In this input, there is just one rule. ‘username’ is the pattern and the ‘printf’
is the action. The ‘%%’ symbol marks the beginning of the rules.
Here’s another simple example:
int num_lines = 0, num_chars = 0;
%%
\n
.
++num_lines; ++num_chars;
++num_chars;
%%
int main()
{
yylex();
printf( "# of lines = %d, # of chars = %d\n",
num_lines, num_chars );
}
This scanner counts the number of characters and the number of lines in its input. It
produces no output other than the final report on the character and line counts. The
first line declares two globals, num_lines and num_chars, which are accessible both inside
yylex() and in the main() routine declared after the second ‘%%’. There are two rules, one
which matches a newline (‘\n’) and increments both the line count and the character count,
and one which matches any character other than a newline (indicated by the ‘.’ regular
expression).
A somewhat more complicated example:
/* scanner for a toy Pascal-like language */
%{
/* need this for the call to atof() below */
#include <math.h>
%}
DIGIT
ID
%%
[0-9]
[a-z][a-z0-9]*
Chapter 4: Some Simple Examples
{DIGIT}+
5
{
printf( "An integer: %s (%d)\n", yytext,
atoi( yytext ) );
}
{DIGIT}+"."{DIGIT}*
{
printf( "A float: %s (%g)\n", yytext,
atof( yytext ) );
}
if|then|begin|end|procedure|function
{
printf( "A keyword: %s\n", yytext );
}
{ID}
printf( "An identifier: %s\n", yytext );
"+"|"-"|"*"|"/"
"{"[\^{}}\n]*"}"
[ \t\n]+
.
printf( "An operator: %s\n", yytext );
/* eat up one-line comments */
/* eat up whitespace */
printf( "Unrecognized character: %s\n", yytext );
%%
int main( int argc, char **argv )
{
++argv, --argc; /* skip over program name */
if ( argc > 0 )
yyin = fopen( argv[0], "r" );
else
yyin = stdin;
yylex();
}
This is the beginnings of a simple scanner for a language like Pascal. It identifies different
types of tokens and reports on what it has seen.
The details of this example will be explained in the following sections.
Chapter 5: Format of the Input File
6
5 Format of the Input File
The flex input file consists of three sections, separated by a line containing only ‘%%’.
definitions
%%
rules
%%
user code
5.1 Format of the Definitions Section
The definitions section contains declarations of simple name definitions to simplify the
scanner specification, and declarations of start conditions, which are explained in a later
section.
Name definitions have the form:
name definition
The ‘name’ is a word beginning with a letter or an underscore (‘_’) followed by zero
or more letters, digits, ‘_’, or ‘-’ (dash). The definition is taken to begin at the first nonwhitespace character following the name and continuing to the end of the line. The definition
can subsequently be referred to using ‘{name}’, which will expand to ‘(definition)’. For
example,
DIGIT
[0-9]
ID
[a-z][a-z0-9]*
Defines ‘DIGIT’ to be a regular expression which matches a single digit, and ‘ID’ to be
a regular expression which matches a letter followed by zero-or-more letters-or-digits. A
subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a ‘.’ followed by zero-or-more digits.
An unindented comment (i.e., a line beginning with ‘/*’) is copied verbatim to the output
up to the next ‘*/’.
Any indented text or text enclosed in ‘%{’ and ‘%}’ is also copied verbatim to the output
(with the %{ and %} symbols removed). The %{ and %} symbols must appear unindented
on lines by themselves.
A %top block is similar to a ‘%{’ ... ‘%}’ block, except that the code in a %top block
is relocated to the top of the generated file, before any flex definitions1 . The %top block
is useful when you want certain preprocessor macros to be defined or certain files to be
included before the generated code. The single characters, ‘{’ and ‘}’ are used to delimit
the %top block, as show in the example below:
%top{
/* This code goes at the "top" of the generated file. */
1
Actually, yyIN_HEADER is defined before the ‘%top’ block.
Chapter 5: Format of the Input File
7
#include <stdint.h>
#include <inttypes.h>
}
Multiple %top blocks are allowed, and their order is preserved.
5.2 Format of the Rules Section
The rules section of the flex input contains a series of rules of the form:
pattern
action
where the pattern must be unindented and the action must begin on the same line. See
Chapter 6 [Patterns], page 9, for a further description of patterns and actions.
In the rules section, any indented or %{ %} enclosed text appearing before the first rule
may be used to declare variables which are local to the scanning routine and (after the
declarations) code which is to be executed whenever the scanning routine is entered. Other
indented or %{ %} text in the rule section is still copied to the output, but its meaning is
not well-defined and it may well cause compile-time errors (this feature is present for POSIX
compliance. See Chapter 20 [Lex and Posix], page 61, for other such features).
Any indented text or text enclosed in ‘%{’ and ‘%}’ is copied verbatim to the output
(with the %{ and %} symbols removed). The %{ and %} symbols must appear unindented
on lines by themselves.
5.3 Format of the User Code Section
The user code section is simply copied to ‘lex.yy.c’ verbatim. It is used for companion
routines which call or are called by the scanner. The presence of this section is optional; if
it is missing, the second ‘%%’ in the input file may be skipped, too.
5.4 Comments in the Input
Flex supports C-style comments, that is, anything between ‘/*’ and ‘*/’ is considered a
comment. Whenever flex encounters a comment, it copies the entire comment verbatim
to the generated source code. Comments may appear just about anywhere, but with the
following exceptions:
• Comments may not appear in the Rules Section wherever flex is expecting a regular expression. This means comments may not appear at the beginning of a line, or
immediately following a list of scanner states.
• Comments may not appear on an ‘%option’ line in the Definitions Section.
If you want to follow a simple rule, then always begin a comment on a new line, with
one or more whitespace characters before the initial ‘/*’). This rule will work anywhere in
the input file.
All the comments in the following example are valid:
%{
/* code block */
%}
/* Definitions Section */
Chapter 5: Format of the Input File
%x STATE_X
%%
/* Rules Section */
ruleA
/* after regex */ { /* code block */ } /* after code block */
/* Rules Section (indented) */
<STATE_X>{
ruleC
ECHO;
ruleD
ECHO;
%{
/* code block */
%}
}
%%
/* User Code Section */
8
Chapter 6: Patterns
9
6 Patterns
The patterns in the input (see Section 5.2 [Rules Section], page 7) are written using an
extended set of regular expressions. These are:
‘x’
match the character ’x’
‘.’
any character (byte) except newline
‘[xyz]’
a character class; in this case, the pattern matches either an ’x’, a ’y’, or a ’z’
‘[abj-oZ]’
a "character class" with a range in it; matches an ’a’, a ’b’, any letter from ’j’
through ’o’, or a ’Z’
‘[^A-Z]’
a "negated character class", i.e., any character but those in the class. In this
case, any character EXCEPT an uppercase letter.
‘[^A-Z\n]’
any character EXCEPT an uppercase letter or a newline
‘[a-z]{-}[aeiou]’
the lowercase consonants
‘r*’
zero or more r’s, where r is any regular expression
‘r+’
one or more r’s
‘r?’
zero or one r’s (that is, “an optional r”)
‘r{2,5}’
anywhere from two to five r’s
‘r{2,}’
two or more r’s
‘r{4}’
exactly 4 r’s
‘{name}’
the expansion of the ‘name’ definition (see Chapter 5 [Format], page 6).
‘"[xyz]\"foo"’
the literal string: ‘[xyz]"foo’
‘\X’
if X is ‘a’, ‘b’, ‘f’, ‘n’, ‘r’, ‘t’, or ‘v’, then the ANSI-C interpretation of ‘\x’.
Otherwise, a literal ‘X’ (used to escape operators such as ‘*’)
‘\0’
a NUL character (ASCII code 0)
‘\123’
the character with octal value 123
‘\x2a’
the character with hexadecimal value 2a
‘(r)’
match an ‘r’; parentheses are used to override precedence (see below)
‘(?r-s:pattern)’
apply option ‘r’ and omit option ‘s’ while interpreting pattern. Options may
be zero or more of the characters ‘i’, ‘s’, or ‘x’.
‘i’ means case-insensitive. ‘-i’ means case-sensitive.
‘s’ alters the meaning of the ‘.’ syntax to match any single byte whatsoever.
‘-s’ alters the meaning of ‘.’ to match any byte except ‘\n’.
Chapter 6: Patterns
10
‘x’ ignores comments and whitespace in patterns. Whitespace is ignored unless
it is backslash-escaped, contained within ‘""’s, or appears inside a character
class.
The following are all valid:
(?:foo)
same as (foo)
(?i:ab7)
same as ([aA][bB]7)
(?-i:ab)
same as (ab)
(?s:.)
same as [\x00-\xFF]
(?-s:.)
same as [^\n]
(?ix-s: a . b) same as ([Aa][^\n][bB])
(?x:a b)
same as ("ab")
(?x:a\ b)
same as ("a b")
(?x:a" "b)
same as ("a b")
(?x:a[ ]b)
same as ("a b")
(?x:a
/* comment */
b
c)
same as (abc)
‘(?# comment )’
omit everything within ‘()’. The first ‘)’ character encountered ends the pattern. It is not possible to for the comment to contain a ‘)’ character. The
comment may span lines.
‘rs’
the regular expression ‘r’ followed by the regular expression ‘s’; called concatenation
‘r|s’
either an ‘r’ or an ‘s’
‘r/s’
an ‘r’ but only if it is followed by an ‘s’. The text matched by ‘s’ is included
when determining whether this rule is the longest match, but is then returned
to the input before the action is executed. So the action only sees the text
matched by ‘r’. This type of pattern is called trailing context. (There are
some combinations of ‘r/s’ that flex cannot match correctly. See Chapter 24
[Limitations], page 74, regarding dangerous trailing context.)
‘^r’
an ‘r’, but only at the beginning of a line (i.e., when just starting to scan, or
right after a newline has been scanned).
‘r$’
an ‘r’, but only at the end of a line (i.e., just before a newline). Equivalent to
‘r/\n’.
Note that flex’s notion of “newline” is exactly whatever the C compiler used
to compile flex interprets ‘\n’ as; in particular, on some DOS systems you
must either filter out ‘\r’s in the input yourself, or explicitly use ‘r/\r\n’ for
‘r$’.
‘<s>r’
an ‘r’, but only in start condition s (see Chapter 10 [Start Conditions], page 21
for discussion of start conditions).
‘<s1,s2,s3>r’
same, but in any of start conditions s1, s2, or s3.
Chapter 6: Patterns
11
‘<*>r’
an ‘r’ in any start condition, even an exclusive one.
‘<<EOF>>’
an end-of-file.
‘<s1,s2><<EOF>>’
an end-of-file when in start condition s1 or s2
Note that inside of a character class, all regular expression operators lose their special
meaning except escape (‘\’) and the character class operators, ‘-’, ‘]]’, and, at the beginning
of the class, ‘^’.
The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom. Those grouped together have equal
precedence (see special note on the precedence of the repeat operator, ‘{}’, under the documentation for the ‘--posix’ POSIX compliance option). For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the ‘*’ operator has higher precedence than concatenation, and concatenation
higher than alternation (‘|’). This pattern therefore matches either the string ‘foo’ or
the string ‘ba’ followed by zero-or-more ‘r’’s. To match ‘foo’ or zero-or-more repetitions of
the string ‘bar’, use:
foo|(bar)*
And to match a sequence of zero or more repetitions of ‘foo’ and ‘bar’:
(foo|bar)*
In addition to characters and ranges of characters, character classes can also contain
character class expressions. These are expressions enclosed inside ‘[’: and ‘:]’ delimiters
(which themselves must appear between the ‘[’ and ‘]’ of the character class. Other elements
may occur inside the character class, too). The valid expressions are:
[:alnum:]
[:cntrl:]
[:lower:]
[:space:]
[:alpha:]
[:digit:]
[:print:]
[:upper:]
[:blank:]
[:graph:]
[:punct:]
[:xdigit:]
These expressions all designate a set of characters equivalent to the corresponding standard C isXXX function. For example, ‘[:alnum:]’ designates those characters for which
isalnum() returns true - i.e., any alphabetic or numeric character. Some systems don’t
provide isblank(), so flex defines ‘[:blank:]’ as a blank or a tab.
For example, the following character classes are all equivalent:
[[:alnum:]]
[[:alpha:][:digit:]]
[[:alpha:][0-9]]
[a-zA-Z0-9]
A word of caution. Character classes are expanded immediately when seen in the flex
input. This means the character classes are sensitive to the locale in which flex is executed,
and the resulting scanner will not be sensitive to the runtime locale. This may or may not
be desirable.
Chapter 6: Patterns
12
• If your scanner is case-insensitive (the ‘-i’ flag), then ‘[:upper:]’ and ‘[:lower:]’ are
equivalent to ‘[:alpha:]’.
• Character classes with ranges, such as ‘[a-Z]’, should be used with caution in a caseinsensitive scanner if the range spans upper or lowercase characters. Flex does not
know if you want to fold all upper and lowercase characters together, or if you want the
literal numeric range specified (with no case folding). When in doubt, flex will assume
that you meant the literal numeric range, and will issue a warning. The exception to
this rule is a character range such as ‘[a-z]’ or ‘[S-W]’ where it is obvious that you
want case-folding to occur. Here are some examples with the ‘-i’ flag enabled:
Range
Result
Literal Range
Alternate Range
‘[a-t]’
ok
‘[a-tA-T]’
‘[A-T]’
ok
‘[a-tA-T]’
‘[A-t]’
ambiguous ‘[A-Z\[\\\]_‘a-t]’ ‘[a-tA-T]’
‘[_-{]’
ambiguous ‘[_‘a-z{]’
‘[_‘a-zA-Z{]’
‘[@-C]’
ambiguous ‘[@ABC]’
‘[@A-Z\[\\\]_‘abc]’
• A negated character class such as the example ‘[^A-Z]’ above will match a newline
unless ‘\n’ (or an equivalent escape sequence) is one of the characters explicitly present
in the negated character class (e.g., ‘[^A-Z\n]’). This is unlike how many other regular
expression tools treat negated character classes, but unfortunately the inconsistency is
historically entrenched. Matching newlines means that a pattern like ‘[^"]*’ can match
the entire input unless there’s another quote in the input.
Flex allows negation of character class expressions by prepending ‘^’ to the POSIX
character class name.
[:^alnum:] [:^alpha:] [:^blank:]
[:^cntrl:] [:^digit:] [:^graph:]
[:^lower:] [:^print:] [:^punct:]
[:^space:] [:^upper:] [:^xdigit:]
Flex will issue a warning if the expressions ‘[:^upper:]’ and ‘[:^lower:]’ appear in
a case-insensitive scanner, since their meaning is unclear. The current behavior is to
skip them entirely, but this may change without notice in future revisions of flex.
•
The ‘{-}’ operator computes the difference of two character classes. For example,
‘[a-c]{-}[b-z]’ represents all the characters in the class ‘[a-c]’ that are not in the
class ‘[b-z]’ (which in this case, is just the single character ‘a’). The ‘{-}’ operator
is left associative, so ‘[abc]{-}[b]{-}[c]’ is the same as ‘[a]’. Be careful not to
accidentally create an empty set, which will never match.
•
The ‘{+}’ operator computes the union of two character classes. For example,
‘[a-z]{+}[0-9]’ is the same as ‘[a-z0-9]’. This operator is useful when preceded
by the result of a difference operation, as in, ‘[[:alpha:]]{-}[[:lower:]]{+}[q]’,
which is equivalent to ‘[A-Zq]’ in the "C" locale.
• A rule can have at most one instance of trailing context (the ‘/’ operator or the ‘$’
operator). The start condition, ‘^’, and ‘<<EOF>>’ patterns can only occur at the
beginning of a pattern, and, as well as with ‘/’ and ‘$’, cannot be grouped inside
parentheses. A ‘^’ which does not occur at the beginning of a rule or a ‘$’ which does
Chapter 6: Patterns
13
not occur at the end of a rule loses its special properties and is treated as a normal
character.
• The following are invalid:
foo/bar$
<sc1>foo<sc2>bar
Note that the first of these can be written ‘foo/bar\n’.
• The following will result in ‘$’ or ‘^’ being treated as a normal character:
foo|(bar$)
foo|^bar
If the desired meaning is a ‘foo’ or a ‘bar’-followed-by-a-newline, the following could
be used (the special | action is explained below, see Chapter 8 [Actions], page 15):
foo
|
bar$
/* action goes here */
A similar trick will work for matching a ‘foo’ or a ‘bar’-at-the-beginning-of-a-line.
Chapter 7: How the Input Is Matched
14
7 How the Input Is Matched
When the generated scanner is run, it analyzes its input looking for strings which match
any of its patterns. If it finds more than one match, it takes the one matching the most
text (for trailing context rules, this includes the length of the trailing part, even though it
will then be returned to the input). If it finds two or more matches of the same length, the
rule listed first in the flex input file is chosen.
Once the match is determined, the text corresponding to the match (called the token) is
made available in the global character pointer yytext, and its length in the global integer
yyleng. The action corresponding to the matched pattern is then executed (see Chapter 8
[Actions], page 15), and then the remaining input is scanned for another match.
If no match is found, then the default rule is executed: the next character in the input
is considered matched and copied to the standard output. Thus, the simplest valid flex
input is:
%%
which generates a scanner that simply copies its input (one character at a time) to its
output.
Note that yytext can be defined in two different ways: either as a character pointer or
as a character array. You can control which definition flex uses by including one of the
special directives %pointer or %array in the first (definitions) section of your flex input.
The default is %pointer, unless you use the ‘-l’ lex compatibility option, in which case
yytext will be an array. The advantage of using %pointer is substantially faster scanning
and no buffer overflow when matching very large tokens (unless you run out of dynamic
memory). The disadvantage is that you are restricted in how your actions can modify
yytext (see Chapter 8 [Actions], page 15), and calls to the unput() function destroys the
present contents of yytext, which can be a considerable porting headache when moving
between different lex versions.
The advantage of %array is that you can then modify yytext to your heart’s content, and
calls to unput() do not destroy yytext (see Chapter 8 [Actions], page 15). Furthermore,
existing lex programs sometimes access yytext externally using declarations of the form:
extern char yytext[];
This definition is erroneous when used with %pointer, but correct for %array.
The %array declaration defines yytext to be an array of YYLMAX characters, which
defaults to a fairly large value. You can change the size by simply #define’ing YYLMAX to a
different value in the first section of your flex input. As mentioned above, with %pointer
yytext grows dynamically to accommodate large tokens. While this means your %pointer
scanner can accommodate very large tokens (such as matching entire blocks of comments),
bear in mind that each time the scanner must resize yytext it also must rescan the entire
token from the beginning, so matching such tokens can prove slow. yytext presently does
not dynamically grow if a call to unput() results in too much text being pushed back;
instead, a run-time error results.
Also note that you cannot use %array with C++ scanner classes (see Chapter 18 [Cxx],
page 50).
Chapter 8: Actions
15
8 Actions
Each pattern in a rule has a corresponding action, which can be any arbitrary C statement.
The pattern ends at the first non-escaped whitespace character; the remainder of the line
is its action. If the action is empty, then when the pattern is matched the input token is
simply discarded. For example, here is the specification for a program which deletes all
occurrences of ‘zap me’ from its input:
%%
"zap me"
This example will copy all other characters in the input to the output since they will be
matched by the default rule.
Here is a program which compresses multiple blanks and tabs down to a single blank,
and throws away whitespace found at the end of a line:
%%
[ \t]+
putchar( ’ ’ );
[ \t]+$
/* ignore this token */
If the action contains a ‘{’, then the action spans till the balancing ‘}’ is found, and
the action may cross multiple lines. flex knows about C strings and comments and won’t
be fooled by braces found within them, but also allows actions to begin with ‘%{’ and will
consider the action to be all the text up to the next ‘%}’ (regardless of ordinary braces inside
the action).
An action consisting solely of a vertical bar (‘|’) means “same as the action for the next
rule”. See below for an illustration.
Actions can include arbitrary C code, including return statements to return a value
to whatever routine called yylex(). Each time yylex() is called it continues processing
tokens from where it last left off until it either reaches the end of the file or executes a
return.
Actions are free to modify yytext except for lengthening it (adding characters to its
end–these will overwrite later characters in the input stream). This however does not apply
when using %array (see Chapter 7 [Matching], page 14). In that case, yytext may be freely
modified in any way.
Actions are free to modify yyleng except they should not do so if the action also includes
use of yymore() (see below).
There are a number of special directives which can be included within an action:
ECHO
copies yytext to the scanner’s output.
BEGIN
followed by the name of a start condition places the scanner in the corresponding
start condition (see below).
REJECT
directs the scanner to proceed on to the “second best” rule which matched
the input (or a prefix of the input). The rule is chosen as described above in
Chapter 7 [Matching], page 14, and yytext and yyleng set up appropriately.
It may either be one which matched as much text as the originally chosen rule
but came later in the flex input file, or one which matched less text. For
example, the following will both count the words in the input and call the
routine special() whenever ‘frob’ is seen:
Chapter 8: Actions
16
int word_count = 0;
%%
frob
special(); REJECT;
[^ \t\n]+
++word_count;
Without the REJECT, any occurrences of ‘frob’ in the input would not be
counted as words, since the scanner normally executes only one action per
token. Multiple uses of REJECT are allowed, each one finding the next best
choice to the currently active rule. For example, when the following scanner
scans the token ‘abcd’, it will write ‘abcdabcaba’ to the output:
%%
a
|
ab
|
abc
|
abcd
ECHO; REJECT;
.|\n
/* eat up any unmatched character */
The first three rules share the fourth’s action since they use the special ‘|’
action.
REJECT is a particularly expensive feature in terms of scanner performance; if
it is used in any of the scanner’s actions it will slow down all of the scanner’s
matching. Furthermore, REJECT cannot be used with the ‘-Cf’ or ‘-CF’ options
(see Chapter 16 [Scanner Options], page 35).
Note also that unlike the other special actions, REJECT is a branch. Code
immediately following it in the action will not be executed.
yymore()
tells the scanner that the next time it matches a rule, the corresponding token should be appended onto the current value of yytext rather than replacing it. For example, given the input ‘mega-kludge’ the following will write
‘mega-mega-kludge’ to the output:
%%
megaECHO; yymore();
kludge
ECHO;
First ‘mega-’ is matched and echoed to the output. Then ‘kludge’ is matched,
but the previous ‘mega-’ is still hanging around at the beginning of yytext so
the ECHO for the ‘kludge’ rule will actually write ‘mega-kludge’.
Two notes regarding use of yymore(). First, yymore() depends on the value of yyleng
correctly reflecting the size of the current token, so you must not modify yyleng if you are
using yymore(). Second, the presence of yymore() in the scanner’s action entails a minor
performance penalty in the scanner’s matching speed.
yyless(n) returns all but the first n characters of the current token back to the input
stream, where they will be rescanned when the scanner looks for the next match. yytext
and yyleng are adjusted appropriately (e.g., yyleng will now be equal to n). For example,
on the input ‘foobar’ the following will write out ‘foobarbar’:
%%
foobar
ECHO; yyless(3);
Chapter 8: Actions
17
[a-z]+
ECHO;
An argument of 0 to yyless() will cause the entire current input string to be scanned
again. Unless you’ve changed how the scanner will subsequently process its input (using
BEGIN, for example), this will result in an endless loop.
Note that yyless() is a macro and can only be used in the flex input file, not from
other source files.
unput(c) puts the character c back onto the input stream. It will be the next character
scanned. The following action will take the current token and cause it to be rescanned
enclosed in parentheses.
{
int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ’)’ );
for ( i = yyleng - 1; i >= 0; --i )
unput( yycopy[i] );
unput( ’(’ );
free( yycopy );
}
Note that since each unput() puts the given character back at the beginning of the input
stream, pushing back strings must be done back-to-front.
An important potential problem when using unput() is that if you are using %pointer
(the default), a call to unput() destroys the contents of yytext, starting with its rightmost
character and devouring one character to the left with each call. If you need the value of
yytext preserved after a call to unput() (as in the above example), you must either first
copy it elsewhere, or build your scanner using %array instead (see Chapter 7 [Matching],
page 14).
Finally, note that you cannot put back ‘EOF’ to attempt to mark the input stream with
an end-of-file.
input() reads the next character from the input stream. For example, the following is
one way to eat up C comments:
%%
"/*"
{
register int c;
for ( ; ; )
{
while ( (c = input()) != ’*’ &&
c != EOF )
;
/* eat up text of comment */
if ( c == ’*’ )
{
while ( (c = input()) == ’*’ )
;
Chapter 8: Actions
18
if ( c == ’/’ )
break;
/* found the end */
}
if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}
(Note that if the scanner is compiled using C++, then input() is instead referred to as
yyinput(), in order to avoid a name clash with the C++ stream by the name of input.)
YY_FLUSH_BUFFER; flushes the scanner’s internal buffer so that the next time the scanner
attempts to match a token, it will first refill the buffer using YY_INPUT() (see Chapter 9
[Generated Scanner], page 19). This action is a special case of the more general yy_flush_
buffer; function, described below (see Chapter 11 [Multiple Input Buffers], page 27)
yyterminate() can be used in lieu of a return statement in an action. It terminates
the scanner and returns a 0 to the scanner’s caller, indicating “all done”. By default,
yyterminate() is also called when an end-of-file is encountered. It is a macro and may be
redefined.
Chapter 9: The Generated Scanner
19
9 The Generated Scanner
The output of flex is the file ‘lex.yy.c’, which contains the scanning routine yylex(), a
number of tables used by it for matching tokens, and a number of auxiliary routines and
macros. By default, yylex() is declared as follows:
int yylex()
{
... various definitions and the actions in here ...
}
(If your environment supports function prototypes, then it will be int yylex( void ).)
This definition may be changed by defining the YY_DECL macro. For example, you could
use:
#define YY_DECL float lexscan( a, b ) float a, b;
to give the scanning routine the name lexscan, returning a float, and taking two floats as
arguments. Note that if you give arguments to the scanning routine using a K&R-style/nonprototyped function declaration, you must terminate the definition with a semi-colon (;).
flex generates ‘C99’ function definitions by default. However flex does have the ability
to generate obsolete, er, ‘traditional’, function definitions. This is to support bootstrapping gcc on old systems. Unfortunately, traditional definitions prevent us from using any
standard data types smaller than int (such as short, char, or bool) as function arguments.
For this reason, future versions of flex may generate standard C99 code only, leaving K&Rstyle functions to the historians. Currently, if you do not want ‘C99’ definitions, then you
must use %option noansi-definitions.
Whenever yylex() is called, it scans tokens from the global input file ‘yyin’ (which
defaults to stdin). It continues until it either reaches an end-of-file (at which point it
returns the value 0) or one of its actions executes a return statement.
If the scanner reaches an end-of-file, subsequent calls are undefined unless either ‘yyin’ is
pointed at a new input file (in which case scanning continues from that file), or yyrestart()
is called. yyrestart() takes one argument, a FILE * pointer (which can be NULL, if you’ve
set up YY_INPUT to scan from a source other than yyin), and initializes ‘yyin’ for scanning
from that file. Essentially there is no difference between just assigning ‘yyin’ to a new input
file or using yyrestart() to do so; the latter is available for compatibility with previous
versions of flex, and because it can be used to switch input files in the middle of scanning.
It can also be used to throw away the current input buffer, by calling it with an argument of
‘yyin’; but it would be better to use YY_FLUSH_BUFFER (see Chapter 8 [Actions], page 15).
Note that yyrestart() does not reset the start condition to INITIAL (see Chapter 10 [Start
Conditions], page 21).
If yylex() stops scanning due to executing a return statement in one of the actions,
the scanner may then be called again and it will resume scanning where it left off.
By default (and for purposes of efficiency), the scanner uses block-reads rather than
simple getc() calls to read characters from ‘yyin’. The nature of how it gets its input
can be controlled by defining the YY_INPUT macro. The calling sequence for YY_INPUT()
is YY_INPUT(buf,result,max_size). Its action is to place up to max_size characters in
the character array buf and return in the integer variable result either the number of
Chapter 9: The Generated Scanner
20
characters read or the constant YY_NULL (0 on Unix systems) to indicate ‘EOF’. The default
YY_INPUT reads from the global file-pointer ‘yyin’.
Here is a sample definition of YY_INPUT (in the definitions section of the input file):
%{
#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
}
%}
This definition will change the input processing to occur one character at a time.
When the scanner receives an end-of-file indication from YY INPUT, it then checks the
yywrap() function. If yywrap() returns false (zero), then it is assumed that the function
has gone ahead and set up ‘yyin’ to point to another input file, and scanning continues. If
it returns true (non-zero), then the scanner terminates, returning 0 to its caller. Note that
in either case, the start condition remains unchanged; it does not revert to INITIAL.
If you do not supply your own version of yywrap(), then you must either use %option
noyywrap (in which case the scanner behaves as though yywrap() returned 1), or you must
link with ‘-lfl’ to obtain the default version of the routine, which always returns 1.
For scanning from in-memory buffers (e.g., scanning strings), see [Scanning Strings],
page 30. See Chapter 11 [Multiple Input Buffers], page 27.
The scanner writes its ECHO output to the ‘yyout’ global (default, ‘stdout’), which may
be redefined by the user simply by assigning it to some other FILE pointer.
Chapter 10: Start Conditions
21
10 Start Conditions
flex provides a mechanism for conditionally activating rules. Any rule whose pattern is
prefixed with ‘<sc>’ will only be active when the scanner is in the start condition named
sc. For example,
<STRING>[^"]*
...
}
{ /* eat up the string body ... */
will be active only when the scanner is in the STRING start condition, and
<INITIAL,STRING,QUOTE>\.
...
}
{ /* handle an escape ... */
will be active only when the current start condition is either INITIAL, STRING, or QUOTE.
Start conditions are declared in the definitions (first) section of the input using unindented lines beginning with either ‘%s’ or ‘%x’ followed by a list of names. The former
declares inclusive start conditions, the latter exclusive start conditions. A start condition
is activated using the BEGIN action. Until the next BEGIN action is executed, rules with the
given start condition will be active and rules with other start conditions will be inactive.
If the start condition is inclusive, then rules with no start conditions at all will also be
active. If it is exclusive, then only rules qualified with the start condition will be active.
A set of rules contingent on the same exclusive start condition describe a scanner which is
independent of any of the other rules in the flex input. Because of this, exclusive start
conditions make it easy to specify “mini-scanners” which scan portions of the input that
are syntactically different from the rest (e.g., comments).
If the distinction between inclusive and exclusive start conditions is still a little vague,
here’s a simple example illustrating the connection between the two. The set of rules:
%s example
%%
<example>foo
do_something();
bar
something_else();
is equivalent to
%x example
%%
<example>foo
do_something();
<INITIAL,example>bar
something_else();
Without the <INITIAL,example> qualifier, the bar pattern in the second example
wouldn’t be active (i.e., couldn’t match) when in start condition example. If we just used
<example> to qualify bar, though, then it would only be active in example and not in
INITIAL, while in the first example it’s active in both, because in the first example the
example start condition is an inclusive (%s) start condition.
Chapter 10: Start Conditions
22
Also note that the special start-condition specifier <*> matches every start condition.
Thus, the above example could also have been written:
%x example
%%
<example>foo
<*>bar
do_something();
something_else();
The default rule (to ECHO any unmatched character) remains active in start conditions.
It is equivalent to:
<*>.|\n
ECHO;
BEGIN(0) returns to the original state where only the rules with no start conditions are
active. This state can also be referred to as the start-condition INITIAL, so BEGIN(INITIAL)
is equivalent to BEGIN(0). (The parentheses around the start condition name are not
required but are considered good style.)
BEGIN actions can also be given as indented code at the beginning of the rules section.
For example, the following will cause the scanner to enter the SPECIAL start condition
whenever yylex() is called and the global variable enter_special is true:
int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
<SPECIAL>blahblahblah
...more rules follow...
To illustrate the uses of start conditions, here is a scanner which provides two different
interpretations of a string like ‘123.456’. By default it will treat it as three tokens, the
integer ‘123’, a dot (‘.’), and the integer ‘456’. But if the string is preceded earlier in
the line by the string ‘expect-floats’ it will treat it as a single token, the floating-point
number ‘123.456’:
%{
#include <math.h>
%}
%s expect
%%
expect-floats
BEGIN(expect);
<expect>[0-9]+.[0-9]+
{
printf( "found a float, = %f\n",
atof( yytext ) );
}
<expect>\n
{
Chapter 10: Start Conditions
23
/* that’s the end of the line, so
* we need another "expect-number"
* before we’ll recognize any more
* numbers
*/
BEGIN(INITIAL);
}
[0-9]+
{
printf( "found an integer, = %d\n",
atoi( yytext ) );
}
"."
printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C comments while maintaining a count
of the current input line.
%x comment
%%
int line_num = 1;
"/*"
BEGIN(comment);
<comment>[^*\n]*
/* eat anything that’s not a ’*’ */
<comment>"*"+[^*/\n]*
/* eat up ’*’s not followed by ’/’s */
<comment>\n
++line_num;
<comment>"*"+"/"
BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much text as possible with each rule.
In general, when attempting to write a high-speed scanner try to match as much possible
in each rule, as it’s a big win.
Note that start-conditions names are really integer values and can be stored as such.
Thus, the above could be extended in the following fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*"
{
comment_caller = INITIAL;
BEGIN(comment);
}
...
<foo>"/*"
{
comment_caller = foo;
BEGIN(comment);
Chapter 10: Start Conditions
24
}
<comment>[^*\n]*
<comment>"*"+[^*/\n]*
<comment>\n
<comment>"*"+"/"
/* eat anything that’s not a ’*’ */
/* eat up ’*’s not followed by ’/’s */
++line_num;
BEGIN(comment_caller);
Furthermore, you can access the current start condition using the integer-valued YY_
START macro. For example, the above assignments to comment_caller could instead be
written
comment_caller = YY_START;
Flex provides YYSTATE as an alias for YY_START (since that is what’s used by AT&T
lex).
For historical reasons, start conditions do not have their own name-space within the
generated scanner. The start condition names are unmodified in the generated scanner and
generated header. See [option-header], page 35. See [option-prefix], page 40.
Finally, here’s an example of how to match C-style quoted strings using exclusive start
conditions, including expanded escape sequences (but not including checking for a string
that’s too long):
%x str
%%
char string_buf[MAX_STR_CONST];
char *string_buf_ptr;
\"
string_buf_ptr = string_buf; BEGIN(str);
<str>\"
{ /* saw closing quote - all done */
BEGIN(INITIAL);
*string_buf_ptr = ’\0’;
/* return string constant token type and
* value to parser
*/
}
<str>\n
{
/* error - unterminated string constant */
/* generate error message */
}
<str>\\[0-7]{1,3} {
/* octal escape sequence */
int result;
(void) sscanf( yytext + 1, "%o", &result );
Chapter 10: Start Conditions
25
if ( result > 0xff )
/* error, constant is out-of-bounds */
*string_buf_ptr++ = result;
}
<str>\\[0-9]+ {
/* generate error - bad escape sequence; something
* like ’\48’ or ’\0777777’
*/
}
<str>\\n
<str>\\t
<str>\\r
<str>\\b
<str>\\f
*string_buf_ptr++
*string_buf_ptr++
*string_buf_ptr++
*string_buf_ptr++
*string_buf_ptr++
<str>\\(.|\n)
=
=
=
=
=
’\n’;
’\t’;
’\r’;
’\b’;
’\f’;
*string_buf_ptr++ = yytext[1];
<str>[^\\\n\"]+
{
char *yptr = yytext;
while ( *yptr )
*string_buf_ptr++ = *yptr++;
}
Often, such as in some of the examples above, you wind up writing a whole bunch of
rules all preceded by the same start condition(s). Flex makes this a little easier and cleaner
by introducing a notion of start condition scope. A start condition scope is begun with:
<SCs>{
where SCs is a list of one or more start conditions. Inside the start condition scope,
every rule automatically has the prefix SCs> applied to it, until a ‘}’ which matches the
initial ‘{’. So, for example,
<ESC>{
"\\n"
"\\r"
"\\f"
"\\0"
}
return
return
return
return
’\n’;
’\r’;
’\f’;
’\0’;
return
return
return
return
’\n’;
’\r’;
’\f’;
’\0’;
is equivalent to:
<ESC>"\\n"
<ESC>"\\r"
<ESC>"\\f"
<ESC>"\\0"
Chapter 10: Start Conditions
26
Start condition scopes may be nested.
The following routines are available for manipulating stacks of start conditions:
void yy_push_state ( int new_state )
[Function]
pushes the current start condition onto the top of the start condition stack and
switches to new_state as though you had used BEGIN new_state (recall that start
condition names are also integers).
void yy_pop_state ()
[Function]
pops the top of the stack and switches to it via BEGIN.
int yy_top_state ()
[Function]
returns the top of the stack without altering the stack’s contents.
The start condition stack grows dynamically and so has no built-in size limitation. If
memory is exhausted, program execution aborts.
To use start condition stacks, your scanner must include a %option stack directive (see
Chapter 16 [Scanner Options], page 35).
Chapter 11: Multiple Input Buffers
27
11 Multiple Input Buffers
Some scanners (such as those which support “include” files) require reading from several
input streams. As flex scanners do a large amount of buffering, one cannot control where
the next input will be read from by simply writing a YY_INPUT() which is sensitive to the
scanning context. YY_INPUT() is only called when the scanner reaches the end of its buffer,
which may be a long time after scanning a statement such as an include statement which
requires switching the input source.
To negotiate these sorts of problems, flex provides a mechanism for creating and switching between multiple input buffers. An input buffer is created by using:
YY_BUFFER_STATE yy_create_buffer ( FILE *file, int size )
[Function]
which takes a FILE pointer and a size and creates a buffer associated with the given file
and large enough to hold size characters (when in doubt, use YY_BUF_SIZE for the size).
It returns a YY_BUFFER_STATE handle, which may then be passed to other routines (see
below). The YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state
structure, so you may safely initialize YY_BUFFER_STATE variables to ((YY_BUFFER_STATE)
0) if you wish, and also refer to the opaque structure in order to correctly declare input
buffers in source files other than that of your scanner. Note that the FILE pointer in the call
to yy_create_buffer is only used as the value of ‘yyin’ seen by YY_INPUT. If you redefine
YY_INPUT() so it no longer uses ‘yyin’, then you can safely pass a NULL FILE pointer to
yy_create_buffer. You select a particular buffer to scan from using:
void yy_switch_to_buffer ( YY BUFFER STATE new buffer )
[Function]
The above function switches the scanner’s input buffer so subsequent tokens will come
from new_buffer. Note that yy_switch_to_buffer() may be used by yywrap() to set
things up for continued scanning, instead of opening a new file and pointing ‘yyin’ at it. If
you are looking for a stack of input buffers, then you want to use yypush_buffer_state()
instead of this function. Note also that switching input sources via either yy_switch_to_
buffer() or yywrap() does not change the start condition.
void yy_delete_buffer ( YY BUFFER STATE buffer )
[Function]
is used to reclaim the storage associated with a buffer. (buffer can be NULL, in which
case the routine does nothing.) You can also clear the current contents of a buffer using:
void yypush_buffer_state ( YY BUFFER STATE buffer )
[Function]
This function pushes the new buffer state onto an internal stack. The pushed state
becomes the new current state. The stack is maintained by flex and will grow as required.
This function is intended to be used instead of yy_switch_to_buffer, when you want to
change states, but preserve the current state for later use.
void yypop_buffer_state ( )
[Function]
This function removes the current state from the top of the stack, and deletes it by
calling yy_delete_buffer. The next state on the stack, if any, becomes the new current
state.
void yy_flush_buffer ( YY BUFFER STATE buffer )
[Function]
This function discards the buffer’s contents, so the next time the scanner attempts to
match a token from the buffer, it will first fill the buffer anew using YY_INPUT().
Chapter 11: Multiple Input Buffers
28
YY_BUFFER_STATE yy_new_buffer ( FILE *file, int size )
[Function]
is an alias for yy_create_buffer(), provided for compatibility with the C++ use of new
and delete for creating and destroying dynamic objects.
YY_CURRENT_BUFFER macro returns a YY_BUFFER_STATE handle to the current buffer. It
should not be used as an lvalue.
Here are two examples of using these features for writing a scanner which expands include
files (the <<EOF>> feature is discussed below).
This first example uses yypush buffer state and yypop buffer state. Flex maintains the
stack internally.
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
%%
include
BEGIN(incl);
[a-z]+
[^a-z\n]*\n?
ECHO;
ECHO;
<incl>[ \t]*
/* eat the whitespace */
<incl>[^ \t\n]+
{ /* got the include file name */
yyin = fopen( yytext, "r" );
if ( ! yyin )
error( ... );
yypush_buffer_state(yy_create_buffer( yyin, YY_BUF_SIZE ));
BEGIN(INITIAL);
}
<<EOF>> {
yypop_buffer_state();
if ( !YY_CURRENT_BUFFER )
{
yyterminate();
}
}
The second example, below, does the same thing as the previous example did, but
manages its own input buffer stack manually (instead of letting flex do it).
/* the "incl" state is used for picking up the name
* of an include file
*/
%x incl
Chapter 11: Multiple Input Buffers
29
%{
#define MAX_INCLUDE_DEPTH 10
YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH];
int include_stack_ptr = 0;
%}
%%
include
BEGIN(incl);
[a-z]+
[^a-z\n]*\n?
ECHO;
ECHO;
<incl>[ \t]*
/* eat the whitespace */
<incl>[^ \t\n]+
{ /* got the include file name */
if ( include_stack_ptr >= MAX_INCLUDE_DEPTH )
{
fprintf( stderr, "Includes nested too deeply" );
exit( 1 );
}
include_stack[include_stack_ptr++] =
YY_CURRENT_BUFFER;
yyin = fopen( yytext, "r" );
if ( ! yyin )
error( ... );
yy_switch_to_buffer(
yy_create_buffer( yyin, YY_BUF_SIZE ) );
BEGIN(INITIAL);
}
<<EOF>> {
if ( --include_stack_ptr
{
yyterminate();
}
0 )
else
{
yy_delete_buffer( YY_CURRENT_BUFFER );
yy_switch_to_buffer(
include_stack[include_stack_ptr] );
}
Chapter 11: Multiple Input Buffers
30
}
The following routines are available for setting up input buffers for scanning in-memory
strings instead of files. All of them create a new input buffer for scanning the string, and
return a corresponding YY_BUFFER_STATE handle (which you should delete with yy_delete_
buffer() when done with it). They also switch to the new buffer using yy_switch_to_
buffer(), so the next call to yylex() will start scanning the string.
YY_BUFFER_STATE yy_scan_string ( const char *str )
[Function]
scans a NUL-terminated string.
YY_BUFFER_STATE yy_scan_bytes ( const char *bytes, int len )
[Function]
scans len bytes (including possibly NULs) starting at location bytes.
Note that both of these functions create and scan a copy of the string or bytes. (This
may be desirable, since yylex() modifies the contents of the buffer it is scanning.) You can
avoid the copy by using:
YY_BUFFER_STATE yy_scan_buffer (char *base, yy size t size)
[Function]
which scans in place the buffer starting at base, consisting of size bytes, the last
two bytes of which must be YY_END_OF_BUFFER_CHAR (ASCII NUL). These last two
bytes are not scanned; thus, scanning consists of base[0] through base[size-2],
inclusive.
If you fail to set up base in this manner (i.e., forget the final two YY_END_OF_BUFFER_
CHAR bytes), then yy_scan_buffer() returns a NULL pointer instead of creating a new
input buffer.
[Data type]
is an integral type to which you can cast an integer expression reflecting the size of
the buffer.
yy_size_t
Chapter 12: End-of-File Rules
31
12 End-of-File Rules
The special rule <<EOF>> indicates actions which are to be taken when an end-of-file is
encountered and yywrap() returns non-zero (i.e., indicates no further files to process). The
action must finish by doing one of the following things:
• assigning ‘yyin’ to a new input file (in previous versions of flex, after doing the
assignment you had to call the special action YY_NEW_FILE. This is no longer necessary.)
• executing a return statement;
• executing the special yyterminate() action.
• or, switching to a new buffer using yy_switch_to_buffer() as shown in the example
above.
<<EOF>> rules may not be used with other patterns; they may only be qualified with
a list of start conditions. If an unqualified <<EOF>> rule is given, it applies to all start
conditions which do not already have <<EOF>> actions. To specify an <<EOF>> rule for
only the initial start condition, use:
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed comments. An example:
%x quote
%%
...other rules for dealing with quotes...
<quote><<EOF>>
{
error( "unterminated quote" );
yyterminate();
}
<<EOF>> {
if ( *++filelist )
yyin = fopen( *filelist, "r" );
else
yyterminate();
}
Chapter 13: Miscellaneous Macros
32
13 Miscellaneous Macros
The macro YY_USER_ACTION can be defined to provide an action which is always executed
prior to the matched rule’s action. For example, it could be #define’d to call a routine to
convert yytext to lower-case. When YY_USER_ACTION is invoked, the variable yy_act gives
the number of the matched rule (rules are numbered starting with 1). Suppose you want
to profile how often each of your rules is matched. The following would do the trick:
#define YY_USER_ACTION ++ctr[yy_act]
where ctr is an array to hold the counts for the different rules. Note that the macro
YY_NUM_RULES gives the total number of rules (including the default rule), even if you use
‘-s)’, so a correct declaration for ctr is:
int ctr[YY_NUM_RULES];
The macro YY_USER_INIT may be defined to provide an action which is always executed
before the first scan (and before the scanner’s internal initializations are done). For example,
it could be used to call a routine to read in a data table or open a logging file.
The macro yy_set_interactive(is_interactive) can be used to control whether the
current buffer is considered interactive. An interactive buffer is processed more slowly, but
must be used when the scanner’s input source is indeed interactive to avoid problems due
to waiting to fill buffers (see the discussion of the ‘-I’ flag in Chapter 16 [Scanner Options],
page 35). A non-zero value in the macro invocation marks the buffer as interactive, a
zero value as non-interactive. Note that use of this macro overrides %option alwaysinteractive or %option never-interactive (see Chapter 16 [Scanner Options], page 35).
yy_set_interactive() must be invoked prior to beginning to scan the buffer that is (or
is not) to be considered interactive.
The macro yy_set_bol(at_bol) can be used to control whether the current buffer’s
scanning context for the next token match is done as though at the beginning of a line.
A non-zero macro argument makes rules anchored with ‘^’ active, while a zero argument
makes ‘^’ rules inactive.
The macro YY_AT_BOL() returns true if the next token scanned from the current buffer
will have ‘^’ rules active, false otherwise.
In the generated scanner, the actions are all gathered in one large switch statement
and separated using YY_BREAK, which may be redefined. By default, it is simply a break,
to separate each rule’s action from the following rule’s. Redefining YY_BREAK allows, for
example, C++ users to #define YY BREAK to do nothing (while being very careful that
every rule ends with a break or a return!) to avoid suffering from unreachable statement
warnings where because a rule’s action ends with return, the YY_BREAK is inaccessible.
Chapter 14: Values Available To the User
33
14 Values Available To the User
This chapter summarizes the various values available to the user in the rule actions.
char *yytext
holds the text of the current token. It may be modified but not lengthened
(you cannot append characters to the end).
If the special directive %array appears in the first section of the scanner description, then yytext is instead declared char yytext[YYLMAX], where YYLMAX is
a macro definition that you can redefine in the first section if you don’t like
the default value (generally 8KB). Using %array results in somewhat slower
scanners, but the value of yytext becomes immune to calls to unput(), which
potentially destroy its value when yytext is a character pointer. The opposite
of %array is %pointer, which is the default.
You cannot use %array when generating C++ scanner classes (the ‘-+’ flag).
int yyleng
holds the length of the current token.
FILE *yyin
is the file which by default flex reads from. It may be redefined but doing so
only makes sense before scanning begins or after an EOF has been encountered.
Changing it in the midst of scanning will have unexpected results since flex
buffers its input; use yyrestart() instead. Once scanning terminates because
an end-of-file has been seen, you can assign ‘yyin’ at the new input file and
then call the scanner again to continue scanning.
void yyrestart( FILE *new_file )
may be called to point ‘yyin’ at the new input file. The switch-over to the new
file is immediate (any previously buffered-up input is lost). Note that calling
yyrestart() with ‘yyin’ as an argument thus throws away the current input
buffer and continues scanning the same input file.
FILE *yyout
is the file to which ECHO actions are done. It can be reassigned by the user.
YY_CURRENT_BUFFER
returns a YY_BUFFER_STATE handle to the current buffer.
YY_START
returns an integer value corresponding to the current start condition. You can
subsequently use this value with BEGIN to return to that start condition.
Chapter 15: Interfacing with Yacc
34
15 Interfacing with Yacc
One of the main uses of flex is as a companion to the yacc parser-generator. yacc parsers
expect to call a routine named yylex() to find the next input token. The routine is
supposed to return the type of the next token as well as putting any associated value in the
global yylval. To use flex with yacc, one specifies the ‘-d’ option to yacc to instruct it to
generate the file ‘y.tab.h’ containing definitions of all the %tokens appearing in the yacc
input. This file is then included in the flex scanner. For example, if one of the tokens is
TOK_NUMBER, part of the scanner might look like:
%{
#include "y.tab.h"
%}
%%
[0-9]+
yylval = atoi( yytext ); return TOK_NUMBER;
Chapter 16: Scanner Options
35
16 Scanner Options
The various flex options are categorized by function in the following menu. If you want to
lookup a particular option by name, See [Index of Scanner Options], page 132.
Even though there are many scanner options, a typical scanner might only specify the
following options:
%option
8bit reentrant bison-bridge
%option
warn nodefault
%option
yylineno
%option
outfile="scanner.c" header-file="scanner.h"
The first line specifies the general type of scanner we want. The second line specifies
that we are being careful. The third line asks flex to track line numbers. The last line tells
flex what to name the files. (The options can be specified in any order. We just divided
them.)
flex also provides a mechanism for controlling options within the scanner specification
itself, rather than from the flex command-line. This is done by including %option directives
in the first section of the scanner specification. You can specify multiple options with a single
%option directive, and multiple directives in the first section of your flex input file.
Most options are given simply as names, optionally preceded by the word ‘no’ (with
no intervening whitespace) to negate their meaning. The names are the same as their
long-option equivalents (but without the leading ‘--’ ).
flex scans your rule actions to determine whether you use the REJECT or yymore()
features. The REJECT and yymore options are available to override its decision as to whether
you use the options, either by setting them (e.g., %option reject) to indicate the feature is
indeed used, or unsetting them to indicate it actually is not used (e.g., %option noyymore).
A number of options are available for lint purists who want to suppress the appearance
of unneeded routines in the generated scanner. Each of the following, if unset (e.g., %option
nounput), results in the corresponding routine not appearing in the generated scanner:
input, unput
yy_push_state, yy_pop_state, yy_top_state
yy_scan_buffer, yy_scan_bytes, yy_scan_string
yyget_extra, yyset_extra, yyget_leng, yyget_text,
yyget_lineno, yyset_lineno, yyget_in, yyset_in,
yyget_out, yyset_out, yyget_lval, yyset_lval,
yyget_lloc, yyset_lloc, yyget_debug, yyset_debug
(though yy_push_state() and friends won’t appear anyway unless you use %option
stack).
16.1 Options for Specifying Filenames
‘--header-file=FILE, %option header-file="FILE"’
instructs flex to write a C header to ‘FILE’. This file contains function prototypes, extern variables, and types used by the scanner. Only the external
API is exported by the header file. Many macros that are usable from within
Chapter 16: Scanner Options
36
scanner actions are not exported to the header file. This is due to namespace
problems and the goal of a clean external API.
While in the header, the macro yyIN_HEADER is defined, where ‘yy’ is substituted
with the appropriate prefix.
The ‘--header-file’ option is not compatible with the ‘--c++’ option, since
the C++ scanner provides its own header in ‘yyFlexLexer.h’.
‘-oFILE, --outfile=FILE, %option outfile="FILE"’
directs flex to write the scanner to the file ‘FILE’ instead of ‘lex.yy.c’. If you
combine ‘--outfile’ with the ‘--stdout’ option, then the scanner is written
to ‘stdout’ but its #line directives (see the ‘-l’ option above) refer to the file
‘FILE’.
‘-t, --stdout, %option stdout’
instructs flex to write the scanner it generates to standard output instead of
‘lex.yy.c’.
‘-SFILE, --skel=FILE’
overrides the default skeleton file from which flex constructs its scanners.
You’ll never need this option unless you are doing flex maintenance or development.
‘--tables-file=FILE’
Write serialized scanner dfa tables to FILE. The generated scanner will not contain the tables, and requires them to be loaded at runtime. See [serialization],
page 67.
‘--tables-verify’
This option is for flex development. We document it here in case you stumble
upon it by accident or in case you suspect some inconsistency in the serialized
tables. Flex will serialize the scanner dfa tables but will also generate the incode tables as it normally does. At runtime, the scanner will verify that the
serialized tables match the in-code tables, instead of loading them.
16.2 Options Affecting Scanner Behavior
‘-i, --case-insensitive, %option case-insensitive’
instructs flex to generate a case-insensitive scanner. The case of letters given
in the flex input patterns will be ignored, and tokens in the input will be
matched regardless of case. The matched text given in yytext will have the
preserved case (i.e., it will not be folded). For tricky behavior, see [case and
character ranges], page 12.
‘-l, --lex-compat, %option lex-compat’
turns on maximum compatibility with the original AT&T lex implementation.
Note that this does not mean full compatibility. Use of this option costs a
considerable amount of performance, and it cannot be used with the ‘--c++’,
‘--full’, ‘--fast’, ‘-Cf’, or ‘-CF’ options. For details on the compatibilities it
provides, see Chapter 20 [Lex and Posix], page 61. This option also results in
the name YY_FLEX_LEX_COMPAT being #define’d in the generated scanner.
Chapter 16: Scanner Options
37
‘-B, --batch, %option batch’
instructs flex to generate a batch scanner, the opposite of interactive scanners
generated by ‘--interactive’ (see below). In general, you use ‘-B’ when you
are certain that your scanner will never be used interactively, and you want to
squeeze a little more performance out of it. If your goal is instead to squeeze
out a lot more performance, you should be using the ‘-Cf’ or ‘-CF’ options,
which turn on ‘--batch’ automatically anyway.
‘-I, --interactive, %option interactive’
instructs flex to generate an interactive scanner. An interactive scanner is one
that only looks ahead to decide what token has been matched if it absolutely
must. It turns out that always looking one extra character ahead, even if the
scanner has already seen enough text to disambiguate the current token, is a bit
faster than only looking ahead when necessary. But scanners that always look
ahead give dreadful interactive performance; for example, when a user types a
newline, it is not recognized as a newline token until they enter another token,
which often means typing in another whole line.
flex scanners default to interactive unless you use the ‘-Cf’ or ‘-CF’ tablecompression options (see Chapter 17 [Performance], page 45). That’s because
if you’re looking for high-performance you should be using one of these options, so if you didn’t, flex assumes you’d rather trade off a bit of run-time
performance for intuitive interactive behavior. Note also that you cannot use
‘--interactive’ in conjunction with ‘-Cf’ or ‘-CF’. Thus, this option is not
really needed; it is on by default for all those cases in which it is allowed.
You can force a scanner to not be interactive by using ‘--batch’
‘-7, --7bit, %option 7bit’
instructs flex to generate a 7-bit scanner, i.e., one which can only recognize 7bit characters in its input. The advantage of using ‘--7bit’ is that the scanner’s
tables can be up to half the size of those generated using the ‘--8bit’. The
disadvantage is that such scanners often hang or crash if their input contains
an 8-bit character.
Note, however, that unless you generate your scanner using the ‘-Cf’ or ‘-CF’
table compression options, use of ‘--7bit’ will save only a small amount of table
space, and make your scanner considerably less portable. Flex’s default behavior is to generate an 8-bit scanner unless you use the ‘-Cf’ or ‘-CF’, in which
case flex defaults to generating 7-bit scanners unless your site was always configured to generate 8-bit scanners (as will often be the case with non-USA sites).
You can tell whether flex generated a 7-bit or an 8-bit scanner by inspecting
the flag summary in the ‘--verbose’ output as described above.
Note that if you use ‘-Cfe’ or ‘-CFe’ flex still defaults to generating an 8-bit
scanner, since usually with these compression options full 8-bit tables are not
much more expensive than 7-bit tables.
‘-8, --8bit, %option 8bit’
instructs flex to generate an 8-bit scanner, i.e., one which can recognize 8-bit
characters. This flag is only needed for scanners generated using ‘-Cf’ or ‘-CF’,
as otherwise flex defaults to generating an 8-bit scanner anyway.
Chapter 16: Scanner Options
38
See the discussion of ‘--7bit’ above for flex’s default behavior and the tradeoffs between 7-bit and 8-bit scanners.
‘--default, %option default’
generate the default rule.
‘--always-interactive, %option always-interactive’
instructs flex to generate a scanner which always considers its input interactive.
Normally, on each new input file the scanner calls isatty() in an attempt to
determine whether the scanner’s input source is interactive and thus should be
read a character at a time. When this option is used, however, then no such
call is made.
‘--never-interactive, --never-interactive’
instructs flex to generate a scanner which never considers its input interactive.
This is the opposite of always-interactive.
‘-X, --posix, %option posix’
turns on maximum compatibility with the POSIX 1003.2-1992 definition of lex.
Since flex was originally designed to implement the POSIX definition of lex
this generally involves very few changes in behavior. At the current writing the
known differences between flex and the POSIX standard are:
• In POSIX and AT&T lex, the repeat operator, ‘{}’, has lower precedence than concatenation (thus ‘ab{3}’ yields ‘ababab’). Most POSIX
utilities use an Extended Regular Expression (ERE) precedence that has
the precedence of the repeat operator higher than concatenation (which
causes ‘ab{3}’ to yield ‘abbb’). By default, flex places the precedence
of the repeat operator higher than concatenation which matches the ERE
processing of other POSIX utilities. When either ‘--posix’ or ‘-l’ are
specified, flex will use the traditional AT&T and POSIX-compliant precedence for the repeat operator where concatenation has higher precedence
than the repeat operator.
‘--stack, %option stack’
enables the use of start condition stacks (see Chapter 10 [Start Conditions],
page 21).
‘--stdinit, %option stdinit’
if set (i.e., %option stdinit) initializes yyin and yyout to ‘stdin’ and ‘stdout’,
instead of the default of ‘NULL’. Some existing lex programs depend on this
behavior, even though it is not compliant with ANSI C, which does not require
‘stdin’ and ‘stdout’ to be compile-time constant. In a reentrant scanner,
however, this is not a problem since initialization is performed in yylex_init
at runtime.
‘--yylineno, %option yylineno’
directs flex to generate a scanner that maintains the number of the current
line read from its input in the global variable yylineno. This option is implied
by %option lex-compat. In a reentrant C scanner, the macro yylineno is
accessible regardless of the value of %option yylineno, however, its value is
not modified by flex unless %option yylineno is enabled.
Chapter 16: Scanner Options
39
‘--yywrap, %option yywrap’
if unset (i.e., --noyywrap), makes the scanner not call yywrap() upon an endof-file, but simply assume that there are no more files to scan (until the user
points ‘yyin’ at a new file and calls yylex() again).
16.3 Code-Level And API Options
‘--ansi-definitions, %option ansi-definitions’
instruct flex to generate ANSI C99 definitions for functions. This option is
enabled by default. If %option noansi-definitions is specified, then the obsolete style is generated.
‘--ansi-prototypes, %option ansi-prototypes’
instructs flex to generate ANSI C99 prototypes for functions. This option is
enabled by default. If noansi-prototypes is specified, then prototypes will
have empty parameter lists.
‘--bison-bridge, %option bison-bridge’
instructs flex to generate a C scanner that is meant to be called by a GNU
bison parser. The scanner has minor API changes for bison compatibility. In
particular, the declaration of yylex is modified to take an additional parameter,
yylval. See Section A.2 [Bison Bridge], page 122.
‘--bison-locations, %option bison-locations’
instruct flex that GNU bison %locations are being used. This means yylex
will be passed an additional parameter, yylloc. This option implies %option
bison-bridge. See Section A.2 [Bison Bridge], page 122.
‘-L, --noline, %option noline’
instructs flex not to generate #line directives. Without this option, flex
peppers the generated scanner with #line directives so error messages in the
actions will be correctly located with respect to either the original flex input
file (if the errors are due to code in the input file), or ‘lex.yy.c’ (if the errors
are flex’s fault – you should report these sorts of errors to the email address
given in Chapter 2 [Reporting Bugs], page 2).
‘-R, --reentrant, %option reentrant’
instructs flex to generate a reentrant C scanner. The generated scanner may
safely be used in a multi-threaded environment. The API for a reentrant scanner is different than for a non-reentrant scanner see Chapter 19 [Reentrant],
page 54). Because of the API difference between reentrant and non-reentrant
flex scanners, non-reentrant flex code must be modified before it is suitable
for use with this option. This option is not compatible with the ‘--c++’ option.
The option ‘--reentrant’ does not affect the performance of the scanner.
‘-+, --c++, %option c++’
specifies that you want flex to generate a C++ scanner class. See Chapter 18
[Cxx], page 50, for details.
‘--array, %option array’
specifies that you want yytext to be an array instead of a char*
Chapter 16: Scanner Options
40
‘--pointer, %option pointer’
specify that yytext should be a char *, not an array. This default is char *.
‘-PPREFIX, --prefix=PREFIX, %option prefix="PREFIX"’
changes the default ‘yy’ prefix used by flex for all globally-visible variable and
function names to instead be ‘PREFIX’. For example, ‘--prefix=foo’ changes
the name of yytext to footext. It also changes the name of the default output
file from ‘lex.yy.c’ to ‘lex.foo.c’. Here is a partial list of the names affected:
yy_create_buffer
yy_delete_buffer
yy_flex_debug
yy_init_buffer
yy_flush_buffer
yy_load_buffer_state
yy_switch_to_buffer
yyin
yyleng
yylex
yylineno
yyout
yyrestart
yytext
yywrap
yyalloc
yyrealloc
yyfree
(If you are using a C++ scanner, then only yywrap and yyFlexLexer are affected.) Within your scanner itself, you can still refer to the global variables
and functions using either version of their name; but externally, they have the
modified name.
This option lets you easily link together multiple flex programs into the same
executable. Note, though, that using this option also renames yywrap(), so
you now must either provide your own (appropriately-named) version of the
routine for your scanner, or use %option noyywrap, as linking with ‘-lfl’ no
longer provides one for you by default.
‘--main, %option main’
directs flex to provide a default main() program for the scanner, which simply
calls yylex(). This option implies noyywrap (see below).
‘--nounistd, %option nounistd’
suppresses inclusion of the non-ANSI header file ‘unistd.h’. This option is
meant to target environments in which ‘unistd.h’ does not exist. Be aware
that certain options may cause flex to generate code that relies on functions
normally found in ‘unistd.h’, (e.g. isatty(), read().) If you wish to use
these functions, you will have to inform your compiler where to find them. See
[option-always-interactive], page 38. See [option-read], page 41.
Chapter 16: Scanner Options
41
‘--yyclass=NAME, %option yyclass="NAME"’
only applies when generating a C++ scanner (the ‘--c++’ option).
It informs flex that you have derived NAME as a subclass of
yyFlexLexer, so flex will place your actions in the member function
foo::yylex() instead of yyFlexLexer::yylex().
It also generates a
yyFlexLexer::yylex() member function that emits a run-time error (by
invoking yyFlexLexer::LexerError()) if called. See Chapter 18 [Cxx],
page 50.
16.4 Options for Scanner Speed and Size
‘-C[aefFmr]’
controls the degree of table compression and, more generally, trade-offs between
small scanners and fast scanners.
‘-C’
A lone ‘-C’ specifies that the scanner tables should be compressed
but neither equivalence classes nor meta-equivalence classes should
be used.
‘-Ca, --align, %option align’
(“align”) instructs flex to trade off larger tables in the generated
scanner for faster performance because the elements of the tables
are better aligned for memory access and computation. On some
RISC architectures, fetching and manipulating longwords is more
efficient than with smaller-sized units such as shortwords. This
option can quadruple the size of the tables used by your scanner.
‘-Ce, --ecs, %option ecs’
directs flex to construct equivalence classes, i.e., sets of characters
which have identical lexical properties (for example, if the only appearance of digits in the flex input is in the character class “[0-9]”
then the digits ’0’, ’1’, ..., ’9’ will all be put in the same equivalence
class). Equivalence classes usually give dramatic reductions in the
final table/object file sizes (typically a factor of 2-5) and are pretty
cheap performance-wise (one array look-up per character scanned).
‘-Cf’
specifies that the full scanner tables should be generated - flex
should not compress the tables by taking advantages of similar
transition functions for different states.
‘-CF’
specifies that the alternate fast scanner representation (described
above under the ‘--fast’ flag) should be used. This option cannot
be used with ‘--c++’.
‘-Cm, --meta-ecs, %option meta-ecs’
directs flex to construct meta-equivalence classes, which are sets
of equivalence classes (or characters, if equivalence classes are not
being used) that are commonly used together. Meta-equivalence
classes are often a big win when using compressed tables, but they
have a moderate performance impact (one or two if tests and one
array look-up per character scanned).
Chapter 16: Scanner Options
42
‘-Cr, --read, %option read’
causes the generated scanner to bypass use of the standard I/O
library (stdio) for input. Instead of calling fread() or getc(), the
scanner will use the read() system call, resulting in a performance
gain which varies from system to system, but in general is probably
negligible unless you are also using ‘-Cf’ or ‘-CF’. Using ‘-Cr’
can cause strange behavior if, for example, you read from ‘yyin’
using stdio prior to calling the scanner (because the scanner will
miss whatever text your previous reads left in the stdio input
buffer). ‘-Cr’ has no effect if you define YY_INPUT() (see Chapter 9
[Generated Scanner], page 19).
The options ‘-Cf’ or ‘-CF’ and ‘-Cm’ do not make sense together - there is no
opportunity for meta-equivalence classes if the table is not being compressed.
Otherwise the options may be freely mixed, and are cumulative.
The default setting is ‘-Cem’, which specifies that flex should generate equivalence classes and meta-equivalence classes. This setting provides the highest
degree of table compression. You can trade off faster-executing scanners at the
cost of larger tables with the following generally being true:
slowest & smallest
-Cem
-Cm
-Ce
-C
-C{f,F}e
-C{f,F}
-C{f,F}a
fastest & largest
Note that scanners with the smallest tables are usually generated and compiled
the quickest, so during development you will usually want to use the default,
maximal compression.
‘-Cfe’ is often a good compromise between speed and size for production scanners.
‘-f, --full, %option full’
specifies fast scanner. No table compression is done and stdio is bypassed.
The result is large but fast. This option is equivalent to ‘--Cfr’
‘-F, --fast, %option fast’
specifies that the fast scanner table representation should be used (and stdio
bypassed). This representation is about as fast as the full table representation
‘--full’, and for some sets of patterns will be considerably smaller (and for
others, larger). In general, if the pattern set contains both keywords and a
catch-all, identifier rule, such as in the set:
"case"
return TOK_CASE;
"switch" return TOK_SWITCH;
...
"default" return TOK_DEFAULT;
Chapter 16: Scanner Options
43
[a-z]+
return TOK_ID;
then you’re better off using the full table representation. If only the identifier rule is present and you then use a hash table or some such to detect the
keywords, you’re better off using ‘--fast’.
This option is equivalent to ‘-CFr’. It cannot be used with ‘--c++’.
16.5 Debugging Options
‘-b, --backup, %option backup’
Generate backing-up information to ‘lex.backup’. This is a list of scanner
states which require backing up and the input characters on which they do so.
By adding rules one can remove backing-up states. If all backing-up states are
eliminated and ‘-Cf’ or -CF is used, the generated scanner will run faster (see
the ‘--perf-report’ flag). Only users who wish to squeeze every last cycle out
of their scanners need worry about this option. (see Chapter 17 [Performance],
page 45).
‘-d, --debug, %option debug’
makes the generated scanner run in debug mode. Whenever a pattern is recognized and the global variable yy_flex_debug is non-zero (which is the default),
the scanner will write to ‘stderr’ a line of the form:
-accepting rule at line 53 ("the matched text")
The line number refers to the location of the rule in the file defining the scanner
(i.e., the file that was fed to flex). Messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or
encounters a NUL; at this point, the two look the same as far as the scanner’s
concerned), or reaches an end-of-file.
‘-p, --perf-report, %option perf-report’
generates a performance report to ‘stderr’. The report consists of comments
regarding features of the flex input file which will cause a serious loss of performance in the resulting scanner. If you give the flag twice, you will also get
comments regarding features that lead to minor performance losses.
Note that the use of REJECT, and variable trailing context (see Chapter 24 [Limitations], page 74) entails a substantial performance penalty; use of yymore(),
the ‘^’ operator, and the ‘--interactive’ flag entail minor performance penalties.
‘-s, --nodefault, %option nodefault’
causes the default rule (that unmatched scanner input is echoed to ‘stdout)’
to be suppressed. If the scanner encounters input that does not match any of
its rules, it aborts with an error. This option is useful for finding holes in a
scanner’s rule set.
‘-T, --trace, %option trace’
makes flex run in trace mode. It will generate a lot of messages to ‘stderr’
concerning the form of the input and the resultant non-deterministic and deterministic finite automata. This option is mostly for use in maintaining flex.
Chapter 16: Scanner Options
44
‘-w, --nowarn, %option nowarn’
suppresses warning messages.
‘-v, --verbose, %option verbose’
specifies that flex should write to ‘stderr’ a summary of statistics regarding
the scanner it generates. Most of the statistics are meaningless to the casual
flex user, but the first line identifies the version of flex (same as reported
by ‘--version’), and the next line the flags used when generating the scanner,
including those that are on by default.
‘--warn, %option warn’
warn about certain things. In particular, if the default rule can be matched but
no default rule has been given, the flex will warn you. We recommend using
this option always.
16.6 Miscellaneous Options
‘-c’
A do-nothing option included for POSIX compliance.
‘-h, -?, --help’
generates a “help” summary of flex’s options to ‘stdout’ and then exits.
‘-n’
Another do-nothing option included for POSIX compliance.
‘-V, --version’
prints the version number to ‘stdout’ and exits.
Chapter 17: Performance Considerations
45
17 Performance Considerations
The main design goal of flex is that it generate high-performance scanners. It has been
optimized for dealing well with large sets of rules. Aside from the effects on scanner speed
of the table compression ‘-C’ options outlined above, there are a number of options/actions
which degrade performance. These are, from most expensive to least:
REJECT
arbitrary trailing context
pattern sets that require backing up
%option yylineno
%array
%option interactive
%option always-interactive
^ beginning-of-line operator
yymore()
with the first two all being quite expensive and the last two being quite cheap. Note
also that unput() is implemented as a routine call that potentially does quite a bit of work,
while yyless() is a quite-cheap macro. So if you are just putting back some excess text
you scanned, use yyless().
REJECT should be avoided at all costs when performance is important. It is a particularly
expensive option.
There is one case when %option yylineno can be expensive. That is when your patterns
match long tokens that could possibly contain a newline character. There is no performance
penalty for rules that can not possibly match newlines, since flex does not need to check
them for newlines. In general, you should avoid rules such as [^f]+, which match very long
tokens, including newlines, and may possibly match your entire file! A better approach is
to separate [^f]+ into two rules:
%option yylineno
%%
[^f\n]+
\n+
The above scanner does not incur a performance penalty.
Getting rid of backing up is messy and often may be an enormous amount of work
for a complicated scanner. In principal, one begins by using the ‘-b’ flag to generate a
‘lex.backup’ file. For example, on the input:
%%
foo
foobar
return TOK_KEYWORD;
return TOK_KEYWORD;
the file looks like:
State #6 is non-accepting associated rule line numbers:
Chapter 17: Performance Considerations
46
2
3
out-transitions: [ o ]
jam-transitions: EOF [ \001-n
p-\177 ]
State #8 is non-accepting associated rule line numbers:
3
out-transitions: [ a ]
jam-transitions: EOF [ \001-‘
b-\177 ]
State #9 is non-accepting associated rule line numbers:
3
out-transitions: [ r ]
jam-transitions: EOF [ \001-q
s-\177 ]
Compressed tables always back up.
The first few lines tell us that there’s a scanner state in which it can make a transition on
an ’o’ but not on any other character, and that in that state the currently scanned text does
not match any rule. The state occurs when trying to match the rules found at lines 2 and 3
in the input file. If the scanner is in that state and then reads something other than an ’o’,
it will have to back up to find a rule which is matched. With a bit of headscratching one
can see that this must be the state it’s in when it has seen ‘fo’. When this has happened,
if anything other than another ‘o’ is seen, the scanner will have to back up to simply match
the ‘f’ (by the default rule).
The comment regarding State #8 indicates there’s a problem when ‘foob’ has been
scanned. Indeed, on any character other than an ‘a’, the scanner will have to back up to
accept "foo". Similarly, the comment for State #9 concerns when ‘fooba’ has been scanned
and an ‘r’ does not follow.
The final comment reminds us that there’s no point going to all the trouble of removing
backing up from the rules unless we’re using ‘-Cf’ or ‘-CF’, since there’s no performance
gain doing so with compressed scanners.
The way to remove the backing up is to add “error” rules:
%%
foo
foobar
fooba
foob
fo
return TOK_KEYWORD;
return TOK_KEYWORD;
|
|
{
/* false alarm, not really a keyword */
return TOK_ID;
}
Eliminating backing up among a list of keywords can also be done using a “catch-all”
rule:
Chapter 17: Performance Considerations
%%
foo
foobar
return TOK_KEYWORD;
return TOK_KEYWORD;
[a-z]+
return TOK_ID;
47
This is usually the best solution when appropriate.
Backing up messages tend to cascade. With a complicated set of rules it’s not uncommon
to get hundreds of messages. If one can decipher them, though, it often only takes a dozen
or so rules to eliminate the backing up (though it’s easy to make a mistake and have
an error rule accidentally match a valid token. A possible future flex feature will be to
automatically add rules to eliminate backing up).
It’s important to keep in mind that you gain the benefits of eliminating backing up only
if you eliminate every instance of backing up. Leaving just one means you gain nothing.
Variable trailing context (where both the leading and trailing parts do not have a fixed
length) entails almost the same performance loss as REJECT (i.e., substantial). So when
possible a rule like:
%%
mouse|rat/(cat|dog)
run();
is better written:
%%
mouse/cat|dog
rat/cat|dog
run();
run();
%%
mouse|rat/cat
mouse|rat/dog
run();
run();
or as
Note that here the special ’|’ action does not provide any savings, and can even make
things worse (see Chapter 24 [Limitations], page 74).
Another area where the user can increase a scanner’s performance (and one that’s easier
to implement) arises from the fact that the longer the tokens matched, the faster the
scanner will run. This is because with long tokens the processing of most input characters
takes place in the (short) inner scanning loop, and does not often have to go through the
additional work of setting up the scanning environment (e.g., yytext) for the action. Recall
the scanner for C comments:
%x comment
%%
int line_num = 1;
"/*"
BEGIN(comment);
<comment>[^*\n]*
<comment>"*"+[^*/\n]*
<comment>\n
<comment>"*"+"/"
++line_num;
BEGIN(INITIAL);
Chapter 17: Performance Considerations
48
This could be sped up by writing it as:
%x comment
%%
int line_num = 1;
"/*"
BEGIN(comment);
<comment>[^*\n]*
<comment>[^*\n]*\n
++line_num;
<comment>"*"+[^*/\n]*
<comment>"*"+[^*/\n]*\n ++line_num;
<comment>"*"+"/"
BEGIN(INITIAL);
Now instead of each newline requiring the processing of another action, recognizing the
newlines is distributed over the other rules to keep the matched text as long as possible. Note
that adding rules does not slow down the scanner! The speed of the scanner is independent
of the number of rules or (modulo the considerations given at the beginning of this section)
how complicated the rules are with regard to operators such as ‘*’ and ‘|’.
A final example in speeding up a scanner: suppose you want to scan through a file
containing identifiers and keywords, one per line and with no other extraneous characters,
and recognize all the keywords. A natural first approach is:
%%
asm
|
auto
|
break
|
... etc ...
volatile |
while
/* it’s a keyword */
.|\n
/* it’s not a keyword */
To eliminate the back-tracking, introduce a catch-all rule:
%%
asm
|
auto
|
break
|
... etc ...
volatile |
while
/* it’s a keyword */
[a-z]+
.|\n
|
/* it’s not a keyword */
Now, if it’s guaranteed that there’s exactly one word per line, then we can reduce the
total number of matches by a half by merging in the recognition of newlines with that of
the other tokens:
%%
asm\n
|
Chapter 17: Performance Considerations
49
auto\n
|
break\n |
... etc ...
volatile\n |
while\n /* it’s a keyword */
[a-z]+\n |
.|\n
/* it’s not a keyword */
One has to be careful here, as we have now reintroduced backing up into the scanner. In
particular, while we know that there will never be any characters in the input stream other
than letters or newlines, flex can’t figure this out, and it will plan for possibly needing to
back up when it has scanned a token like ‘auto’ and then the next character is something
other than a newline or a letter. Previously it would then just match the ‘auto’ rule and
be done, but now it has no ‘auto’ rule, only a ‘auto\n’ rule. To eliminate the possibility of
backing up, we could either duplicate all rules but without final newlines, or, since we never
expect to encounter such an input and therefore don’t how it’s classified, we can introduce
one more catch-all rule, this one which doesn’t include a newline:
%%
asm\n
|
auto\n
|
break\n |
... etc ...
volatile\n |
while\n /* it’s a keyword */
[a-z]+\n |
[a-z]+
|
.|\n
/* it’s not a keyword */
Compiled with ‘-Cf’, this is about as fast as one can get a flex scanner to go for this
particular problem.
A final note: flex is slow when matching NULs, particularly when a token contains
multiple NULs. It’s best to write rules which match short amounts of text if it’s anticipated
that the text will often include NULs.
Another final note regarding performance: as mentioned in Chapter 7 [Matching],
page 14, dynamically resizing yytext to accommodate huge tokens is a slow process
because it presently requires that the (huge) token be rescanned from the beginning. Thus
if performance is vital, you should attempt to match “large” quantities of text but not
“huge” quantities, where the cutoff between the two is at about 8K characters per token.
Chapter 18: Generating C++ Scanners
50
18 Generating C++ Scanners
IMPORTANT: the present form of the scanning class is experimental and may change
considerably between major releases.
flex provides two different ways to generate scanners for use with C++. The first way is
to simply compile a scanner generated by flex using a C++ compiler instead of a C compiler.
You should not encounter any compilation errors (see Chapter 2 [Reporting Bugs], page 2).
You can then use C++ code in your rule actions instead of C code. Note that the default
input source for your scanner remains ‘yyin’, and default echoing is still done to ‘yyout’.
Both of these remain FILE * variables and not C++ streams.
You can also use flex to generate a C++ scanner class, using the ‘-+’ option (or, equivalently, %option c++), which is automatically specified if the name of the flex executable
ends in a ’+’, such as flex++. When using this option, flex defaults to generating the
scanner to the file ‘lex.yy.cc’ instead of ‘lex.yy.c’. The generated scanner includes the
header file ‘FlexLexer.h’, which defines the interface to two C++ classes.
The first class, FlexLexer, provides an abstract base class defining the general scanner
class interface. It provides the following member functions:
const char* YYText()
returns the text of the most recently matched token, the equivalent of yytext.
int YYLeng()
returns the length of the most recently matched token, the equivalent of yyleng.
int lineno() const
returns the current input line number (see %option yylineno), or 1 if %option
yylineno was not used.
void set_debug( int flag )
sets the debugging flag for the scanner, equivalent to assigning to yy_flex_
debug (see Chapter 16 [Scanner Options], page 35). Note that you must build
the scanner using %option debug to include debugging information in it.
int debug() const
returns the current setting of the debugging flag.
Also provided are member functions equivalent to yy_switch_to_buffer(), yy_create_
buffer() (though the first argument is an istream* object pointer and not a FILE*), yy_
flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is a
istream* object pointer).
The second class defined in ‘FlexLexer.h’ is yyFlexLexer, which is derived from
FlexLexer. It defines the following additional member functions:
yyFlexLexer( istream* arg_yyin = 0, ostream* arg_yyout = 0 )
constructs a yyFlexLexer object using the given streams for input and output.
If not specified, the streams default to cin and cout, respectively.
virtual int yylex()
performs the same role is yylex() does for ordinary flex scanners: it scans the
input stream, consuming tokens, until a rule’s action returns a value. If you
Chapter 18: Generating C++ Scanners
51
derive a subclass S from yyFlexLexer and want to access the member functions
and variables of S inside yylex(), then you need to use %option yyclass="S"
to inform flex that you will be using that subclass instead of yyFlexLexer.
In this case, rather than generating yyFlexLexer::yylex(), flex generates
S::yylex() (and also generates a dummy yyFlexLexer::yylex() that calls
yyFlexLexer::LexerError() if called).
virtual void switch_streams(istream* new_in = 0, ostream* new_out = 0)
reassigns yyin to new_in (if non-null) and yyout to new_out (if non-null),
deleting the previous input buffer if yyin is reassigned.
int yylex( istream* new_in, ostream* new_out = 0 )
first switches the input streams via switch_streams( new_in, new_out ) and
then returns the value of yylex().
In addition, yyFlexLexer defines the following protected virtual functions which you
can redefine in derived classes to tailor the scanner:
virtual int LexerInput( char* buf, int max_size )
reads up to max_size characters into buf and returns the number of characters
read. To indicate end-of-input, return 0 characters. Note that interactive
scanners (see the ‘-B’ and ‘-I’ flags in Chapter 16 [Scanner Options], page 35)
define the macro YY_INTERACTIVE. If you redefine LexerInput() and need
to take different actions depending on whether or not the scanner might be
scanning an interactive input source, you can test for the presence of this name
via #ifdef statements.
virtual void LexerOutput( const char* buf, int size )
writes out size characters from the buffer buf, which, while NUL-terminated,
may also contain internal NULs if the scanner’s rules can match text with NULs
in them.
virtual void LexerError( const char* msg )
reports a fatal error message. The default version of this function writes the
message to the stream cerr and exits.
Note that a yyFlexLexer object contains its entire scanning state. Thus you can use
such objects to create reentrant scanners, but see also Chapter 19 [Reentrant], page 54.
You can instantiate multiple instances of the same yyFlexLexer class, and you can also
combine multiple C++ scanner classes together in the same program using the ‘-P’ option
discussed above.
Finally, note that the %array feature is not available to C++ scanner classes; you must
use %pointer (the default).
Here is an example of a simple C++ scanner:
// An example of using the flex C++ scanner class.
%{
#include <iostream>
using namespace std;
int mylineno = 0;
Chapter 18: Generating C++ Scanners
%}
%option noyywrap
string
\"[^\n"]+\"
ws
[ \t]+
alpha
dig
name
num1
num2
number
[A-Za-z]
[0-9]
({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
[-+]?{dig}+\.?([eE][-+]?{dig}+)?
[-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
{num1}|{num2}
%%
{ws}
/* skip blanks and tabs */
"/*"
{
int c;
while((c = yyinput()) != 0)
{
if(c == ’\n’)
++mylineno;
else if(c == ’*’)
{
if((c = yyinput()) == ’/’)
break;
else
unput(c);
}
}
}
{number}
cout << "number " << YYText() << ’\n’;
\n
mylineno++;
{name}
cout << "name " << YYText() << ’\n’;
{string}
cout << "string " << YYText() << ’\n’;
%%
52
Chapter 18: Generating C++ Scanners
53
int main( int /* argc */, char** /* argv */ )
{
FlexLexer* lexer = new yyFlexLexer;
while(lexer->yylex() != 0)
;
return 0;
}
If you want to create multiple (different) lexer classes, you use the ‘-P’ flag (or the
prefix= option) to rename each yyFlexLexer to some other ‘xxFlexLexer’. You then
can include ‘<FlexLexer.h>’ in your other sources once per lexer class, first renaming
yyFlexLexer as follows:
#undef yyFlexLexer
#define yyFlexLexer xxFlexLexer
#include <FlexLexer.h>
#undef yyFlexLexer
#define yyFlexLexer zzFlexLexer
#include <FlexLexer.h>
if, for example, you used %option prefix="xx" for one of your scanners and %option
prefix="zz" for the other.
Chapter 19: Reentrant C Scanners
54
19 Reentrant C Scanners
flex has the ability to generate a reentrant C scanner. This is accomplished by specifying
%option reentrant (‘-R’) The generated scanner is both portable, and safe to use in one
or more separate threads of control. The most common use for reentrant scanners is from
within multi-threaded applications. Any thread may create and execute a reentrant flex
scanner without the need for synchronization with other threads.
19.1 Uses for Reentrant Scanners
However, there are other uses for a reentrant scanner. For example, you could scan two
or more files simultaneously to implement a diff at the token level (i.e., instead of at the
character level):
/* Example of maintaining more than one active scanner. */
do {
int tok1, tok2;
tok1 = yylex( scanner_1 );
tok2 = yylex( scanner_2 );
if( tok1 != tok2 )
printf("Files are different.");
} while ( tok1 && tok2 );
Another use for a reentrant scanner is recursion. (Note that a recursive scanner can also
be created using a non-reentrant scanner and buffer states. See Chapter 11 [Multiple Input
Buffers], page 27.)
The following crude scanner supports the ‘eval’ command by invoking another instance
of itself.
/* Example of recursive invocation. */
%option reentrant
%%
"eval(".+")"
{
yyscan_t scanner;
YY_BUFFER_STATE buf;
yylex_init( &scanner );
yytext[yyleng-1] = ’ ’;
buf = yy_scan_string( yytext + 5, scanner );
yylex( scanner );
yy_delete_buffer(buf,scanner);
Chapter 19: Reentrant C Scanners
55
yylex_destroy( scanner );
}
...
%%
19.2 An Overview of the Reentrant API
The API for reentrant scanners is different than for non-reentrant scanners. Here is a quick
overview of the API:
%option reentrant must be specified.
• All functions take one additional argument: yyscanner
• All global variables are replaced by their macro equivalents. (We tell you this because
it may be important to you during debugging.)
• yylex_init and yylex_destroy must be called before and after yylex, respectively.
• Accessor methods (get/set functions) provide access to common flex variables.
• User-specific data can be stored in yyextra.
19.3 Reentrant Example
First, an example of a reentrant scanner:
/* This scanner prints "//" comments. */
%option reentrant stack noyywrap
%x COMMENT
%%
"//"
.|\n
yy_push_state( COMMENT, yyscanner);
<COMMENT>\n
<COMMENT>[^\n]+
yy_pop_state( yyscanner );
fprintf( yyout, "%s\n", yytext);
%%
int main ( int argc, char * argv[] )
{
yyscan_t scanner;
yylex_init ( &scanner );
yylex ( scanner );
yylex_destroy ( scanner );
return 0;
}
Chapter 19: Reentrant C Scanners
56
19.4 The Reentrant API in Detail
Here are the things you need to do or know to use the reentrant C API of flex.
19.4.1 Declaring a Scanner As Reentrant
%option reentrant (–reentrant) must be specified.
Notice that %option reentrant is specified in the above example (see Section 19.3 [Reentrant Example], page 55. Had this option not been specified, flex would have happily generated a non-reentrant scanner without complaining. You may explicitly specify %option
noreentrant, if you do not want a reentrant scanner, although it is not necessary. The
default is to generate a non-reentrant scanner.
19.4.2 The Extra Argument
All functions take one additional argument: yyscanner.
Notice that the calls to yy_push_state and yy_pop_state both have an argument,
yyscanner , that is not present in a non-reentrant scanner. Here are the declarations of
yy_push_state and yy_pop_state in the reentrant scanner:
static void yy_push_state ( int new_state , yyscan_t yyscanner ) ;
static void yy_pop_state ( yyscan_t yyscanner ) ;
Notice that the argument yyscanner appears in the declaration of both functions. In
fact, all flex functions in a reentrant scanner have this additional argument. It is always
the last argument in the argument list, it is always of type yyscan_t (which is typedef’d
to void *) and it is always named yyscanner. As you may have guessed, yyscanner
is a pointer to an opaque data structure encapsulating the current state of the scanner.
For a list of function declarations, see Section 19.5 [Reentrant Functions], page 59. Note
that preprocessor macros, such as BEGIN, ECHO, and REJECT, do not take this additional
argument.
19.4.3 Global Variables Replaced By Macros
All global variables in traditional flex have been replaced by macro equivalents.
Note that in the above example, yyout and yytext are not plain variables. These are
macros that will expand to their equivalent lvalue. All of the familiar flex globals have
been replaced by their macro equivalents. In particular, yytext, yyleng, yylineno, yyin,
yyout, yyextra, yylval, and yylloc are macros. You may safely use these macros in
actions as if they were plain variables. We only tell you this so you don’t expect to link
to these variables externally. Currently, each macro expands to a member of an internal
struct, e.g.,
#define yytext (((struct yyguts_t*)yyscanner)->yytext_r)
One important thing to remember about yytext and friends is that yytext is not a
global variable in a reentrant scanner, you can not access it directly from outside an action
or from other functions. You must use an accessor method, e.g., yyget_text, to accomplish
this. (See below).
19.4.4 Init and Destroy Functions
yylex_init and yylex_destroy must be called before and after yylex, respectively.
Chapter 19: Reentrant C Scanners
int
int
int
int
57
yylex_init ( yyscan_t * ptr_yy_globals ) ;
yylex_init_extra ( YY_EXTRA_TYPE user_defined, yyscan_t * ptr_yy_globals ) ;
yylex ( yyscan_t yyscanner ) ;
yylex_destroy ( yyscan_t yyscanner ) ;
The function yylex_init must be called before calling any other function. The argument
to yylex_init is the address of an uninitialized pointer to be filled in by yylex_init,
overwriting any previous contents. The function yylex_init_extra may be used instead,
taking as its first argument a variable of type YY_EXTRA_TYPE. See the section on yyextra,
below, for more details.
The value stored in ptr_yy_globals should thereafter be passed to yylex and yylex_
destroy. Flex does not save the argument passed to yylex_init, so it is safe to pass the
address of a local pointer to yylex_init so long as it remains in scope for the duration of
all calls to the scanner, up to and including the call to yylex_destroy.
The function yylex should be familiar to you by now. The reentrant version takes one
argument, which is the value returned (via an argument) by yylex_init. Otherwise, it
behaves the same as the non-reentrant version of yylex.
Both yylex_init and yylex_init_extra returns 0 (zero) on success, or non-zero on
failure, in which case errno is set to one of the following values:
• ENOMEM Memory allocation error. See [memory-management], page 64.
• EINVAL Invalid argument.
The function yylex_destroy should be called to free resources used by the scanner.
After yylex_destroy is called, the contents of yyscanner should not be used. Of course,
there is no need to destroy a scanner if you plan to reuse it. A flex scanner (both reentrant
and non-reentrant) may be restarted by calling yyrestart.
Below is an example of a program that creates a scanner, uses it, then destroys it when
done:
int main ()
{
yyscan_t scanner;
int tok;
yylex_init(&scanner);
while ((tok=yylex(scanner)) > 0)
printf("tok=%d yytext=%s\n", tok, yyget_text(scanner));
yylex_destroy(scanner);
return 0;
}
19.4.5 Accessing Variables with Reentrant Scanners
Accessor methods (get/set functions) provide access to common flex variables.
Many scanners that you build will be part of a larger project. Portions of your project
will need access to flex values, such as yytext. In a non-reentrant scanner, these values
Chapter 19: Reentrant C Scanners
58
are global, so there is no problem accessing them. However, in a reentrant scanner, there
are no global flex values. You can not access them directly. Instead, you must access
flex values using accessor methods (get/set functions). Each accessor method is named
yyget_NAME or yyset_NAME, where NAME is the name of the flex variable you want. For
example:
/* Set the last character of yytext to NULL. */
void chop ( yyscan_t scanner )
{
int len = yyget_leng( scanner );
yyget_text( scanner )[len - 1] = ’\0’;
}
The above code may be called from within an action like this:
%%
.+\n
{ chop( yyscanner );}
You may find that %option header-file is particularly useful for generating prototypes
of all the accessor functions. See [option-header], page 35.
19.4.6 Extra Data
User-specific data can be stored in yyextra.
In a reentrant scanner, it is unwise to use global variables to communicate with or
maintain state between different pieces of your program. However, you may need access
to external data or invoke external functions from within the scanner actions. Likewise,
you may need to pass information to your scanner (e.g., open file descriptors, or database
connections). In a non-reentrant scanner, the only way to do this would be through the use
of global variables. Flex allows you to store arbitrary, “extra” data in a scanner. This data
is accessible through the accessor methods yyget_extra and yyset_extra from outside the
scanner, and through the shortcut macro yyextra from within the scanner itself. They are
defined as follows:
#define YY_EXTRA_TYPE void*
YY_EXTRA_TYPE yyget_extra ( yyscan_t scanner );
void
yyset_extra ( YY_EXTRA_TYPE arbitrary_data , yyscan_t scanner);
In addition, an extra form of yylex_init is provided, yylex_init_extra. This function
is provided so that the yyextra value can be accessed from within the very first yyalloc,
used to allocate the scanner itself.
By default, YY_EXTRA_TYPE is defined as type void *. You may redefine this type using
%option extra-type="your_type" in the scanner:
/* An example of overriding YY_EXTRA_TYPE. */
%{
#include <sys/stat.h>
#include <unistd.h>
%}
%option reentrant
%option extra-type="struct stat *"
%%
Chapter 19: Reentrant C Scanners
59
__filesize__
printf( "%ld", yyextra->st_size );
__lastmod__
printf( "%ld", yyextra->st_mtime );
%%
void scan_file( char* filename )
{
yyscan_t scanner;
struct stat buf;
FILE *in;
in = fopen( filename, "r" );
stat( filename, &buf );
yylex_init_extra( buf, &scanner );
yyset_in( in, scanner );
yylex( scanner );
yylex_destroy( scanner );
fclose( in );
}
19.4.7 About yyscan t
yyscan_t is defined as:
typedef void* yyscan_t;
It is initialized by yylex_init() to point to an internal structure. You should never
access this value directly. In particular, you should never attempt to free it (use yylex_
destroy() instead.)
19.5 Functions and Macros Available in Reentrant C
Scanners
The following Functions are available in a reentrant scanner:
char *yyget_text ( yyscan_t scanner );
int yyget_leng ( yyscan_t scanner );
FILE *yyget_in ( yyscan_t scanner );
FILE *yyget_out ( yyscan_t scanner );
int yyget_lineno ( yyscan_t scanner );
YY_EXTRA_TYPE yyget_extra ( yyscan_t scanner );
int yyget_debug ( yyscan_t scanner );
void yyset_debug ( int flag, yyscan_t scanner );
void yyset_in ( FILE * in_str , yyscan_t scanner );
void yyset_out ( FILE * out_str , yyscan_t scanner );
void yyset_lineno ( int line_number , yyscan_t scanner );
void yyset_extra ( YY_EXTRA_TYPE user_defined , yyscan_t scanner );
There are no “set” functions for yytext and yyleng. This is intentional.
The following Macro shortcuts are available in actions in a reentrant scanner:
Chapter 19: Reentrant C Scanners
60
yytext
yyleng
yyin
yyout
yylineno
yyextra
yy_flex_debug
In a reentrant C scanner, support for yylineno is always present (i.e., you may access
yylineno), but the value is never modified by flex unless %option yylineno is enabled.
This is to allow the user to maintain the line count independently of flex.
The following functions and macros are made available when %option bison-bridge
(‘--bison-bridge’) is specified:
YYSTYPE * yyget_lval ( yyscan_t scanner );
void yyset_lval ( YYSTYPE * yylvalp , yyscan_t scanner );
yylval
The following functions and macros are made available when %option bison-locations
(‘--bison-locations’) is specified:
YYLTYPE *yyget_lloc ( yyscan_t scanner );
void yyset_lloc ( YYLTYPE * yyllocp , yyscan_t scanner );
yylloc
Support for yylval assumes that YYSTYPE is a valid type. Support for yylloc assumes that
YYSLYPE is a valid type. Typically, these types are generated by bison, and are included in
section 1 of the flex input.
Chapter 20: Incompatibilities with Lex and Posix
61
20 Incompatibilities with Lex and Posix
flex is a rewrite of the AT&T Unix lex tool (the two implementations do not share any code,
though), with some extensions and incompatibilities, both of which are of concern to those
who wish to write scanners acceptable to both implementations. flex is fully compliant
with the POSIX lex specification, except that when using %pointer (the default), a call to
unput() destroys the contents of yytext, which is counter to the POSIX specification. In
this section we discuss all of the known areas of incompatibility between flex, AT&T lex,
and the POSIX specification. flex’s ‘-l’ option turns on maximum compatibility with the
original AT&T lex implementation, at the cost of a major loss in the generated scanner’s
performance. We note below which incompatibilities can be overcome using the ‘-l’ option.
flex is fully compatible with lex with the following exceptions:
• The undocumented lex scanner internal variable yylineno is not supported unless ‘-l’
or %option yylineno is used.
• yylineno should be maintained on a per-buffer basis, rather than a per-scanner (single
global variable) basis.
• yylineno is not part of the POSIX specification.
• The input() routine is not redefinable, though it may be called to read characters
following whatever has been matched by a rule. If input() encounters an end-of-file
the normal yywrap() processing is done. A “real” end-of-file is returned by input()
as EOF.
• Input is instead controlled by defining the YY_INPUT() macro.
• The flex restriction that input() cannot be redefined is in accordance with the POSIX
specification, which simply does not specify any way of controlling the scanner’s input
other than by making an initial assignment to ‘yyin’.
• The unput() routine is not redefinable. This restriction is in accordance with POSIX.
• flex scanners are not as reentrant as lex scanners. In particular, if you have an
interactive scanner and an interrupt handler which long-jumps out of the scanner, and
the scanner is subsequently called again, you may get the following message:
fatal flex scanner internal error--end of buffer missed
To reenter the scanner, first use:
yyrestart( yyin );
Note that this call will throw away any buffered input; usually this isn’t a problem
with an interactive scanner. See Chapter 19 [Reentrant], page 54, for flex’s reentrant
API.
• Also note that flex C++ scanner classes are reentrant, so if using C++ is an option
for you, you should use them instead. See Chapter 18 [Cxx], page 50, and Chapter 19
[Reentrant], page 54 for details.
• output() is not supported. Output from the ECHO macro is done to the file-pointer
yyout (default ‘stdout)’.
• output() is not part of the POSIX specification.
• lex does not support exclusive start conditions (%x), though they are in the POSIX
specification.
Chapter 20: Incompatibilities with Lex and Posix
62
• When definitions are expanded, flex encloses them in parentheses. With lex, the
following:
NAME
[A-Z][A-Z0-9]*
%%
foo{NAME}?
printf( "Found it\n" );
%%
will not match the string ‘foo’ because when the macro is expanded the rule is equivalent to ‘foo[A-Z][A-Z0-9]*?’ and the precedence is such that the ‘?’ is associated
with ‘[A-Z0-9]*’. With flex, the rule will be expanded to ‘foo([A-Z][A-Z0-9]*)?’
and so the string ‘foo’ will match.
• Note that if the definition begins with ‘^’ or ends with ‘$’ then it is not expanded
with parentheses, to allow these operators to appear in definitions without losing their
special meanings. But the ‘<s>’, ‘/’, and <<EOF>> operators cannot be used in a flex
definition.
• Using ‘-l’ results in the lex behavior of no parentheses around the definition.
• The POSIX specification is that the definition be enclosed in parentheses.
• Some implementations of lex allow a rule’s action to begin on a separate line, if the
rule’s pattern has trailing whitespace:
%%
foo|bar<space here>
{ foobar_action();}
flex does not support this feature.
• The lex %r (generate a Ratfor scanner) option is not supported. It is not part of the
POSIX specification.
• After a call to unput(), yytext is undefined until the next token is matched, unless
the scanner was built using %array. This is not the case with lex or the POSIX
specification. The ‘-l’ option does away with this incompatibility.
• The precedence of the ‘{,}’ (numeric range) operator is different. The AT&T and
POSIX specifications of lex interpret ‘abc{1,3}’ as match one, two, or three occurrences of ‘abc’”, whereas flex interprets it as “match ‘ab’ followed by one, two, or three
occurrences of ‘c’”. The ‘-l’ and ‘--posix’ options do away with this incompatibility.
• The precedence of the ‘^’ operator is different. lex interprets ‘^foo|bar’ as “match
either ’foo’ at the beginning of a line, or ’bar’ anywhere”, whereas flex interprets it
as “match either ‘foo’ or ‘bar’ if they come at the beginning of a line”. The latter is
in agreement with the POSIX specification.
• The special table-size declarations such as %a supported by lex are not required by
flex scanners.. flex ignores them.
• The name FLEX_SCANNER is #define’d so scanners may be written for use with either
flex or lex. Scanners also include YY_FLEX_MAJOR_VERSION, YY_FLEX_MINOR_VERSION
and YY_FLEX_SUBMINOR_VERSION indicating which version of flex generated the scanner. For example, for the 2.5.22 release, these defines would be 2, 5 and 22 respectively.
If the version of flex being used is a beta version, then the symbol FLEX_BETA is defined.
Chapter 20: Incompatibilities with Lex and Posix
63
• The symbols ‘[[’ and ‘]]’ in the code sections of the input may conflict with the m4
delimiters. See Section A.3 [M4 Dependency], page 124.
The following flex features are not included in lex or the POSIX specification:
• C++ scanners
• %option
• start condition scopes
• start condition stacks
• interactive/non-interactive scanners
• yy scan string() and friends
• yyterminate()
• yy set interactive()
• yy set bol()
• YY AT BOL() <<EOF>>
• <*>
• YY DECL
• YY START
• YY USER ACTION
• YY USER INIT
• #line directives
• %{}’s around actions
• reentrant C API
• multiple actions on a line
• almost all of the flex command-line options
The feature “multiple actions on a line” refers to the fact that with flex you can put
multiple actions on the same line, separated with semi-colons, while with lex, the following:
foo
handle_foo(); ++num_foos_seen;
is (rather surprisingly) truncated to
foo
handle_foo();
flex does not truncate the action. Actions that are not enclosed in braces are simply
terminated at the end of the line.
Chapter 21: Memory Management
64
21 Memory Management
This chapter describes how flex handles dynamic memory, and how you can override the
default behavior.
21.1 The Default Memory Management
Flex allocates dynamic memory during initialization, and once in a while from within a call
to yylex(). Initialization takes place during the first call to yylex(). Thereafter, flex may
reallocate more memory if it needs to enlarge a buffer. As of version 2.5.9 Flex will clean
up all memory when you call yylex_destroy See [faq-memory-leak], page 84.
Flex allocates dynamic memory for four purposes, listed below1
16kB for the input buffer.
Flex allocates memory for the character buffer used to perform pattern matching. Flex must read ahead from the input stream and store it in a large character
buffer. This buffer is typically the largest chunk of dynamic memory flex consumes. This buffer will grow if necessary, doubling the size each time. Flex
frees this memory when you call yylex destroy(). The default size of this buffer
(16384 bytes) is almost always too large. The ideal size for this buffer is the
length of the longest token expected, in bytes, plus a little more. Flex will
allocate a few extra bytes for housekeeping. Currently, to override the size of
the input buffer you must #define YY_BUF_SIZE to whatever number of bytes
you want. We don’t plan to change this in the near future, but we reserve the
right to do so if we ever add a more robust memory management API.
64kb for the REJECT state. This will only be allocated if you use REJECT.
The size is large enough to hold the same number of states as characters in the
input buffer. If you override the size of the input buffer (via YY_BUF_SIZE),
then you automatically override the size of this buffer as well.
100 bytes for the start condition stack.
Flex allocates memory for the start condition stack. This is the stack used
for pushing start states, i.e., with yy push state(). It will grow if necessary.
Since the states are simply integers, this stack doesn’t consume much memory.
This stack is not present if %option stack is not specified. You will rarely
need to tune this buffer. The ideal size for this stack is the maximum depth
expected. The memory for this stack is automatically destroyed when you call
yylex destroy(). See [option-stack], page 38.
40 bytes for each YY BUFFER STATE.
Flex allocates memory for each YY BUFFER STATE. The buffer state itself
is about 40 bytes, plus an additional large character buffer (described above.)
The initial buffer state is created during initialization, and with each call to
yy create buffer(). You can’t tune the size of this, but you can tune the character buffer as described above. Any buffer state that you explicitly create
by calling yy create buffer() is NOT destroyed automatically. You must call
1
The quantities given here are approximate, and may vary due to host architecture, compiler configuration,
or due to future enhancements to flex.
Chapter 21: Memory Management
65
yy delete buffer() to free the memory. The exception to this rule is that flex
will delete the current buffer automatically when you call yylex destroy(). If
you delete the current buffer, be sure to set it to NULL. That way, flex will not
try to delete the buffer a second time (possibly crashing your program!) At the
time of this writing, flex does not provide a growable stack for the buffer states.
You have to manage that yourself. See Chapter 11 [Multiple Input Buffers],
page 27.
84 bytes for the reentrant scanner guts
Flex allocates about 84 bytes for the reentrant scanner structure when you call
yylex init(). It is destroyed when the user calls yylex destroy().
21.2 Overriding The Default Memory Management
Flex calls the functions yyalloc, yyrealloc, and yyfree when it needs to allocate or free
memory. By default, these functions are wrappers around the standard C functions, malloc,
realloc, and free, respectively. You can override the default implementations by telling
flex that you will provide your own implementations.
To override the default implementations, you must do two things:
1. Suppress the default implementations by specifying one or more of the following options:
• %option noyyalloc
• %option noyyrealloc
• %option noyyfree.
2. Provide your own implementation of the following functions:2
// For a non-reentrant scanner
void * yyalloc (size_t bytes);
void * yyrealloc (void * ptr, size_t bytes);
void
yyfree (void * ptr);
// For
void *
void *
void
a reentrant scanner
yyalloc (size_t bytes, void * yyscanner);
yyrealloc (void * ptr, size_t bytes, void * yyscanner);
yyfree (void * ptr, void * yyscanner);
In the following example, we will override all three memory routines. We assume that
there is a custom allocator with garbage collection. In order to make this example interesting, we will use a reentrant scanner, passing a pointer to the custom allocator through
yyextra.
%{
#include "some_allocator.h"
%}
/* Suppress the default implementations. */
%option noyyalloc noyyrealloc noyyfree
2
It is not necessary to override all (or any) of the memory management routines. You may, for example,
override yyrealloc, but not yyfree or yyalloc.
Chapter 21: Memory Management
66
%option reentrant
/* Initialize the allocator. */
#define YY_EXTRA_TYPE struct allocator*
#define YY_USER_INIT yyextra = allocator_create();
%%
.|\n
%%
;
/* Provide our own implementations. */
void * yyalloc (size_t bytes, void* yyscanner) {
return allocator_alloc (yyextra, bytes);
}
void * yyrealloc (void * ptr, size_t bytes, void* yyscanner) {
return allocator_realloc (yyextra, bytes);
}
void yyfree (void * ptr, void * yyscanner) {
/* Do nothing -- we leave it to the garbage collector. */
}
21.3 A Note About yytext And Memory
When flex finds a match, yytext points to the first character of the match in the input
buffer. The string itself is part of the input buffer, and is NOT allocated separately. The
value of yytext will be overwritten the next time yylex() is called. In short, the value of
yytext is only valid from within the matched rule’s action.
Often, you want the value of yytext to persist for later processing, i.e., by a parser with
non-zero lookahead. In order to preserve yytext, you will have to copy it with strdup() or a
similar function. But this introduces some headache because your parser is now responsible
for freeing the copy of yytext. If you use a yacc or bison parser, (commonly used with flex),
you will discover that the error recovery mechanisms can cause memory to be leaked.
To prevent memory leaks from strdup’d yytext, you will have to track the memory
somehow. Our experience has shown that a garbage collection mechanism or a pooled
memory mechanism will save you a lot of grief when writing parsers.
Chapter 22: Serialized Tables
67
22 Serialized Tables
A flex scanner has the ability to save the DFA tables to a file, and load them at runtime
when needed. The motivation for this feature is to reduce the runtime memory footprint.
Traditionally, these tables have been compiled into the scanner as C arrays, and are sometimes quite large. Since the tables are compiled into the scanner, the memory used by the
tables can never be freed. This is a waste of memory, especially if an application uses several
scanners, but none of them at the same time.
The serialization feature allows the tables to be loaded at runtime, before scanning
begins. The tables may be discarded when scanning is finished.
22.1 Creating Serialized Tables
You may create a scanner with serialized tables by specifying:
%option tables-file=FILE
or
--tables-file=FILE
These options instruct flex to save the DFA tables to the file FILE. The tables will not
be embedded in the generated scanner. The scanner will not function on its own. The
scanner will be dependent upon the serialized tables. You must load the tables from this
file at runtime before you can scan anything.
If you do not specify a filename to --tables-file, the tables will be saved to
‘lex.yy.tables’, where ‘yy’ is the appropriate prefix.
If your project uses several different scanners, you can concatenate the serialized tables
into one file, and flex will find the correct set of tables, using the scanner prefix as part of
the lookup key. An example follows:
$ flex --tables-file --prefix=cpp cpp.l
$ flex --tables-file --prefix=c
c.l
$ cat lex.cpp.tables lex.c.tables > all.tables
The above example created two scanners, ‘cpp’, and ‘c’. Since we did not specify a
filename, the tables were serialized to ‘lex.c.tables’ and ‘lex.cpp.tables’, respectively.
Then, we concatenated the two files together into ‘all.tables’, which we will distribute
with our project. At runtime, we will open the file and tell flex to load the tables from it.
Flex will find the correct tables automatically. (See next section).
22.2 Loading and Unloading Serialized Tables
If you’ve built your scanner with %option tables-file, then you must load the scanner
tables at runtime. This can be accomplished with the following function:
int yytables_fload (FILE* fp [, yyscan t scanner ])
[Function]
Locates scanner tables in the stream pointed to by fp and loads them. Memory for
the tables is allocated via yyalloc. You must call this function before the first call to
yylex. The argument scanner only appears in the reentrant scanner. This function
returns ‘0’ (zero) on success, or non-zero on error.
Chapter 22: Serialized Tables
68
The loaded tables are not automatically destroyed (unloaded) when you call yylex_
destroy. The reason is that you may create several scanners of the same type (in a reentrant
scanner), each of which needs access to these tables. To avoid a nasty memory leak, you
must call the following function:
int yytables_destroy ([yyscan t scanner ])
[Function]
Unloads the scanner tables. The tables must be loaded again before you can scan
any more data. The argument scanner only appears in the reentrant scanner. This
function returns ‘0’ (zero) on success, or non-zero on error.
The functions yytables_fload and yytables_destroy are not thread-safe. You must
ensure that these functions are called exactly once (for each scanner type) in a threaded
program, before any thread calls yylex. After the tables are loaded, they are never written
to, and no thread protection is required thereafter – until you destroy them.
22.3 Tables File Format
This section defines the file format of serialized flex tables.
The tables format allows for one or more sets of tables to be specified, where each set
corresponds to a given scanner. Scanners are indexed by name, as described below. The
file format is as follows:
TABLE SET 1
+-------------------------------+
Header | uint32
th_magic;
|
| uint32
th_hsize;
|
| uint32
th_ssize;
|
| uint16
th_flags;
|
| char
th_version[]; |
| char
th_name[];
|
| uint8
th_pad64[];
|
+-------------------------------+
Table 1 | uint16
td_id;
|
| uint16
td_flags;
|
| uint32
td_hilen;
|
| uint32
td_lolen;
|
| void
td_data[];
|
| uint8
td_pad64[];
|
+-------------------------------+
Table 2 |
|
.
.
.
.
.
.
.
.
.
.
.
.
Table n |
|
+-------------------------------+
TABLE SET 2
.
.
Chapter 22: Serialized Tables
69
.
TABLE SET N
The above diagram shows that a complete set of tables consists of a header followed by
multiple individual tables. Furthermore, multiple complete sets may be present in the same
file, each set with its own header and tables. The sets are contiguous in the file. The only
way to know if another set follows is to check the next four bytes for the magic number (or
check for EOF). The header and tables sections are padded to 64-bit boundaries. Below we
describe each field in detail. This format does not specify how the scanner will expand the
given data, i.e., data may be serialized as int8, but expanded to an int32 array at runtime.
This is to reduce the size of the serialized data where possible. Remember, all integer values
are in network byte order.
Fields of a table header:
th_magic
Magic number, always 0xF13C57B1.
th_hsize
Size of this entire header, in bytes, including all fields plus any padding.
th_ssize
Size of this entire set, in bytes, including the header, all tables, plus any padding.
th_flags
Bit flags for this table set. Currently unused.
th_version[]
Flex version in NULL-terminated string format. e.g., ‘2.5.13a’. This is the
version of flex that was used to create the serialized tables.
th_name[]
Contains the name of this table set. The default is ‘yytables’, and is prefixed
accordingly, e.g., ‘footables’. Must be NULL-terminated.
th_pad64[]
Zero or more NULL bytes, padding the entire header to the next 64-bit boundary as calculated from the beginning of the header.
Fields of a table:
td_id
Specifies the table identifier. Possible values are:
YYTD_ID_ACCEPT (0x01)
yy_accept
YYTD_ID_BASE (0x02)
yy_base
YYTD_ID_CHK (0x03)
yy_chk
YYTD_ID_DEF (0x04)
yy_def
YYTD_ID_EC (0x05)
yy_ec
YYTD_ID_META (0x06)
yy_meta
Chapter 22: Serialized Tables
70
YYTD_ID_NUL_TRANS (0x07)
yy_NUL_trans
YYTD_ID_NXT (0x08)
yy_nxt. This array may be two dimensional. See the td_hilen
field below.
YYTD_ID_RULE_CAN_MATCH_EOL (0x09)
yy_rule_can_match_eol
YYTD_ID_START_STATE_LIST (0x0A)
yy_start_state_list. This array is handled specially because it
is an array of pointers to structs. See the td_flags field below.
YYTD_ID_TRANSITION (0x0B)
yy_transition. This array is handled specially because it is an
array of structs. See the td_lolen field below.
YYTD_ID_ACCLIST (0x0C)
yy_acclist
td_flags
Bit flags describing how to interpret the data in td_data. The data arrays
are one-dimensional by default, but may be two dimensional as specified in the
td_hilen field.
YYTD_DATA8 (0x01)
The data is serialized as an array of type int8.
YYTD_DATA16 (0x02)
The data is serialized as an array of type int16.
YYTD_DATA32 (0x04)
The data is serialized as an array of type int32.
YYTD_PTRANS (0x08)
The data is a list of indexes of entries in the expanded
yy_transition array. Each index should be expanded to a pointer
to the corresponding entry in the yy_transition array. We count
on the fact that the yy_transition array has already been seen.
YYTD_STRUCT (0x10)
The data is a list of yy trans info structs, each of which consists
of two integers. There is no padding between struct elements or
between structs. The type of each member is determined by the
YYTD_DATA* bits.
td_hilen
If td_hilen is non-zero, then the data is a two-dimensional array. Otherwise,
the data is a one-dimensional array. td_hilen contains the number of elements
in the higher dimensional array, and td_lolen contains the number of elements
in the lowest dimension.
Conceptually, td_data is either sometype td_data[td_lolen], or sometype
td_data[td_hilen][td_lolen], where sometype is specified by the td_flags
field. It is possible for both td_lolen and td_hilen to be zero, in which case
Chapter 22: Serialized Tables
71
td_data is a zero length array, and no data is loaded, i.e., this table is simply
skipped. Flex does not currently generate tables of zero length.
td_lolen
Specifies the number of elements in the lowest dimension array. If this is a
one-dimensional array, then it is simply the number of elements in this array.
The element size is determined by the td_flags field.
td_data[]
The table data. This array may be a one- or two-dimensional array, of type
int8, int16, int32, struct yy_trans_info, or struct yy_trans_info*, depending upon the values in the td_flags, td_hilen, and td_lolen fields.
td_pad64[]
Zero or more NULL bytes, padding the entire table to the next 64-bit boundary
as calculated from the beginning of this table.
Chapter 23: Diagnostics
72
23 Diagnostics
The following is a list of flex diagnostic messages:
• ‘warning, rule cannot be matched’ indicates that the given rule cannot be matched
because it follows other rules that will always match the same text as it. For example,
in the following ‘foo’ cannot be matched because it comes after an identifier “catch-all”
rule:
[a-z]+
foo
got_identifier();
got_foo();
Using REJECT in a scanner suppresses this warning.
• ‘warning, -s option given but default rule can be matched’ means that it is possible (perhaps only in a particular start condition) that the default rule (match any
single character) is the only one that will match a particular input. Since ‘-s’ was
given, presumably this is not intended.
• reject_used_but_not_detected undefined or yymore_used_but_not_detected
undefined. These errors can occur at compile time. They indicate that the scanner
uses REJECT or yymore() but that flex failed to notice the fact, meaning that flex
scanned the first two sections looking for occurrences of these actions and failed to find
any, but somehow you snuck some in (via a #include file, for example). Use %option
reject or %option yymore to indicate to flex that you really do use these features.
• ‘flex scanner jammed’. a scanner compiled with ‘-s’ has encountered an input string
which wasn’t matched by any of its rules. This error can also occur due to internal
problems.
• ‘token too large, exceeds YYLMAX’. your scanner uses %array and one of its rules
matched a string longer than the YYLMAX constant (8K bytes by default). You can
increase the value by #define’ing YYLMAX in the definitions section of your flex input.
• ‘scanner requires -8 flag to use the character ’x’’. Your scanner specification
includes recognizing the 8-bit character ‘’x’’ and you did not specify the -8 flag, and
your scanner defaulted to 7-bit because you used the ‘-Cf’ or ‘-CF’ table compression
options. See the discussion of the ‘-7’ flag, Chapter 16 [Scanner Options], page 35, for
details.
• ‘flex scanner push-back overflow’. you used unput() to push back so much text
that the scanner’s buffer could not hold both the pushed-back text and the current
token in yytext. Ideally the scanner should dynamically resize the buffer in this case,
but at present it does not.
• ‘input buffer overflow, can’t enlarge buffer because scanner uses REJECT’.
the scanner was working on matching an extremely large token and needed to expand
the input buffer. This doesn’t work with scanners that use REJECT.
• ‘fatal flex scanner internal error--end of buffer missed’. This can occur in a
scanner which is reentered after a long-jump has jumped out (or over) the scanner’s
activation frame. Before reentering the scanner, use:
yyrestart( yyin );
or, as noted above, switch to using the C++ scanner class.
Chapter 23: Diagnostics
73
• ‘too many start conditions in <> construct!’ you listed more start conditions in a
<> construct than exist (so you must have listed at least one of them twice).
Chapter 24: Limitations
74
24 Limitations
Some trailing context patterns cannot be properly matched and generate warning messages
(‘dangerous trailing context’). These are patterns where the ending of the first part of
the rule matches the beginning of the second part, such as ‘zx*/xy*’, where the ’x*’ matches
the ’x’ at the beginning of the trailing context. (Note that the POSIX draft states that the
text matched by such patterns is undefined.) For some trailing context rules, parts which are
actually fixed-length are not recognized as such, leading to the abovementioned performance
loss. In particular, parts using ‘|’ or ‘{n}’ (such as ‘foo{3}’) are always considered variablelength. Combining trailing context with the special ‘|’ action can result in fixed trailing
context being turned into the more expensive variable trailing context. For example, in the
following:
%%
abc
|
xyz/def
Use of unput() invalidates yytext and yyleng, unless the %array directive or the ‘-l’
option has been used. Pattern-matching of NULs is substantially slower than matching other
characters. Dynamic resizing of the input buffer is slow, as it entails rescanning all the text
matched so far by the current (generally huge) token. Due to both buffering of input and
read-ahead, you cannot intermix calls to ‘<stdio.h>’ routines, such as, getchar(), with
flex rules and expect it to work. Call input() instead. The total table entries listed by
the ‘-v’ flag excludes the number of table entries needed to determine what rule has been
matched. The number of entries is equal to the number of DFA states if the scanner does
not use REJECT, and somewhat greater than the number of states if it does. REJECT cannot
be used with the ‘-f’ or ‘-F’ options.
The flex internal algorithms need documentation.
Chapter 25: Additional Reading
75
25 Additional Reading
You may wish to read more about the following programs:
• lex
• yacc
• sed
• awk
The following books may contain material of interest:
John Levine, Tony Mason, and Doug Brown, Lex & Yacc, O’Reilly and Associates. Be
sure to get the 2nd edition.
M. E. Lesk and E. Schmidt, LEX – Lexical Analyzer Generator
Alfred Aho, Ravi Sethi and Jeffrey Ullman, Compilers: Principles, Techniques and Tools,
Addison-Wesley (1986). Describes the pattern-matching techniques used by flex (deterministic finite automata).
FAQ
76
FAQ
From time to time, the flex maintainer receives certain questions. Rather than repeat
answers to well-understood problems, we publish them here.
When was flex born?
Vern Paxson took over the Software Tools lex project from Jef Poskanzer in 1982. At that
point it was written in Ratfor. Around 1987 or so, Paxson translated it into C, and a legend
was born :-).
How do I expand backslash-escape sequences in C-style
quoted strings?
A key point when scanning quoted strings is that you cannot (easily) write a single rule
that will precisely match the string if you allow things like embedded escape sequences and
newlines. If you try to match strings with a single rule then you’ll wind up having to rescan
the string anyway to find any escape sequences.
Instead you can use exclusive start conditions and a set of rules, one for matching nonescaped text, one for matching a single escape, one for matching an embedded newline, and
one for recognizing the end of the string. Each of these rules is then faced with the question
of where to put its intermediary results. The best solution is for the rules to append their
local value of yytext to the end of a “string literal” buffer. A rule like the escape-matcher
will append to the buffer the meaning of the escape sequence rather than the literal text in
yytext. In this way, yytext does not need to be modified at all.
Why do flex scanners call fileno if it is not ANSI
compatible?
Flex scanners call fileno() in order to get the file descriptor corresponding to yyin. The
file descriptor may be passed to isatty() or read(), depending upon which %options you
specified. If your system does not have fileno() support, to get rid of the read() call, do
not specify %option read. To get rid of the isatty() call, you must specify one of %option
always-interactive or %option never-interactive.
Does flex support recursive pattern definitions?
e.g.,
%%
block
"{"({block}|{statement})*"}"
No. You cannot have recursive definitions. The pattern-matching power of regular expressions in general (and therefore flex scanners, too) is limited. In particular, regular
expressions cannot “balance” parentheses to an arbitrary degree. For example, it’s impossible to write a regular expression that matches all strings containing the same number of
’{’s as ’}’s. For more powerful pattern matching, you need a parser, such as GNU bison.
FAQ
77
How do I skip huge chunks of input (tens of megabytes)
while using flex?
Use fseek() (or lseek()) to position yyin, then call yyrestart().
Flex is not matching my patterns in the same order that I
defined them.
flex picks the rule that matches the most text (i.e., the longest possible input string).
This is because flex uses an entirely different matching technique (“deterministic finite
automata”) that actually does all of the matching simultaneously, in parallel. (Seems
impossible, but it’s actually a fairly simple technique once you understand the principles.)
A side-effect of this parallel matching is that when the input matches more than one
rule, flex scanners pick the rule that matched the most text. This is explained further in
the manual, in the section See Chapter 7 [Matching], page 14.
If you want flex to choose a shorter match, then you can work around this behavior by
expanding your short rule to match more text, then put back the extra:
data_.*
yyless( 5 ); BEGIN BLOCKIDSTATE;
Another fix would be to make the second rule active only during the <BLOCKIDSTATE>
start condition, and make that start condition exclusive by declaring it with %x instead of
%s.
A final fix is to change the input language so that the ambiguity for ‘data_’ is removed,
by adding characters to it that don’t match the identifier rule, or by removing characters
(such as ‘_’) from the identifier rule so it no longer matches ‘data_’. (Of course, you might
also not have the option of changing the input language.)
My actions are executing out of order or sometimes not at
all.
Most likely, you have (in error) placed the opening ‘{’ of the action block on a different line
than the rule, e.g.,
^(foo|bar)
{ <<<--- WRONG!
}
flex requires that the opening ‘{’ of an action associated with a rule begin on the same
line as does the rule. You need instead to write your rules as follows:
^(foo|bar)
{ // CORRECT!
}
How can I have multiple input sources feed into the same
scanner at the same time?
If . . .
• your scanner is free of backtracking (verified using flex’s ‘-b’ flag),
• AND you run your scanner interactively (‘-I’ option; default unless using special table
compression options),
FAQ
78
• AND you feed it one character at a time by redefining YY_INPUT to do so,
then every time it matches a token, it will have exhausted its input buffer (because the
scanner is free of backtracking). This means you can safely use select() at the point and
only call yylex() for another token if select() indicates there’s data available.
That is, move the select() out from the input function to a point where it determines
whether yylex() gets called for the next token.
With this approach, you will still have problems if your input can arrive piecemeal;
select() could inform you that the beginning of a token is available, you call yylex() to
get it, but it winds up blocking waiting for the later characters in the token.
Here’s another way: Move your input multiplexing inside of YY_INPUT. That is, whenever
YY_INPUT is called, it select()’s to see where input is available. If input is available for
the scanner, it reads and returns the next byte. If input is available from another source, it
calls whatever function is responsible for reading from that source. (If no input is available,
it blocks until some input is available.) I’ve used this technique in an interpreter I wrote
that both reads keyboard input using a flex scanner and IPC traffic from sockets, and it
works fine.
Can I build nested parsers that work with the same input
file?
This is not going to work without some additional effort. The reason is that flex blockbuffers the input it reads from yyin. This means that the “outermost” yylex(), when
called, will automatically slurp up the first 8K of input available on yyin, and subsequent
calls to other yylex()’s won’t see that input. You might be tempted to work around this
problem by redefining YY_INPUT to only return a small amount of text, but it turns out
that that approach is quite difficult. Instead, the best solution is to combine all of your
scanners into one large scanner, using a different exclusive start condition for each.
How can I match text only at the end of a file?
There is no way to write a rule which is “match this text, but only if it comes at the end of
the file”. You can fake it, though, if you happen to have a character lying around that you
don’t allow in your input. Then you redefine YY_INPUT to call your own routine which, if
it sees an ‘EOF’, returns the magic character first (and remembers to return a real EOF next
time it’s called). Then you could write:
<COMMENT>(.|\n)*{EOF_CHAR}
/* saw comment at EOF */
How can I make REJECT cascade across start condition
boundaries?
You can do this as follows. Suppose you have a start condition ‘A’, and after exhausting all
of the possible matches in ‘<A>’, you want to try matches in ‘<INITIAL>’. Then you could
use the following:
%x A
%%
<A>rule_that_is_long
...; REJECT;
<A>rule
...; REJECT; /* shorter rule */
FAQ
79
<A>etc.
...
<A>.|\n {
/* Shortest and last rule in <A>, so
* cascaded REJECTs will eventually
* wind up matching this rule. We want
* to now switch to the initial state
* and try matching from there instead.
*/
yyless(0);
/* put back matched text */
BEGIN(INITIAL);
}
Why can’t I use fast or full tables with interactive mode?
One of the assumptions flex makes is that interactive applications are inherently slow
(they’re waiting on a human after all). It has to do with how the scanner detects that
it must be finished scanning a token. For interactive scanners, after scanning each character the current state is looked up in a table (essentially) to see whether there’s a chance
of another input character possibly extending the length of the match. If not, the scanner
halts. For non-interactive scanners, the end-of-token test is much simpler, basically a compare with 0, so no memory bus cycles. Since the test occurs in the innermost scanning loop,
one would like to make it go as fast as possible.
Still, it seems reasonable to allow the user to choose to trade off a bit of performance in
this area to gain the corresponding flexibility. There might be another reason, though, why
fast scanners don’t support the interactive option.
How much faster is -F or -f than -C?
Much faster (factor of 2-3).
If I have a simple grammar can’t I just parse it with flex?
Is your grammar recursive? That’s almost always a sign that you’re better off using a
parser/scanner rather than just trying to use a scanner alone.
Why doesn’t yyrestart() set the start state back to
INITIAL?
There are two reasons. The first is that there might be programs that rely on the start state
not changing across file changes. The second is that beginning with flex version 2.4, use
of yyrestart() is no longer required, so fixing the problem there doesn’t solve the more
general problem.
How can I match C-style comments?
You might be tempted to try something like this:
"/*".*"*/"
// WRONG!
or, worse, this:
FAQ
80
"/*"(.|\n)"*/"
// WRONG!
The above rules will eat too much input, and blow up on things like:
/* a comment */ do_my_thing( "oops */" );
Here is one way which allows you to track line information:
<INITIAL>{
"/*"
BEGIN(IN_COMMENT);
}
<IN_COMMENT>{
"*/"
BEGIN(INITIAL);
[^*\n]+
// eat comment in chunks
"*"
// eat the lone star
\n
yylineno++;
}
The ’.’ isn’t working the way I expected.
Here are some tips for using ‘.’:
• A common mistake is to place the grouping parenthesis AFTER an operator, when you
really meant to place the parenthesis BEFORE the operator, e.g., you probably want
this (foo|bar)+ and NOT this (foo|bar+).
The first pattern matches the words ‘foo’ or ‘bar’ any number of times, e.g., it matches
the text ‘barfoofoobarfoo’. The second pattern matches a single instance of foo or a
single instance of bar followed by one or more ‘r’s, e.g., it matches the text barrrr .
• A ‘.’ inside ‘[]’’s just means a literal‘.’ (period), and NOT “any character except
newline”.
• Remember that ‘.’ matches any character EXCEPT ‘\n’ (and ‘EOF’). If you really
want to match ANY character, including newlines, then use (.|\n) Beware that the
regex (.|\n)+ will match your entire input!
• Finally, if you want to match a literal ‘.’ (a period), then use ‘[.]’ or ‘"."’
Can I get the flex manual in another format?
The flex source distribution includes a texinfo manual. You are free to convert that texinfo
into whatever format you desire. The texinfo package includes tools for conversion to a
number of formats.
Does there exist a "faster" NDFA->DFA algorithm?
There’s no way around the potential exponential running time - it can take you exponential
time just to enumerate all of the DFA states. In practice, though, the running time is closer
to linear, or sometimes quadratic.
How does flex compile the DFA so quickly?
There are two big speed wins that flex uses:
1. It analyzes the input rules to construct equivalence classes for those characters that
always make the same transitions. It then rewrites the NFA using equivalence classes
FAQ
81
for transitions instead of characters. This cuts down the NFA->DFA computation time
dramatically, to the point where, for uncompressed DFA tables, the DFA generation is
often I/O bound in writing out the tables.
2. It maintains hash values for previously computed DFA states, so testing whether a
newly constructed DFA state is equivalent to a previously constructed state can be
done very quickly, by first comparing hash values.
How can I use more than 8192 rules?
Flex is compiled with an upper limit of 8192 rules per scanner. If you need more than
8192 rules in your scanner, you’ll have to recompile flex with the following changes in
‘flexdef.h’:
<
<
->
>
#define YY_TRAILING_MASK 0x2000
#define YY_TRAILING_HEAD_MASK 0x4000
#define YY_TRAILING_MASK 0x20000000
#define YY_TRAILING_HEAD_MASK 0x40000000
This should work okay as long as your C compiler uses 32 bit integers. But you might
want to think about whether using such a huge number of rules is the best way to solve
your problem.
The following may also be relevant:
With luck, you should be able to increase the definitions in flexdef.h for:
#define JAMSTATE -32766 /* marks a reference to the state that always jams */
#define MAXIMUM_MNS 31999
#define BAD_SUBSCRIPT -32767
recompile everything, and it’ll all work. Flex only has these 16-bit-like values built into
it because a long time ago it was developed on a machine with 16-bit ints. I’ve given this
advice to others in the past but haven’t heard back from them whether it worked okay or
not...
How do I abandon a file in the middle of a scan and switch
to a new file?
Just call yyrestart(newfile). Be sure to reset the start state if you want a “fresh start,
since yyrestart does NOT reset the start state back to INITIAL.
How do I execute code only during initialization (only
before the first scan)?
You can specify an initial action by defining the macro YY_USER_INIT (though note that
yyout may not be available at the time this macro is executed). Or you can add to the
beginning of your rules section:
%%
/* Must be indented! */
static int did_init = 0;
FAQ
82
if ( ! did_init ){
do_my_init();
did_init = 1;
}
How do I execute code at termination?
You can specify an action for the <<EOF>> rule.
Where else can I find help?
You can find the flex homepage on the web at http://flex.sourceforge.net/. See that
page for details about flex mailing lists as well.
Can I include comments in the "rules" section of the file?
Yes, just about anywhere you want to. See the manual for the specific syntax.
I get an error about undefined yywrap().
You must supply a yywrap() function of your own, or link to ‘libfl.a’ (which provides
one), or use
%option noyywrap
in your source to say you don’t want a yywrap() function.
How can I change the matching pattern at run time?
You can’t, it’s compiled into a static table when flex builds the scanner.
How can I expand macros in the input?
The best way to approach this problem is at a higher level, e.g., in the parser.
However, you can do this using multiple input buffers.
%%
macro/[a-z]+
{
/* Saw the macro "macro" followed by extra stuff. */
main_buffer = YY_CURRENT_BUFFER;
expansion_buffer = yy_scan_string(expand(yytext));
yy_switch_to_buffer(expansion_buffer);
}
<<EOF>> {
if ( expansion_buffer )
{
// We were doing an expansion, return to where
// we were.
yy_switch_to_buffer(main_buffer);
yy_delete_buffer(expansion_buffer);
expansion_buffer = 0;
FAQ
83
}
else
yyterminate();
}
You probably will want a stack of expansion buffers to allow nested macros. From the
above though hopefully the idea is clear.
How can I build a two-pass scanner?
One way to do it is to filter the first pass to a temporary file, then process the temporary
file on the second pass. You will probably see a performance hit, due to all the disk I/O.
When you need to look ahead far forward like this, it almost always means that the right
solution is to build a parse tree of the entire input, then walk it after the parse in order
to generate the output. In a sense, this is a two-pass approach, once through the text and
once through the parse tree, but the performance hit for the latter is usually an order of
magnitude smaller, since everything is already classified, in binary format, and residing in
memory.
How do I match any string not matched in the preceding
rules?
One way to assign precedence, is to place the more specific rules first. If two rules would
match the same input (same sequence of characters) then the first rule listed in the flex
input wins, e.g.,
%%
foo[a-zA-Z_]+
bar[a-zA-Z_]+
[a-zA-Z_]+
return FOO_ID;
return BAR_ID;
return GENERIC_ID;
Note that the rule [a-zA-Z_]+ must come *after* the others. It will match the same
amount of text as the more specific rules, and in that case the flex scanner will pick the
first rule listed in your scanner as the one to match.
I am trying to port code from AT&T lex that uses yysptr
and yysbuf.
Those are internal variables pointing into the AT&T scanner’s input buffer. I imagine
they’re being manipulated in user versions of the input() and unput() functions. If so,
what you need to do is analyze those functions to figure out what they’re doing, and then
replace input() with an appropriate definition of YY_INPUT. You shouldn’t need to (and
must not) replace flex’s unput() function.
Is there a way to make flex treat NULL like a regular
character?
Yes, ‘\0’ and ‘\x00’ should both do the trick. Perhaps you have an ancient version of flex.
The latest release is version 2.5.37.
FAQ
84
Whenever flex can not match the input it says "flex scanner
jammed".
You need to add a rule that matches the otherwise-unmatched text, e.g.,
%option yylineno
%%
[[a bunch of rules here]]
.
printf("bad input character ’%s’ at line %d\n", yytext, yylineno);
See %option default for more information.
Why doesn’t flex have non-greedy operators like perl does?
A DFA can do a non-greedy match by stopping the first time it enters an accepting state,
instead of consuming input until it determines that no further matching is possible (a “jam”
state). This is actually easier to implement than longest leftmost match (which flex does).
But it’s also much less useful than longest leftmost match. In general, when you find
yourself wishing for non-greedy matching, that’s usually a sign that you’re trying to make
the scanner do some parsing. That’s generally the wrong approach, since it lacks the power
to do a decent job. Better is to either introduce a separate parser, or to split the scanner
into multiple scanners using (exclusive) start conditions.
You might have a separate start state once you’ve seen the ‘BEGIN’. In that state, you
might then have a regex that will match ‘END’ (to kick you out of the state), and perhaps
‘(.|\n)’ to get a single character within the chunk ...
This approach also has much better error-reporting properties.
Memory leak - 16386 bytes allocated by malloc.
UPDATED 2002-07-10: As of flex version 2.5.9, this leak means that you did not call
yylex_destroy(). If you are using an earlier version of flex, then read on.
The leak is about 16426 bytes. That is, (8192 * 2 + 2) for the read-buffer, and about 40 for
struct yy_buffer_state (depending upon alignment). The leak is in the non-reentrant C
scanner only (NOT in the reentrant scanner, NOT in the C++ scanner). Since flex doesn’t
know when you are done, the buffer is never freed.
However, the leak won’t multiply since the buffer is reused no matter how many times
you call yylex().
If you want to reclaim the memory when you are completely done scanning, then you
might try this:
/* For non-reentrant C scanner only. */
yy_delete_buffer(YY_CURRENT_BUFFER);
yy_init = 1;
Note: yy_init is an "internal variable", and hasn’t been tested in this situation. It is
possible that some other globals may need resetting as well.
FAQ
85
How do I track the byte offset for lseek()?
>
>
>
>
We thought that it would be possible to have this number through the
evaluation of the following expression:
seek_position = (no_buffers)*YY_READ_BUF_SIZE + yy_c_buf_p - YY_CURRENT_BUFFER->yy_
While this is the right idea, it has two problems. The first is that it’s possible that flex
will request less than YY_READ_BUF_SIZE during an invocation of YY_INPUT (or that your
input source will return less even though YY_READ_BUF_SIZE bytes were requested). The
second problem is that when refilling its internal buffer, flex keeps some characters from the
previous buffer (because usually it’s in the middle of a match, and needs those characters
to construct yytext for the match once it’s done). Because of this, yy_c_buf_p - YY_
CURRENT_BUFFER->yy_ch_buf won’t be exactly the number of characters already read from
the current buffer.
An alternative solution is to count the number of characters you’ve matched since starting
to scan. This can be done by using YY_USER_ACTION. For example,
#define YY_USER_ACTION num_chars += yyleng;
(You need to be careful to update your bookkeeping if you use yymore(), yyless(),
unput(), or input().)
How do I use my own I/O classes in a C++ scanner?
When the flex C++ scanning class rewrite finally happens, then this sort of thing should
become much easier.
You can do this by passing the various functions (such as LexerInput() and
LexerOutput()) NULL iostream*’s, and then dealing with your own I/O classes
surreptitiously (i.e., stashing them in special member variables). This works because
the only assumption about the lexer regarding what’s done with the iostream’s is that
they’re ultimately passed to LexerInput() and LexerOutput, which then do whatever is
necessary with them.
How do I skip as many chars as possible?
How do I skip as many chars as possible – without interfering with the other patterns?
In the example below, we want to skip over characters until we see the phrase "endskip".
The following will NOT work correctly (do you see why not?)
/* INCORRECT SCANNER */
%x SKIP
%%
<INITIAL>startskip
BEGIN(SKIP);
...
<SKIP>"endskip"
BEGIN(INITIAL);
<SKIP>.*
;
The problem is that the pattern .* will eat up the word "endskip." The simplest (but
slow) fix is:
<SKIP>"endskip"
BEGIN(INITIAL);
FAQ
86
<SKIP>.
;
The fix involves making the second rule match more, without making it match "endskip"
plus something else. So for example:
<SKIP>"endskip"
<SKIP>[^e]+
<SKIP>.
BEGIN(INITIAL);
;
;/* so you eat up e’s, too */
deleteme00
QUESTION:
When was flex born?
Vern Paxson took over
the Software Tools lex project from Jef Poskanzer in 1982. At that point it
was written in Ratfor. Around 1987 or so, Paxson translated it into C, and
a legend was born :-).
Are certain equivalent patterns faster than others?
To: Adoram Rogel <adoram@orna.hybridge.com>
Subject: Re: Flex 2.5.2 performance questions
In-reply-to: Your message of Wed, 18 Sep 96 11:12:17 EDT.
Date: Wed, 18 Sep 96 10:51:02 PDT
From: Vern Paxson <vern>
[Note, the most recent flex release is 2.5.4, which you can get from
ftp.ee.lbl.gov. It has bug fixes over 2.5.2 and 2.5.3.]
> 1. Using the pattern
>
([Ff](oot)?)?[Nn](ote)?(\.)?
>
instead of
>
(((F|f)oot(N|n)ote)|((N|n)ote)|((N|n)\.)|((F|f)(N|n)(\.)))
>
(in a very complicated flex program) caused the program to slow from
>
300K+/min to 100K/min (no other changes were done).
These two are not equivalent. For example, the first can match "footnote."
but the second can only match "footnote". This is almost certainly the
cause in the discrepancy - the slower scanner run is matching more tokens,
and/or having to do more backing up.
> 2. Which of these two are better: [Ff]oot or (F|f)oot ?
From a performance point of view, they’re equivalent (modulo presumably
minor effects such as memory cache hit rates; and the presence of trailing
context, see below). From a space point of view, the first is slightly
preferable.
FAQ
87
> 3. I have a pattern that look like this:
>
pats {p1}|{p2}|{p3}|...|{p50}
(50 patterns ORd)
>
>
running yet another complicated program that includes the following rule:
>
<snext>{and}/{no4}{bb}{pats}
>
>
gets me to "too complicated - over 32,000 states"...
I can’t tell from this example whether the trailing context is variable-length
or fixed-length (it could be the latter if {and} is fixed-length). If it’s
variable length, which flex -p will tell you, then this reflects a basic
performance problem, and if you can eliminate it by restructuring your
scanner, you will see significant improvement.
>
>
>
so I divided {pats} to {pats1}, {pats2},..., {pats5} each consists of about
10 patterns and changed the rule to be 5 rules.
This did compile, but what is the rule of thumb here ?
The rule is to avoid trailing context other than fixed-length, in which for
a/b, either the ’a’ pattern or the ’b’ pattern have a fixed length. Use
of the ’|’ operator automatically makes the pattern variable length, so in
this case ’[Ff]oot’ is preferred to ’(F|f)oot’.
> 4. I changed a rule that looked like this:
>
<snext8>{and}{bb}/{ROMAN}[^A-Za-z] { BEGIN...
>
>
to the next 2 rules:
>
<snext8>{and}{bb}/{ROMAN}[A-Za-z] { ECHO;}
>
<snext8>{and}{bb}/{ROMAN}
{ BEGIN...
>
>
Again, I understand the using [^...] will cause a great performance loss
Actually, it doesn’t cause any sort of performance loss. It’s a surprising
fact about regular expressions that they always match in linear time
regardless of how complex they are.
>
but are there any specific rules about it ?
See the "Performance Considerations" section of the man page, and also
the example in MISC/fastwc/.
Vern
Is backing up a big deal?
To: Adoram Rogel <adoram@hybridge.com>
Subject: Re: Flex 2.5.2 performance questions
FAQ
88
In-reply-to: Your message of Thu, 19 Sep 96 10:16:04 EDT.
Date: Thu, 19 Sep 96 09:58:00 PDT
From: Vern Paxson <vern>
> a lot about the backing up problem.
> I believe that there lies my biggest problem, and I’ll try to improve
> it.
Since you have variable trailing context, this is a bigger performance
problem. Fixing it is usually easier than fixing backing up, which in a
complicated scanner (yours seems to fit the bill) can be extremely
difficult to do correctly.
You also don’t mention what flags you are using for your scanner.
-f makes a large speed difference, and -Cfe buys you nearly as much
speed but the resulting scanner is considerably smaller.
> I have an | operator in {and} and in {pats} so both of them are variable
> length.
-p should have reported this.
> Is changing one of them to fixed-length is enough ?
Yes.
> Is it possible to change the 32,000 states limit ?
Yes. I’ve appended instructions on how. Before you make this change,
though, you should think about whether there are ways to fundamentally
simplify your scanner - those are certainly preferable!
Vern
To increase the 32K limit (on a machine with 32 bit integers), you increase
the magnitude of the following in flexdef.h:
#define
#define
#define
#define
JAMSTATE -32766 /* marks a reference to the state that always jams */
MAXIMUM_MNS 31999
BAD_SUBSCRIPT -32767
MAX_SHORT 32700
Adding a 0 or two after each should do the trick.
Can I fake multi-byte character support?
To: Heeman_Lee@hp.com
FAQ
89
Subject: Re: flex
In-reply-to: Your
Date: Fri, 04 Oct
From: Vern Paxson
>
>
>
>
- multi-byte support?
message of Thu, 03 Oct 1996 17:24:04 PDT.
1996 11:42:18 PDT
<vern>
I assume as long as my *.l file defines the
range of expected character code values (in octal format), flex will
scan the file and read multi-byte characters correctly. But I have no
confidence in this assumption.
Your lack of confidence is justified - this won’t work.
Flex has in it a widespread assumption that the input is processed
one byte at a time. Fixing this is on the to-do list, but is involved,
so it won’t happen any time soon. In the interim, the best I can suggest
(unless you want to try fixing it yourself) is to write your rules in
terms of pairs of bytes, using definitions in the first section:
X
\xfe\xc2
...
%%
foo{X}bar
found_foo_fe_c2_bar();
etc.
Definitely a pain - sorry about that.
By the way, the email address you used for me is ancient, indicating you
have a very old version of flex. You can get the most recent, 2.5.4, from
ftp.ee.lbl.gov.
Vern
deleteme01
To: moleary@primus.com
Subject: Re: Flex / Unicode compatibility question
In-reply-to: Your message of Tue, 22 Oct 1996 10:15:42 PDT.
Date: Tue, 22 Oct 1996 11:06:13 PDT
From: Vern Paxson <vern>
Unfortunately flex at the moment has a widespread assumption within it
that characters are processed 8 bits at a time. I don’t see any easy
fix for this (other than writing your rules in terms of double characters a pain). I also don’t know of a wider lex, though you might try surfing
the Plan 9 stuff because I know it’s a Unicode system, and also the PCCT
toolkit (try searching say Alta Vista for "Purdue Compiler Construction
Toolkit").
FAQ
90
Fixing flex to handle wider characters is on the long-term to-do list.
But since flex is a strictly spare-time project these days, this probably
won’t happen for quite a while, unless someone else does it first.
Vern
Can you discuss some flex internals?
To: Johan Linde <jl@theophys.kth.se>
Subject: Re: translation of flex
In-reply-to: Your message of Sun, 10 Nov 1996 09:16:36 PST.
Date: Mon, 11 Nov 1996 10:33:50 PST
From: Vern Paxson <vern>
> I’m working for the Swedish team translating GNU program, and I’m currently
> working with flex. I have a few questions about some of the messages which
> I hope you can answer.
All of the things you’re wondering about, by the way, concerning flex
internals - probably the only person who understands what they mean in
English is me! So I wouldn’t worry too much about getting them right.
That said ...
> #: main.c:545
> msgid " %d protos created\n"
>
> Does proto mean prototype?
Yes - prototypes of state compression tables.
>
>
>
>
>
>
#: main.c:539
msgid " %d/%d (peak %d) template nxt-chk entries created\n"
Here I’m mainly puzzled by ’nxt-chk’. I guess it means ’next-check’. (?)
However, ’template next-check entries’ doesn’t make much sense to me. To be
able to find a good translation I need to know a little bit more about it.
There is a scheme in the Aho/Sethi/Ullman compiler book for compressing
scanner tables. It involves creating two pairs of tables. The first has
"base" and "default" entries, the second has "next" and "check" entries.
The "base" entry is indexed by the current state and yields an index into
the next/check table. The "default" entry gives what to do if the state
transition isn’t found in next/check. The "next" entry gives the next
state to enter, but only if the "check" entry verifies that this entry is
correct for the current state. Flex creates templates of series of
next/check entries and then encodes differences from these templates as a
way to compress the tables.
FAQ
91
> #: main.c:533
> msgid " %d/%d base-def entries created\n"
>
> The same problem here for ’base-def’.
See above.
Vern
unput() messes up yy at bol
To: Xinying Li <xli@npac.syr.edu>
Subject: Re: FLEX ?
In-reply-to: Your message of Wed, 13 Nov 1996 17:28:38 PST.
Date: Wed, 13 Nov 1996 19:51:54 PST
From: Vern Paxson <vern>
> "unput()" them to input flow, question occurs. If I do this after I scan
> a carriage, the variable "YY_CURRENT_BUFFER->yy_at_bol" is changed. That
> means the carriage flag has gone.
You can control this by calling yy_set_bol().
It’s described in the manual.
>
And if in pre-reading it goes to the end of file, is anything done
> to control the end of curren buffer and end of file?
No, there’s no way to put back an end-of-file.
>
By the way I am using flex 2.5.2 and using the "-l".
The latest release is 2.5.4, by the way. It fixes some bugs in 2.5.2 and
2.5.3. You can get it from ftp.ee.lbl.gov.
Vern
The | operator is not doing what I want
To: Alain.ISSARD@st.com
Subject: Re: Start condition with FLEX
In-reply-to: Your message of Mon, 18 Nov 1996 09:45:02 PST.
Date: Mon, 18 Nov 1996 10:41:34 PST
From: Vern Paxson <vern>
> I am not able to use the start condition scope and to use the | (OR) with
> rules having start conditions.
The problem is that if you use ’|’ as a regular expression operator, for
FAQ
92
example "a|b" meaning "match either ’a’ or ’b’", then it must *not* have
any blanks around it. If you instead want the special ’|’ *action* (which
from your scanner appears to be the case), which is a way of giving two
different rules the same action:
foo
bar
|
matched_foo_or_bar();
then ’|’ *must* be separated from the first rule by whitespace and *must*
be followed by a new line. You *cannot* write it as:
foo | bar
matched_foo_or_bar();
even though you might think you could because yacc supports this syntax.
The reason for this unfortunately incompatibility is historical, but it’s
unlikely to be changed.
Your problems with start condition scope are simply due to syntax errors
from your use of ’|’ later confusing flex.
Let me know if you still have problems.
Vern
Why can’t flex understand this variable trailing context
pattern?
To: Gregory Margo <gmargo@newton.vip.best.com>
Subject: Re: flex-2.5.3 bug report
In-reply-to: Your message of Sat, 23 Nov 1996 16:50:09 PST.
Date: Sat, 23 Nov 1996 17:07:32 PST
From: Vern Paxson <vern>
> Enclosed is a lex file that "real" lex will process, but I cannot get
> flex to process it. Could you try it and maybe point me in the right direction?
Your problem is that some of the definitions in the scanner use the ’/’
trailing context operator, and have it enclosed in ()’s. Flex does not
allow this operator to be enclosed in ()’s because doing so allows undefined
regular expressions such as "(a/b)+". So the solution is to remove the
parentheses. Note that you must also be building the scanner with the -l
option for AT&T lex compatibility. Without this option, flex automatically
encloses the definitions in parentheses.
Vern
FAQ
93
The ^ operator isn’t working
To: Thomas Hadig <hadig@toots.physik.rwth-aachen.de>
Subject: Re: Flex Bug ?
In-reply-to: Your message of Tue, 26 Nov 1996 14:35:01 PST.
Date: Tue, 26 Nov 1996 11:15:05 PST
From: Vern Paxson <vern>
>
>
>
>
>
In my lexer code, i have the line :
^\*.*
{ }
Thus all lines starting with an astrix (*) are comment lines.
This does not work !
I can’t get this problem to reproduce - it works fine for me.
though that if what you have is slightly different:
COMMENT ^\*.*
%%
{COMMENT}
Note
{ }
then it won’t work, because flex pushes back macro definitions enclosed
in ()’s, so the rule becomes
(^\*.*)
{ }
and now that the ’^’ operator is not at the immediate beginning of the
line, it’s interpreted as just a regular character. You can avoid this
behavior by using the "-l" lex-compatibility flag, or "%option lex-compat".
Vern
Trailing context is getting confused with trailing optional
patterns
To: Adoram Rogel <adoram@hybridge.com>
Subject: Re: Flex 2.5.4 BOF ???
In-reply-to: Your message of Tue, 26 Nov 1996 16:10:41 PST.
Date: Wed, 27 Nov 1996 10:56:25 PST
From: Vern Paxson <vern>
>
Organization(s)?/[a-z]
>
> This matched "Organizations" (looking in debug mode, the trailing s
> was matched with trailing context instead of the optional (s) in the
> end of the word.
That should only happen with lex.
Flex can properly match this pattern.
FAQ
94
(That might be what you’re saying, I’m just not sure.)
> Is there a way to avoid this dangerous trailing context problem ?
Unfortunately, there’s no easy way. On the other hand, I don’t see why
it should be a problem. Lex’s matching is clearly wrong, and I’d hope
that usually the intent remains the same as expressed with the pattern,
so flex’s matching will be correct.
Vern
Is flex GNU or not?
To: Cameron MacKinnon <mackin@interlog.com>
Subject: Re: Flex documentation bug
In-reply-to: Your message of Mon, 02 Dec 1996 00:07:08 PST.
Date: Sun, 01 Dec 1996 22:29:39 PST
From: Vern Paxson <vern>
> I’m not sure how or where to submit bug reports (documentation or
> otherwise) for the GNU project stuff ...
Well, strictly speaking flex isn’t part of the GNU project. They just
distribute it because no one’s written a decent GPL’d lex replacement.
So you should send bugs directly to me. Those sent to the GNU folks
sometimes find there way to me, but some may drop between the cracks.
>
>
>
>
In GNU Info, under the section ’Start Conditions’, and also in the man
page (mine’s dated April ’95) is a nice little snippet showing how to
parse C quoted strings into a buffer, defined to be MAX_STR_CONST in
size. Unfortunately, no overflow checking is ever done ...
This is already mentioned in the manual:
Finally, here’s an example of how to match C-style quoted
strings using exclusive start conditions, including expanded
escape sequences (but not including checking for a string
that’s too long):
The reason for not doing the overflow checking is that it will needlessly
clutter up an example whose main purpose is just to demonstrate how to
use flex.
The latest release is 2.5.4, by the way, available from ftp.ee.lbl.gov.
Vern
FAQ
95
ERASEME53
To: tsv@cs.UManitoba.CA
Subject: Re: Flex (reg)..
In-reply-to: Your message of Thu, 06 Mar 1997 23:50:16 PST.
Date: Thu, 06 Mar 1997 15:54:19 PST
From: Vern Paxson <vern>
> [:alpha:] ([:alnum:] | \\_)*
If your rule really has embedded blanks as shown above, then it won’t
work, as the first blank delimits the rule from the action. (It wouldn’t
even compile ...) You need instead:
[:alpha:]([:alnum:]|\\_)*
and that should work fine - there’s no restriction on what can go inside
of ()’s except for the trailing context operator, ’/’.
Vern
I need to scan if-then-else blocks and while loops
To: "Mike Stolnicki" <mstolnic@ford.com>
Subject: Re: FLEX help
In-reply-to: Your message of Fri, 30 May 1997 13:33:27 PDT.
Date: Fri, 30 May 1997 10:46:35 PDT
From: Vern Paxson <vern>
>
>
>
>
We’d like to add "if-then-else", "while", and "for" statements to our
language ...
We’ve investigated many possible solutions. The one solution that seems
the most reasonable involves knowing the position of a TOKEN in yyin.
I strongly advise you to instead build a parse tree (abstract syntax tree)
and loop over that instead. You’ll find this has major benefits in keeping
your interpreter simple and extensible.
That said, the functionality you mention for get_position and set_position
have been on the to-do list for a while. As flex is a purely spare-time
project for me, no guarantees when this will be added (in particular, it
for sure won’t be for many months to come).
Vern
ERASEME55
To: Colin Paul Adams <colin@colina.demon.co.uk>
FAQ
96
Subject: Re: Flex
In-reply-to: Your
Date: Fri, 15 Aug
From: Vern Paxson
>
>
>
>
>
>
>
>
>
>
C++ classes and Bison
message of 09 Aug 1997 17:11:41 PDT.
1997 10:48:19 PDT
<vern>
#define YY_DECL
*parm)
int yylex (YYSTYPE *lvalp, struct parser_control
I have been trying to get this to work as a C++ scanner, but it does
not appear to be possible (warning that it matches no declarations in
yyFlexLexer, or something like that).
Is this supposed to be possible, or is it being worked on (I DID
notice the comment that scanner classes are still experimental, so I’m
not too hopeful)?
What you need to do is derive a subclass from yyFlexLexer that provides
the above yylex() method, squirrels away lvalp and parm into member
variables, and then invokes yyFlexLexer::yylex() to do the regular scanning.
Vern
ERASEME56
To: Mikael.Latvala@lmf.ericsson.se
Subject: Re: Possible mistake in Flex v2.5 document
In-reply-to: Your message of Fri, 05 Sep 1997 16:07:24 PDT.
Date: Fri, 05 Sep 1997 10:01:54 PDT
From: Vern Paxson <vern>
>
>
>
>
In that example you show how to count comment lines when using
C style /* ... */ comments. My question is, shouldn’t you take into
account a scenario where end of a comment marker occurs inside
character or string literals?
The scanner certainly needs to also scan character and string literals.
However it does that (there’s an example in the man page for strings), the
lexer will recognize the beginning of the literal before it runs across the
embedded "/*". Consequently, it will finish scanning the literal before it
even considers the possibility of matching "/*".
Example:
’([^’]*|{ESCAPE_SEQUENCE})’
will match all the text between the ’’s (inclusive). So the lexer
considers this as a token beginning at the first ’, and doesn’t even
FAQ
97
attempt to match other tokens inside it.
I thinnk this subtlety is not worth putting in the manual, as I suspect
it would confuse more people than it would enlighten.
Vern
ERASEME57
To: "Marty Leisner" <leisner@sdsp.mc.xerox.com>
Subject: Re: flex limitations
In-reply-to: Your message of Sat, 06 Sep 1997 11:27:21 PDT.
Date: Mon, 08 Sep 1997 11:38:08 PDT
From: Vern Paxson <vern>
> %%
> [a-zA-Z]+
>
> %%
/* skip a line */
{ printf("got %s\n", yytext); }
What version of flex are you using?
If I feed this to 2.5.4, it complains:
"bug.l", line 5: EOF encountered inside an action
"bug.l", line 5: unrecognized rule
"bug.l", line 5: fatal parse error
Not the world’s greatest error message, but it manages to flag the problem.
(With the introduction of start condition scopes, flex can’t accommodate
an action on a separate line, since it’s ambiguous with an indented rule.)
You can get 2.5.4 from ftp.ee.lbl.gov.
Vern
Is there a repository for flex scanners?
Not that we know of. You might try asking on comp.compilers.
How can I conditionally compile or preprocess my flex input
file?
Flex doesn’t have a preprocessor like C does. You might try using m4, or the C preprocessor
plus a sed script to clean up the result.
Where can I find grammars for lex and yacc?
In the sources for flex and bison.
FAQ
98
I get an end-of-buffer message for each character scanned.
This will happen if your LexerInput() function returns only one character at a time, which
can happen either if you’re scanner is "interactive", or if the streams library on your platform
always returns 1 for yyin->gcount().
Solution: override LexerInput() with a version that returns whole buffers.
unnamed-faq-62
To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
Subject: Re: Flex maximums
In-reply-to: Your message of Mon, 17 Nov 1997 17:16:06 PST.
Date: Mon, 17 Nov 1997 17:16:15 PST
From: Vern Paxson <vern>
> I took a quick look into the flex-sources and altered some #defines in
> flexdefs.h:
>
>
#define INITIAL_MNS 64000
>
#define MNS_INCREMENT 1024000
>
#define MAXIMUM_MNS 64000
The things to fix are to add a couple of zeroes to:
#define
#define
#define
#define
JAMSTATE -32766 /* marks a reference to the state that always jams */
MAXIMUM_MNS 31999
BAD_SUBSCRIPT -32767
MAX_SHORT 32700
and, if you get complaints about too many rules, make the following change too:
#define YY_TRAILING_MASK 0x200000
#define YY_TRAILING_HEAD_MASK 0x400000
- Vern
unnamed-faq-63
To: jimmey@lexis-nexis.com (Jimmey Todd)
Subject: Re: FLEX question regarding istream vs ifstream
In-reply-to: Your message of Mon, 08 Dec 1997 15:54:15 PST.
Date: Mon, 15 Dec 1997 13:21:35 PST
From: Vern Paxson <vern>
>
>
>
>
stdin_handle = YY_CURRENT_BUFFER;
ifstream fin( "aFile" );
yy_switch_to_buffer( yy_create_buffer( fin, YY_BUF_SIZE ) );
FAQ
99
>
>
>
>
>
>
>
>
>
What I’m wanting to do, is pass the contents of a file thru one set
of rules and then pass stdin thru another set... It works great if, I
don’t use the C++ classes. But since everything else that I’m doing is
in C++, I thought I’d be consistent.
The problem is that ’yy_create_buffer’ is expecting an istream* as it’s
first argument (as stated in the man page). However, fin is a ifstream
object. Any ideas on what I might be doing wrong? Any help would be
appreciated. Thanks!!
You need to pass &fin, to turn it into an ifstream* instead of an ifstream.
Then its type will be compatible with the expected istream*, because ifstream
is derived from istream.
Vern
unnamed-faq-64
To: Enda Fadian <fadiane@piercom.ie>
Subject: Re: Question related to Flex man page?
In-reply-to: Your message of Tue, 16 Dec 1997 15:17:34 PST.
Date: Tue, 16 Dec 1997 14:17:09 PST
From: Vern Paxson <vern>
> Can you explain to me what is ment by a long-jump in relation to flex?
Using the longjmp() function while inside yylex() or a routine called by it.
> what is the flex activation frame.
Just yylex()’s stack frame.
> As far as I can see yyrestart will bring me back to the sart of the input
> file and using flex++ isnot really an option!
No, yyrestart() doesn’t imply a rewind, even though its name might sound
like it does. It tells the scanner to flush its internal buffers and
start reading from the given file at its present location.
Vern
unnamed-faq-65
To: hassan@larc.info.uqam.ca (Hassan Alaoui)
Subject: Re: Need urgent Help
In-reply-to: Your message of Sat, 20 Dec 1997 19:38:19 PST.
Date: Sun, 21 Dec 1997 21:30:46 PST
From: Vern Paxson <vern>
FAQ
100
> /usr/lib/yaccpar: In function ‘int yyparse()’:
> /usr/lib/yaccpar:184: warning: implicit declaration of function ‘int yylex(...)’
>
> ld: Undefined symbol
>
_yylex
>
_yyparse
>
_yyin
This is a known problem with Solaris C++ (and/or Solaris yacc). I believe
the fix is to explicitly insert some ’extern "C"’ statements for the
corresponding routines/symbols.
Vern
unnamed-faq-66
To: mc0307@mclink.it
Cc: gnu@prep.ai.mit.edu
Subject: Re: [mc0307@mclink.it: Help request]
In-reply-to: Your message of Fri, 12 Dec 1997 17:57:29 PST.
Date: Sun, 21 Dec 1997 22:33:37 PST
From: Vern Paxson <vern>
>
>
>
>
>
>
>
This is my definition for float and integer types:
. . .
NZD
[1-9]
...
I’ve tested my program on other lex version (on UNIX Sun Solaris an HP
UNIX) and it work well, so I think that my definitions are correct.
There are any differences between Lex and Flex?
There are indeed differences, as discussed in the man page. The one
you are probably running into is that when flex expands a name definition,
it puts parentheses around the expansion, while lex does not. There’s
an example in the man page of how this can lead to different matching.
Flex’s behavior complies with the POSIX standard (or at least with the
last POSIX draft I saw).
Vern
unnamed-faq-67
To: hassan@larc.info.uqam.ca (Hassan Alaoui)
Subject: Re: Thanks
In-reply-to: Your message of Mon, 22 Dec 1997 16:06:35 PST.
Date: Mon, 22 Dec 1997 14:35:05 PST
From: Vern Paxson <vern>
FAQ
101
>
>
>
>
>
Thank you very much for your help. I compile and link well with C++ while
declaring ’yylex ...’ extern, But a little problem remains. I get a
segmentation default when executing ( I linked with lfl library) while it
works well when using LEX instead of flex. Do you have some ideas about the
reason for this ?
The one possible reason for this that comes to mind is if you’ve defined
yytext as "extern char yytext[]" (which is what lex uses) instead of
"extern char *yytext" (which is what flex uses). If it’s not that, then
I’m afraid I don’t know what the problem might be.
Vern
unnamed-faq-68
To: "Bart Niswonger" <NISWONGR@almaden.ibm.com>
Subject: Re: flex 2.5: c++ scanners & start conditions
In-reply-to: Your message of Tue, 06 Jan 1998 10:34:21 PST.
Date: Tue, 06 Jan 1998 19:19:30 PST
From: Vern Paxson <vern>
> The problem is that when I do this (using %option c++) start
> conditions seem to not apply.
The BEGIN macro modifies the yy_start variable. For C scanners, this
is a static with scope visible through the whole file. For C++ scanners,
it’s a member variable, so it only has visible scope within a member
function. Your lexbegin() routine is not a member function when you
build a C++ scanner, so it’s not modifying the correct yy_start. The
diagnostic that indicates this is that you found you needed to add
a declaration of yy_start in order to get your scanner to compile when
using C++; instead, the correct fix is to make lexbegin() a member
function (by deriving from yyFlexLexer).
Vern
unnamed-faq-69
To: "Boris Zinin" <boris@ippe.rssi.ru>
Subject: Re: current position in flex buffer
In-reply-to: Your message of Mon, 12 Jan 1998 18:58:23 PST.
Date: Mon, 12 Jan 1998 12:03:15 PST
From: Vern Paxson <vern>
> The problem is how to determine the current position in flex active
> buffer when a rule is matched....
FAQ
102
You will need to keep track of this explicitly, such as by redefining
YY_USER_ACTION to count the number of characters matched.
The latest flex release, by the way, is 2.5.4, available from ftp.ee.lbl.gov.
Vern
unnamed-faq-70
To: Bik.Dhaliwal@bis.org
Subject: Re: Flex question
In-reply-to: Your message of Mon, 26 Jan 1998 13:05:35 PST.
Date: Tue, 27 Jan 1998 22:41:52 PST
From: Vern Paxson <vern>
> That requirement involves knowing
> the character position at which a particular token was matched
> in the lexer.
The way you have to do this is by explicitly keeping track of where
you are in the file, by counting the number of characters scanned
for each token (available in yyleng). It may prove convenient to
do this by redefining YY_USER_ACTION, as described in the manual.
Vern
unnamed-faq-71
To: Vladimir Alexiev <vladimir@cs.ualberta.ca>
Subject: Re: flex: how to control start condition from parser?
In-reply-to: Your message of Mon, 26 Jan 1998 05:50:16 PST.
Date: Tue, 27 Jan 1998 22:45:37 PST
From: Vern Paxson <vern>
> It seems useful for the parser to be able to tell the lexer about such
> context dependencies, because then they don’t have to be limited to
> local or sequential context.
One way to do this is to have the parser call a stub routine that’s
included in the scanner’s .l file, and consequently that has access ot
BEGIN. The only ugliness is that the parser can’t pass in the state
it wants, because those aren’t visible - but if you don’t have many
such states, then using a different set of names doesn’t seem like
to much of a burden.
While generating a .h file like you suggests is certainly cleaner,
flex development has come to a virtual stand-still :-(, so a workaround
like the above is much more pragmatic than waiting for a new feature.
FAQ
103
Vern
unnamed-faq-72
To: Barbara Denny <denny@3com.com>
Subject: Re: freebsd flex bug?
In-reply-to: Your message of Fri, 30 Jan 1998 12:00:43 PST.
Date: Fri, 30 Jan 1998 12:42:32 PST
From: Vern Paxson <vern>
> lex.yy.c:1996: parse error before ‘=’
This is the key, identifying this error. (It may help to pinpoint
it by using flex -L, so it doesn’t generate #line directives in its
output.) I will bet you heavy money that you have a start condition
name that is also a variable name, or something like that; flex spits
out #define’s for each start condition name, mapping them to a number,
so you can wind up with:
%x foo
%%
...
%%
void bar()
{
int foo = 3;
}
and the penultimate will turn into "int 1 = 3" after C preprocessing,
since flex will put "#define foo 1" in the generated scanner.
Vern
unnamed-faq-73
To: Maurice Petrie <mpetrie@infoscigroup.com>
Subject: Re: Lost flex .l file
In-reply-to: Your message of Mon, 02 Feb 1998 14:10:01 PST.
Date: Mon, 02 Feb 1998 11:15:12 PST
From: Vern Paxson <vern>
> I am curious as to
> whether there is a simple way to backtrack from the generated source to
> reproduce the lost list of tokens we are searching on.
In theory, it’s straight-forward to go from the DFA representation
back to a regular-expression representation - the two are isomorphic.
FAQ
104
In practice, a huge headache, because you have to unpack all the tables
back into a single DFA representation, and then write a program to munch
on that and translate it into an RE.
Sorry for the less-than-happy news ...
Vern
unnamed-faq-74
To: jimmey@lexis-nexis.com (Jimmey Todd)
Subject: Re: Flex performance question
In-reply-to: Your message of Thu, 19 Feb 1998 11:01:17 PST.
Date: Thu, 19 Feb 1998 08:48:51 PST
From: Vern Paxson <vern>
> What I have found, is that the smaller the data chunk, the faster the
> program executes. This is the opposite of what I expected. Should this be
> happening this way?
This is exactly what will happen if your input file has embedded NULs.
From the man page:
A final note: flex is slow when matching NUL’s, particularly
when a token contains multiple NUL’s. It’s best to write
rules which match short amounts of text if it’s anticipated
that the text will often include NUL’s.
So that’s the first thing to look for.
Vern
unnamed-faq-75
To: jimmey@lexis-nexis.com (Jimmey Todd)
Subject: Re: Flex performance question
In-reply-to: Your message of Thu, 19 Feb 1998 11:01:17 PST.
Date: Thu, 19 Feb 1998 15:42:25 PST
From: Vern Paxson <vern>
So there are several problems.
First, to go fast, you want to match as much text as possible, which
your scanners don’t in the case that what they’re scanning is *not*
a <RN> tag. So you want a rule like:
[^<]+
FAQ
105
Second, C++ scanners are particularly slow if they’re interactive,
which they are by default. Using -B speeds it up by a factor of 3-4
on my workstation.
Third, C++ scanners that use the istream interface are slow, because
of how poorly implemented istream’s are. I built two versions of
the following scanner:
%%
.*\n
.*
%%
and the C version inhales a 2.5MB file on my workstation in 0.8 seconds.
The C++ istream version, using -B, takes 3.8 seconds.
Vern
unnamed-faq-76
To: "Frescatore, David (CRD, TAD)" <frescatore@exc01crdge.crd.ge.com>
Subject: Re: FLEX 2.5 & THE YEAR 2000
In-reply-to: Your message of Wed, 03 Jun 1998 11:26:22 PDT.
Date: Wed, 03 Jun 1998 10:22:26 PDT
From: Vern Paxson <vern>
> I am researching the Y2K problem with General Electric R&D
> and need to know if there are any known issues concerning
> the above mentioned software and Y2K regardless of version.
There shouldn’t be, all it ever does with the date is ask the system
for it and then print it out.
Vern
unnamed-faq-77
To: "Hans Dermot Doran" <htd@ibhdoran.com>
Subject: Re: flex problem
In-reply-to: Your message of Wed, 15 Jul 1998 21:30:13 PDT.
Date: Tue, 21 Jul 1998 14:23:34 PDT
From: Vern Paxson <vern>
> To overcome this, I gets() the stdin into a string and lex the string. The
> string is lexed OK except that the end of string isn’t lexed properly
> (yy_scan_string()), that is the lexer dosn’t recognise the end of string.
Flex doesn’t contain mechanisms for recognizing buffer endpoints.
But if
FAQ
106
you use fgets instead (which you should anyway, to protect against buffer
overflows), then the final \n will be preserved in the string, and you can
scan that in order to find the end of the string.
Vern
unnamed-faq-78
To: soumen@almaden.ibm.com
Subject: Re: Flex++ 2.5.3 instance member vs. static member
In-reply-to: Your message of Mon, 27 Jul 1998 02:10:04 PDT.
Date: Tue, 28 Jul 1998 01:10:34 PDT
From: Vern Paxson <vern>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
%{
int mylineno = 0;
%}
ws
[ \t]+
alpha
[A-Za-z]
dig
[0-9]
%%
Now you’d expect mylineno to
yyFlexLexer, but is this the
indicate otherwise; unless I
mylineno seems to be outside
be a member of each instance of class
case? A look at the lex.yy.cc file seems to
am missing something the declaration of
any class scope.
How will this work if I want to run a multi-threaded application with each
thread creating a FlexLexer instance?
Derive your own subclass and make mylineno a member variable of it.
Vern
unnamed-faq-79
To: Adoram Rogel <adoram@hybridge.com>
Subject: Re: More than 32K states change hangs
In-reply-to: Your message of Tue, 04 Aug 1998 16:55:39 PDT.
Date: Tue, 04 Aug 1998 22:28:45 PDT
From: Vern Paxson <vern>
>
>
>
>
>
>
Vern Paxson,
I followed your advice, posted on Usenet bu you, and emailed to me
personally by you, on how to overcome the 32K states limit. I’m running
on Linux machines.
I took the full source of version 2.5.4 and did the following changes in
FAQ
107
>
>
>
>
>
>
>
>
flexdef.h:
#define JAMSTATE -327660
#define MAXIMUM_MNS 319990
#define BAD_SUBSCRIPT -327670
#define MAX_SHORT 327000
and compiled.
All looked fine, including check and bigcheck, so I installed.
Hmmm, you shouldn’t increase MAX_SHORT, though looking through my email
archives I see that I did indeed recommend doing so. Try setting it back
to 32700; that should suffice that you no longer need -Ca. If it still
hangs, then the interesting question is - where?
> Compiling the same hanged program with a out-of-the-box (RedHat 4.2
> distribution of Linux)
> flex 2.5.4 binary works.
Since Linux comes with source code, you should diff it against what
you have to see what problems they missed.
> Should I always compile with the -Ca option now ? even short and simple
> filters ?
No, definitely not. It’s meant to be for those situations where you
absolutely must squeeze every last cycle out of your scanner.
Vern
unnamed-faq-80
To: "Schmackpfeffer, Craig" <Craig.Schmackpfeffer@usa.xerox.com>
Subject: Re: flex output for static code portion
In-reply-to: Your message of Tue, 11 Aug 1998 11:55:30 PDT.
Date: Mon, 17 Aug 1998 23:57:42 PDT
From: Vern Paxson <vern>
> I would like to use flex under the hood to generate a binary file
> containing the data structures that control the parse.
This has been on the wish-list for a long time. In principle it’s
straight-forward - you redirect mkdata() et al’s I/O to another file,
and modify the skeleton to have a start-up function that slurps these
into dynamic arrays. The concerns are (1) the scanner generation code
is hairy and full of corner cases, so it’s easy to get surprised when
going down this path :-( ; and (2) being careful about buffering so
that when the tables change you make sure the scanner starts in the
FAQ
108
correct state and reading at the right point in the input file.
> I was wondering if you know of anyone who has used flex in this way.
I don’t - but it seems like a reasonable project to undertake (unlike
numerous other flex tweaks :-).
Vern
unnamed-faq-81
Received: from 131.173.17.11 (131.173.17.11 [131.173.17.11])
by ee.lbl.gov (8.9.1/8.9.1) with ESMTP id AAA03838
for <vern@ee.lbl.gov>; Thu, 20 Aug 1998 00:47:57 -0700 (PDT)
Received: from hal.cl-ki.uni-osnabrueck.de (hal.cl-ki.Uni-Osnabrueck.DE [131.173.141.2]
by deimos.rz.uni-osnabrueck.de (8.8.7/8.8.8) with ESMTP id JAA34694
for <vern@ee.lbl.gov>; Thu, 20 Aug 1998 09:47:55 +0200
Received: (from georg@localhost) by hal.cl-ki.uni-osnabrueck.de (8.6.12/8.6.12) id JAA3
From: Georg Rehm <georg@hal.cl-ki.uni-osnabrueck.de>
Message-Id: <199808200747.JAA34834@hal.cl-ki.uni-osnabrueck.de>
Subject: "flex scanner push-back overflow"
To: vern@ee.lbl.gov
Date: Thu, 20 Aug 1998 09:47:54 +0200 (MEST)
Reply-To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
X-NoJunk: Do NOT send commercial mail, spam or ads to this address!
X-URL: http://www.cl-ki.uni-osnabrueck.de/~georg/
X-Mailer: ELM [version 2.4ME+ PL28 (25)]
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Hi Vern,
Yesterday, I encountered a strange problem: I use the macro processor m4
to include some lengthy lists into a .l file. Following is a flex macro
definition that causes some serious pain in my neck:
AUTHOR
("A. Boucard / L. Boucard"|"A. Dastarac / M. Levent"|"A.Boucaud / L.Bo
The complete list contains about 10kB. When I try to "flex" this file
(on a Solaris 2.6 machine, using a modified flex 2.5.4 (I only increased
some of the predefined values in flexdefs.h) I get the error:
myflex/flex -8 sentag.tmp.l
flex scanner push-back overflow
When I remove the slashes in the macro definition everything works fine.
FAQ
109
As I understand it, the double quotes escape the slash-character so it
really means "/" and not "trailing context". Furthermore, I tried to
escape the slashes with backslashes, but with no use, the same error message
appeared when flexing the code.
Do you have an idea what’s going on here?
Greetings from Germany,
Georg
-Georg Rehm
georg@cl-ki.uni-osnabrueck.de
Institute for Semantic Information Processing, University of Osnabrueck, FRG
unnamed-faq-82
To: Georg.Rehm@CL-KI.Uni-Osnabrueck.DE
Subject: Re: "flex scanner push-back overflow"
In-reply-to: Your message of Thu, 20 Aug 1998 09:47:54 PDT.
Date: Thu, 20 Aug 1998 07:05:35 PDT
From: Vern Paxson <vern>
> myflex/flex -8 sentag.tmp.l
> flex scanner push-back overflow
Flex itself uses a flex scanner. That scanner is running out of buffer
space when it tries to unput() the humongous macro you’ve defined. When
you remove the ’/’s, you make it small enough so that it fits in the buffer;
removing spaces would do the same thing.
The fix is to either rethink how come you’re using such a big macro and
perhaps there’s another/better way to do it; or to rebuild flex’s own
scan.c with a larger value for
#define YY_BUF_SIZE 16384
- Vern
unnamed-faq-83
To: Jan Kort <jan@research.techforce.nl>
Subject: Re: Flex
In-reply-to: Your message of Fri, 04 Sep 1998 12:18:43 +0200.
Date: Sat, 05 Sep 1998 00:59:49 PDT
From: Vern Paxson <vern>
> %%
>
> "TEST1\n"
{ fprintf(stderr, "TEST1\n"); yyless(5); }
FAQ
110
>
>
>
>
>
>
>
>
>
>
>
^\n
.
\n
{ fprintf(stderr, "empty line\n"); }
{ }
{ fprintf(stderr, "new line\n"); }
%%
-- input --------------------------------------TEST1
-- output -------------------------------------TEST1
empty line
------------------------------------------------
IMHO, it’s not clear whether or not this is in fact a bug. It depends
on whether you view yyless() as backing up in the input stream, or as
pushing new characters onto the beginning of the input stream. Flex
interprets it as the latter (for implementation convenience, I’ll admit),
and so considers the newline as in fact matching at the beginning of a
line, as after all the last token scanned an entire line and so the
scanner is now at the beginning of a new line.
I agree that this is counter-intuitive for yyless(), given its
functional description (it’s less so for unput(), depending on whether
you’re unput()’ing new text or scanned text). But I don’t plan to
change it any time soon, as it’s a pain to do so. Consequently,
you do indeed need to use yy_set_bol() and YY_AT_BOL() to tweak
your scanner into the behavior you desire.
Sorry for the less-than-completely-satisfactory answer.
Vern
unnamed-faq-84
To: Patrick Krusenotto <krusenot@mac-info-link.de>
Subject: Re: Problems with restarting flex-2.5.2-generated scanner
In-reply-to: Your message of Thu, 24 Sep 1998 10:14:07 PDT.
Date: Thu, 24 Sep 1998 23:28:43 PDT
From: Vern Paxson <vern>
>
>
>
>
>
I am using flex-2.5.2 and bison 1.25 for Solaris and I am desperately
trying to make my scanner restart with a new file after my parser stops
with a parse error. When my compiler restarts, the parser always
receives the token after the token (in the old file!) that caused the
parser error.
I suspect the problem is that your parser has read ahead in order
to attempt to resolve an ambiguity, and when it’s restarted it picks
FAQ
111
up with that token rather than reading a fresh one. If you’re using
yacc, then the special "error" production can sometimes be used to
consume tokens in an attempt to get the parser into a consistent state.
Vern
unnamed-faq-85
To: Henric Jungheim <junghelh@pe-nelson.com>
Subject: Re: flex 2.5.4a
In-reply-to: Your message of Tue, 27 Oct 1998 16:41:42 PST.
Date: Tue, 27 Oct 1998 16:50:14 PST
From: Vern Paxson <vern>
>
>
>
>
This brings up a feature request: How about a command line
option to specify the filename when reading from stdin? That way one
doesn’t need to create a temporary file in order to get the "#line"
directives to make sense.
Use -o combined with -t (per the man page description of -o).
> P.S., Is there any simple way to use non-blocking IO to parse multiple
> streams?
Simple, no.
One approach might be to return a magic character on EWOULDBLOCK and
have a rule
.*<magic-character>
// put back .*, eat magic character
This is off the top of my head, not sure it’ll work.
Vern
unnamed-faq-86
To: "Repko, Billy D" <billy.d.repko@intel.com>
Subject: Re: Compiling scanners
In-reply-to: Your message of Wed, 13 Jan 1999 10:52:47 PST.
Date: Thu, 14 Jan 1999 00:25:30 PST
From: Vern Paxson <vern>
> It appears that maybe it cannot find the lfl library.
The Makefile in the distribution builds it, so you should have it.
It’s exceedingly trivial, just a main() that calls yylex() and
a yyrap() that always returns 1.
FAQ
112
> %%
>
>
\n
.
++num_lines; ++num_chars;
++num_chars;
You can’t indent your rules like this - that’s where the errors are coming
from. Flex copies indented text to the output file, it’s how you do things
like
int num_lines_seen = 0;
to declare local variables.
Vern
unnamed-faq-87
To: Erick Branderhorst <Erick.Branderhorst@asml.nl>
Subject: Re: flex input buffer
In-reply-to: Your message of Tue, 09 Feb 1999 13:53:46 PST.
Date: Tue, 09 Feb 1999 21:03:37 PST
From: Vern Paxson <vern>
> In the flex.skl file the size of the default input buffers is set.
> explain why this size is set and why it is such a high number.
It’s large to optimize performance when scanning large files.
safely make it a lot lower if needed.
Can you
You can
Vern
unnamed-faq-88
To: "Guido Minnen" <guidomi@cogs.susx.ac.uk>
Subject: Re: Flex error message
In-reply-to: Your message of Wed, 24 Feb 1999 15:31:46 PST.
Date: Thu, 25 Feb 1999 00:11:31 PST
From: Vern Paxson <vern>
> I’m extending a larger scanner written in Flex and I keep running into
> problems. More specifically, I get the error message:
> "flex: input rules are too complicated (>= 32000 NFA states)"
Increase the definitions in flexdef.h for:
#define JAMSTATE -32766 /* marks a reference to the state that always j
ams */
#define MAXIMUM_MNS 31999
FAQ
113
#define BAD_SUBSCRIPT -32767
recompile everything, and it should all work.
Vern
unnamed-faq-90
To: "Dmitriy Goldobin" <gold@ems.chel.su>
Subject: Re: FLEX trouble
In-reply-to: Your message of Mon, 31 May 1999 18:44:49 PDT.
Date: Tue, 01 Jun 1999 00:15:07 PDT
From: Vern Paxson <vern>
>
I have a trouble with FLEX. Why rule "/*".*"*/" work properly,=20
> but rule "/*"(.|\n)*"*/" don’t work ?
The second of these will have to scan the entire input stream (because
"(.|\n)*" matches an arbitrary amount of any text) in order to see if
it ends with "*/", terminating the comment. That potentially will overflow
the input buffer.
>
More complex rule "/*"([^*]|(\*/[^/]))*"*/ give an error
> ’unrecognized rule’.
You can’t use the ’/’ operator inside parentheses.
what "(a/b)*" actually means.
It’s not clear
>
I now use workaround with state <comment>, but single-rule is
> better, i think.
Single-rule is nice but will always have the problem of either setting
restrictions on comments (like not allowing multi-line comments) and/or
running the risk of consuming the entire input stream, as noted above.
Vern
unnamed-faq-91
Received: from mc-qout4.whowhere.com (mc-qout4.whowhere.com [209.185.123.18])
by ee.lbl.gov (8.9.3/8.9.3) with SMTP id IAA05100
for <vern@ee.lbl.gov>; Tue, 15 Jun 1999 08:56:06 -0700 (PDT)
Received: from Unknown/Local ([?.?.?.?]) by my-deja.com; Tue Jun 15 08:55:43 1999
To: vern@ee.lbl.gov
Date: Tue, 15 Jun 1999 08:55:43 -0700
From: "Aki Niimura" <neko@my-deja.com>
Message-ID: <KNONDOHDOBGAEAAA@my-deja.com>
Mime-Version: 1.0
FAQ
114
Cc:
X-Sent-Mail: on
Reply-To:
X-Mailer: MailCity Service
Subject: A question on flex C++ scanner
X-Sender-Ip: 12.72.207.61
Organization: My Deja Email (http://www.my-deja.com:80)
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Dear Dr. Paxon,
I have been using flex for years.
It works very well on many projects.
Most case, I used it to generate a scanner on C language.
However, one project I needed to generate a scanner
on C++ lanuage. Thanks to your enhancement, flex did
the job.
Currently, I’m working on enhancing my previous project.
I need to deal with multiple input streams (recursive
inclusion) in this scanner (C++).
I did similar thing for another scanner (C) as you
explained in your documentation.
The generated scanner (C++) has necessary methods:
- switch_to_buffer(struct yy_buffer_state *b)
- yy_create_buffer(istream *is, int sz)
- yy_delete_buffer(struct yy_buffer_state *b)
However, I couldn’t figure out how to access current
buffer (yy_current_buffer).
yy_current_buffer is a protected member of yyFlexLexer.
I can’t access it directly.
Then, I thought yy_create_buffer() with is = 0 might
return current stream buffer. But it seems not as far
as I checked the source. (flex 2.5.4)
I went through the Web in addition to Flex documentation.
However, it hasn’t been successful, so far.
It is not my intention to bother you, but, can you
comment about how to obtain the current stream buffer?
Your response would be highly appreciated.
FAQ
115
Best regards,
Aki Niimura
--== Sent via Deja.com http://www.deja.com/ ==-Share what you know. Learn what you don’t.
unnamed-faq-92
To: neko@my-deja.com
Subject: Re: A question on flex C++ scanner
In-reply-to: Your message of Tue, 15 Jun 1999 08:55:43 PDT.
Date: Tue, 15 Jun 1999 09:04:24 PDT
From: Vern Paxson <vern>
> However, I couldn’t figure out how to access current
> buffer (yy_current_buffer).
Derive your own subclass from yyFlexLexer.
Vern
unnamed-faq-93
To: "Stones, Darren" <Darren.Stones@nectech.co.uk>
Subject: Re: You’re the man to see?
In-reply-to: Your message of Wed, 23 Jun 1999 11:10:29 PDT.
Date: Wed, 23 Jun 1999 09:01:40 PDT
From: Vern Paxson <vern>
>
>
>
>
>
I hope you can help me. I am using Flex and Bison to produce an interpreted
language. However all goes well until I try to implement an IF statement or
a WHILE. I cannot get this to work as the parser parses all the conditions
eg. the TRUE and FALSE conditons to check for a rule match. So I cannot
make a decision!!
You need to use the parser to build a parse tree (= abstract syntax trwee),
and when that’s all done you recursively evaluate the tree, binding variables
to values at that time.
Vern
unnamed-faq-94
To: Petr Danecek <petr@ics.cas.cz>
Subject: Re: flex - question
In-reply-to: Your message of Mon, 28 Jun 1999 19:21:41 PDT.
Date: Fri, 02 Jul 1999 16:52:13 PDT
From: Vern Paxson <vern>
FAQ
116
> file, it takes an enormous amount of time. It is funny, because the
> source code has only 12 rules!!! I think it looks like an exponencial
> growth.
Right, that’s the problem - some patterns (those with a lot of
ambiguity, where yours has because at any given time the scanner can
be in the middle of all sorts of combinations of the different
rules) blow up exponentially.
For your rules, there is an easy fix. Change the ".*" that comes fater
the directory name to "[^ ]*". With that in place, the rules are no
longer nearly so ambiguous, because then once one of the directories
has been matched, no other can be matched (since they all require a
leading blank).
If that’s not an acceptable solution, then you can enter a start state
to pick up the .*\n after each directory is matched.
Also note that for speed, you’ll want to add a ".*" rule at the end,
otherwise rules that don’t match any of the patterns will be matched
very slowly, a character at a time.
Vern
unnamed-faq-95
To: Tielman Koekemoer <tielman@spi.co.za>
Subject: Re: Please help.
In-reply-to: Your message of Thu, 08 Jul 1999 13:20:37 PDT.
Date: Thu, 08 Jul 1999 08:20:39 PDT
From: Vern Paxson <vern>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
I was hoping you could help me with my problem.
I tried compiling (gnu)flex on a Solaris 2.4 machine
but when I ran make (after configure) I got an error.
-------------------------------------------------------------gcc -c -I. -I. -g -O parse.c
./flex -t -p ./scan.l >scan.c
sh: ./flex: not found
*** Error code 1
make: Fatal error: Command failed for target ‘scan.c’
------------------------------------------------------------What’s strange to me is that I’m only
FAQ
117
>
>
>
>
>
>
>
trying to install flex now. I then edited the Makefile to
and changed where it says "FLEX = flex" to "FLEX = lex"
( lex: the native Solaris one ) but then it complains about
the "-p" option. Is there any way I can compile flex without
using flex or lex?
Thanks so much for your time.
You managed to step on the bootstrap sequence, which first copies
initscan.c to scan.c in order to build flex. Try fetching a fresh
distribution from ftp.ee.lbl.gov. (Or you can first try removing
".bootstrap" and doing a make again.)
Vern
unnamed-faq-96
To: Tielman Koekemoer <tielman@spi.co.za>
Subject: Re: Please help.
In-reply-to: Your message of Fri, 09 Jul 1999 09:16:14 PDT.
Date: Fri, 09 Jul 1999 00:27:20 PDT
From: Vern Paxson <vern>
> First I removed .bootstrap (and ran make) - no luck. I downloaded the
> software but I still have the same problem. Is there anything else I
> could try.
Try:
cp initscan.c scan.c
touch scan.c
make scan.o
If this last tries to first build scan.c from scan.l using ./flex, then
your "make" is broken, in which case compile scan.c to scan.o by hand.
Vern
unnamed-faq-97
To: Sumanth Kamenani <skamenan@crl.nmsu.edu>
Subject: Re: Error
In-reply-to: Your message of Mon, 19 Jul 1999 23:08:41 PDT.
Date: Tue, 20 Jul 1999 00:18:26 PDT
From: Vern Paxson <vern>
> I am getting a compilation error. The error is given as "unknown symbol- yylex".
FAQ
118
The parser relies on calling yylex(), but you’re instead using the C++ scanning
class, so you need to supply a yylex() "glue" function that calls an instance
scanner of the scanner (e.g., "scanner->yylex()").
Vern
unnamed-faq-98
To: daniel@synchrods.synchrods.COM (Daniel Senderowicz)
Subject: Re: lex
In-reply-to: Your message of Mon, 22 Nov 1999 11:19:04 PST.
Date: Tue, 23 Nov 1999 15:54:30 PST
From: Vern Paxson <vern>
Well, your problem is the
switch (yybgin-yysvec-1) {
/* witchcraft */
at the beginning of lex rules. "witchcraft" == "non-portable".
assuming knowledge of the AT&T lex’s internal variables.
It’s
For flex, you can probably do the equivalent using a switch on YYSTATE.
Vern
unnamed-faq-99
To: archow@hss.hns.com
Subject: Re: Regarding distribution of flex and yacc based grammars
In-reply-to: Your message of Sun, 19 Dec 1999 17:50:24 +0530.
Date: Wed, 22 Dec 1999 01:56:24 PST
From: Vern Paxson <vern>
>
>
>
>
When we provide the customer with an object code distribution, is it
necessary for us to provide source
for the generated C files from flex and bison since they are generated by
flex and bison ?
For flex, no.
I don’t know what the current state of this is for bison.
> Also, is there any requrirement for us to neccessarily
> the grammar files which are fed into flex and bison ?
provide source for
Again, for flex, no.
See the file "COPYING" in the flex distribution for the legalese.
Vern
FAQ
119
unnamed-faq-100
To: Martin Gallwey <gallweym@hyperion.moe.ul.ie>
Subject: Re: Flex, and self referencing rules
In-reply-to: Your message of Sun, 20 Feb 2000 01:01:21 PST.
Date: Sat, 19 Feb 2000 18:33:16 PST
From: Vern Paxson <vern>
> However, I do not use unput anywhere. I do use self-referencing
> rules like this:
>
> UnaryExpr
({UnionExpr})|("-"{UnaryExpr})
You can’t do this - flex is *not* a parser like yacc (which does indeed
allow recursion), it is a scanner that’s confined to regular expressions.
Vern
unnamed-faq-101
To: slg3@lehigh.edu (SAMUEL L. GULDEN)
Subject: Re: Flex problem
In-reply-to: Your message of Thu, 02 Mar 2000 12:29:04 PST.
Date: Thu, 02 Mar 2000 23:00:46 PST
From: Vern Paxson <vern>
If this is exactly your program:
>
>
>
>
>
>
>
>
>
>
>
digit [0-9]
digits {digit}+
whitespace [ \t\n]+
%%
"[" { printf("open_brac\n");}
"]" { printf("close_brac\n");}
"+" { printf("addop\n");}
"*" { printf("multop\n");}
{digits} { printf("NUMBER = %s\n", yytext);}
whitespace ;
then the problem is that the last rule needs to be "{whitespace}" !
Vern
FAQ
120
What is the difference between YYLEX PARAM and
YY DECL?
YYLEX PARAM is not a flex symbol. It is for Bison. It tells Bison to pass extra params
when it calls yylex() from the parser.
YY DECL is the Flex declaration of yylex. The default is similar to this:
#define int yy_lex ()
Why do I get "conflicting types for yylex" error?
This is a compiler error regarding a generated Bison parser, not a Flex scanner. It means
you need a prototype of yylex() in the top of the Bison file. Be sure the prototype matches
YY DECL.
How do I access the values set in a Flex action from within
a Bison action?
With $1, $2, $3, etc. These are called "Semantic Values" in the Bison manual. See Section
“Top” in the GNU Bison Manual.
Appendix A: Appendices
121
Appendix A Appendices
A.1 Makefiles and Flex
In this appendix, we provide tips for writing Makefiles to build your scanners.
In a traditional build environment, we say that the ‘.c’ files are the sources, and the ‘.o’
files are the intermediate files. When using flex, however, the ‘.l’ files are the sources, and
the generated ‘.c’ files (along with the ‘.o’ files) are the intermediate files. This requires
you to carefully plan your Makefile.
Modern make programs understand that ‘foo.l’ is intended to generate ‘lex.yy.c’ or
‘foo.c’, and will behave accordingly12 . The following Makefile does not explicitly instruct
make how to build ‘foo.c’ from ‘foo.l’. Instead, it relies on the implicit rules of the make
program to build the intermediate file, ‘scan.c’:
# Basic Makefile -- relies on implicit rules
# Creates "myprogram" from "scan.l" and "myprogram.c"
#
LEX=flex
myprogram: scan.o myprogram.o
scan.o: scan.l
For simple cases, the above may be sufficient. For other cases, you may have to explicitly instruct make how to build your scanner. The following is an example of a Makefile
containing explicit rules:
# Basic Makefile -- provides explicit rules
# Creates "myprogram" from "scan.l" and "myprogram.c"
#
LEX=flex
myprogram: scan.o myprogram.o
$(CC) -o $@ $(LDFLAGS) $^
myprogram.o: myprogram.c
$(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^
scan.o: scan.c
$(CC) $(CPPFLAGS) $(CFLAGS) -o $@ -c $^
scan.c: scan.l
$(LEX) $(LFLAGS) -o $@ $^
clean:
$(RM) *.o scan.c
1
2
GNU make and GNU automake are two such programs that provide implicit rules for flex-generated
scanners.
GNU automake may generate code to execute flex in lex-compatible mode, or to stdout. If this is not
what you want, then you should provide an explicit rule in your Makefile.am
Appendix A: Appendices
122
Notice in the above example that ‘scan.c’ is in the clean target. This is because we
consider the file ‘scan.c’ to be an intermediate file.
Finally, we provide a realistic example of a flex scanner used with a bison parser3 .
There is a tricky problem we have to deal with. Since a flex scanner will typically include
a header file (e.g., ‘y.tab.h’) generated by the parser, we need to be sure that the header
file is generated BEFORE the scanner is compiled. We handle this case in the following
example:
# Makefile example -- scanner and parser.
# Creates "myprogram" from "scan.l", "parse.y", and "myprogram.c"
#
LEX
= flex
YACC
= bison -y
YFLAGS = -d
objects = scan.o parse.o myprogram.o
myprogram: $(objects)
scan.o: scan.l parse.c
parse.o: parse.y
myprogram.o: myprogram.c
In the above example, notice the line,
scan.o: scan.l parse.c
, which lists the file ‘parse.c’ (the generated parser) as a dependency of ‘scan.o’. We
want to ensure that the parser is created before the scanner is compiled, and the above line
seems to do the trick. Feel free to experiment with your specific implementation of make.
For more details on writing Makefiles, see Section “Top” in The GNU Make Manual.
A.2 C Scanners with Bison Parsers
This section describes the flex features useful when integrating flex with GNU bison4 .
Skip this section if you are not using bison with your scanner. Here we discuss only the
flex half of the flex and bison pair. We do not discuss bison in any detail. For more
information about generating bison parsers, see Section “Top” in the GNU Bison Manual.
A compatible bison scanner is generated by declaring ‘%option bison-bridge’ or by
supplying ‘--bison-bridge’ when invoking flex from the command line. This instructs
flex that the macro yylval may be used. The data type for yylval, YYSTYPE, is typically
defined in a header file, included in section 1 of the flex input file. For a list of functions
and macros available, See [bison-functions], page 60.
The declaration of yylex becomes,
int yylex ( YYSTYPE * lvalp, yyscan_t scanner );
If %option bison-locations is specified, then the declaration becomes,
3
4
This example also applies to yacc parsers.
The features described here are purely optional, and are by no means the only way to use flex with bison.
We merely provide some glue to ease development of your parser-scanner pair.
Appendix A: Appendices
123
int yylex ( YYSTYPE * lvalp, YYLTYPE * llocp, yyscan_t scanner );
Note that the macros yylval and yylloc evaluate to pointers. Support for yylloc is
optional in bison, so it is optional in flex as well. The following is an example of a flex
scanner that is compatible with bison.
/* Scanner for "C" assignment statements... sort of. */
%{
#include "y.tab.h" /* Generated by bison. */
%}
%option bison-bridge bison-locations
%
[[:digit:]]+
[[:alnum:]]+
"="|";"
. {}
%
{ yylval->num = atoi(yytext);
return NUMBER;}
{ yylval->str = strdup(yytext); return STRING;}
{ return yytext[0];}
As you can see, there really is no magic here. We just use yylval as we would any other
variable. The data type of yylval is generated by bison, and included in the file ‘y.tab.h’.
Here is the corresponding bison parser:
/* Parser to convert "C" assignments to lisp. */
%{
/* Pass the argument to yyparse through to yylex. */
#define YYPARSE_PARAM scanner
#define YYLEX_PARAM
scanner
%}
%locations
%pure_parser
%union {
int num;
char* str;
}
%token <str> STRING
%token <num> NUMBER
%%
assignment:
STRING ’=’ NUMBER ’;’ {
printf( "(setf %s %d)", $1, $3 );
}
;
Appendix A: Appendices
124
A.3 M4 Dependency
The macro processor m45 must be installed wherever flex is installed. flex invokes ‘m4’,
found by searching the directories in the PATH environment variable. Any code you place
in section 1 or in the actions will be sent through m4. Please follow these rules to protect
your code from unwanted m4 processing.
• Do not use symbols that begin with, ‘m4_’, such as, ‘m4_define’, or ‘m4_include’, since
those are reserved for m4 macro names. If for some reason you need m4 as a prefix,
use a preprocessor #define to get your symbol past m4 unmangled.
• Do not use the strings ‘[[’ or ‘]]’ anywhere in your code. The former is not valid in C,
except within comments and strings, but the latter is valid in code such as x[y[z]].
The solution is simple. To get the literal string "]]", use "]""]". To get the array
notation x[y[z]], use x[y[z] ]. Flex will attempt to detect these sequences in user
code, and escape them. However, it’s best to avoid this complexity where possible, by
removing such sequences from your code.
m4 is only required at the time you run flex. The generated scanner is ordinary C or
C++, and does not require m4.
A.4 Common Patterns
This appendix provides examples of common regular expressions you might use in your
scanner.
A.4.1 Numbers
C99 decimal constant
([[:digit:]]{-}[0])[[:digit:]]*
C99 hexadecimal constant
0[xX][[:xdigit:]]+
C99 octal constant
0[01234567]*
C99 floating point constant
{dseq}
([[:digit:]]+)
{dseq_opt} ([[:digit:]]*)
{frac}
(({dseq_opt}"."{dseq})|{dseq}".")
{exp}
([eE][+-]?{dseq})
{exp_opt}
({exp}?)
{fsuff}
[flFL]
{fsuff_opt} ({fsuff}?)
{hpref}
(0[xX])
{hdseq}
([[:xdigit:]]+)
{hdseq_opt} ([[:xdigit:]]*)
{hfrac}
(({hdseq_opt}"."{hdseq})|({hdseq}"."))
{bexp}
([pP][+-]?{dseq})
5
The use of m4 is subject to change in future revisions of flex. It is not part of the public API of flex. Do
not depend on it.
Appendix A: Appendices
125
{dfc}
{hfc}
(({frac}{exp_opt}{fsuff_opt})|({dseq}{exp}{fsuff_opt}))
(({hpref}{hfrac}{bexp}{fsuff_opt})|({hpref}{hdseq}{bexp}{fsuff_opt}))
{c99_floating_point_constant}
({dfc}|{hfc})
See C99 section 6.4.4.2 for the gory details.
A.4.2 Identifiers
C99 Identifier
ucn
nondigit
c99_id
((\\u([[:xdigit:]]{4}))|(\\U([[:xdigit:]]{8})))
[_[:alpha:]]
([_[:alpha:]]|{ucn})([_[:alnum:]]|{ucn})*
Technically, the above pattern does not encompass all possible C99 identifiers,
since C99 allows for "implementation-defined" characters. In practice, C compilers follow the above pattern, with the addition of the ‘$’ character.
UTF-8 Encoded Unicode Code Point
[\x09\x0A\x0D\x20-\x7E]|[\xC2-\xDF][\x80-\xBF]|\xE0[\xA0-\xBF][\x80-\xBF]|[\xE1-\x
A.4.3 Quoted Constructs
C99 String Literal
L?\"([^\"\\\n]|(\\[’\"?\\abfnrtv])|(\\([0123456]{1,3}))|(\\x[[:xdigit:]]+)|(\\u([[
C99 Comment
("/*"([^*]|"*"[^/])*"*/")|("/"(\\\n)*"/"[^\n]*)
Note that in C99, a ‘//’-style comment may be split across lines, and, contrary
to popular belief, does not include the trailing ‘\n’ character.
A better way to scan ‘/* */’ comments is by line, rather than matching possibly
huge comments all at once. This will allow you to scan comments of unlimited
length, as long as line breaks appear at sane intervals. This is also more efficient when used with automatic line number processing. See [option-yylineno],
page 38.
<INITIAL>{
"/*"
}
<COMMENT>{
"*/"
[^*\n]+
"*"[^/]
\n
}
BEGIN(COMMENT);
BEGIN(0);
;
;
;
A.4.4 Addresses
IPv4 Address
dec-octet
IPv4address
[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]
{dec-octet}\.{dec-octet}\.{dec-octet}\.{dec-octet}
Appendix A: Appendices
126
IPv6 Address
h16
ls32
IPv6address
[0-9A-Fa-f]{1,4}
{h16}:{h16}|{IPv4address}
({h16}:){6}{ls32}|
::({h16}:){5}{ls32}|
({h16})?::({h16}:){4}{ls32}|
(({h16}:){0,1}{h16})?::({h16}:){3}{ls32}|
(({h16}:){0,2}{h16})?::({h16}:){2}{ls32}|
(({h16}:){0,3}{h16})?::{h16}:{ls32}|
(({h16}:){0,4}{h16})?::{ls32}|
(({h16}:){0,5}{h16})?::{h16}|
(({h16}:){0,6}{h16})?::
See RFC 2373 for details. Note that you have to fold the definition of
IPv6address into one line and that it also matches the “unspecified address”
“::”.
URI
(([^:/?#]+):)?("//"([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
This pattern is nearly useless, since it allows just about any character to appear
in a URI, including spaces and control characters. See RFC 2396 for details.
Indices
127
Indices
Concept Index
$
$ as normal character in patterns . . . . . . . . . . . . . . 12
%
%{ and %}, in Definitions Section. . . . . . . . . . . . . . . 6
%{ and %}, in Rules Section . . . . . . . . . . . . . . . . . . . 15
%array, advantages of. . . . . . . . . . . . . . . . . . . . . . . . . . 14
%array, use of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
%array, with C++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
%option noyywrapp . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
%pointer, and unput() . . . . . . . . . . . . . . . . . . . . . . . . . 17
%pointer, use of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
%top . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
<
<<EOF>>, use of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
[
[] in patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
^
^ as non-special character in patterns . . . . . . . . . . 12
|
|, in actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
|, use of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
A
accessor functions, use of . . . . . . . . . . . . . . . . . . . . . . 58
actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
actions, embedded C strings . . . . . . . . . . . . . . . . . . . 15
actions, redefining YY BREAK . . . . . . . . . . . . . . . . 32
actions, use of { and } . . . . . . . . . . . . . . . . . . . . . . . . . 15
aliases, how to define . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
arguments, command-line . . . . . . . . . . . . . . . . . . . . . . 35
array, default size for yytext . . . . . . . . . . . . . . . . . . . 33
BEGIN, explanation . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
beginning of line, in patterns. . . . . . . . . . . . . . . . . . . 10
bison, bridging with flex . . . . . . . . . . . . . . . . . . . . . . 122
bison, parser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
bison, scanner to be called from bison . . . . . . . . . 123
BOL, checking the BOL flag . . . . . . . . . . . . . . . . . . . 32
BOL, in patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
BOL, setting it . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
braces in patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
bugs, reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
C
C code in flex input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
C++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
C++ and %array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
C++ I/O, customizing . . . . . . . . . . . . . . . . . . . . . . . . . . 85
C++ scanners, including multiple scanners . . . . . . 53
C++ scanners, use of . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
c++, experimental form of scanner class . . . . . . . . 50
C++, multiple different scanners . . . . . . . . . . . . . . . . 53
C-strings, in actions . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
case-insensitive, effect on character classes . . . . . 12
character classes in patterns . . . . . . . . . . . . . . . . . . . 11
character classes in patterns, syntax of . . . . . . . . . . 9
character classes, equivalence of . . . . . . . . . . . . . . . . 11
clearing an input buffer . . . . . . . . . . . . . . . . . . . . . . . . 27
command-line options . . . . . . . . . . . . . . . . . . . . . . . . . 35
comments in flex input. . . . . . . . . . . . . . . . . . . . . . . . . . 6
comments in the input . . . . . . . . . . . . . . . . . . . . . . . . . . 7
comments, discarding . . . . . . . . . . . . . . . . . . . . . . . . . . 17
comments, example of scanning C comments . . . 23
comments, in actions . . . . . . . . . . . . . . . . . . . . . . . . . . 15
comments, in rules section . . . . . . . . . . . . . . . . . . . . . . 7
comments, syntax of . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
comments, valid uses of . . . . . . . . . . . . . . . . . . . . . . . . . 7
compressing whitespace . . . . . . . . . . . . . . . . . . . . . . . . 15
concatenation, in patterns . . . . . . . . . . . . . . . . . . . . . 10
copyright of flex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
counting characters and lines. . . . . . . . . . . . . . . . . . . . 4
customizing I/O in C++ scanners . . . . . . . . . . . . . . . 85
D
B
backing up, eliminating . . . . . . . . . . . . . . . . . . . . . . . .
backing up, eliminating by adding error rules. . .
backing up, eliminating with catch-all rule . . . . .
backing up, example of eliminating. . . . . . . . . . . . .
BEGIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
45
46
46
45
15
default rule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 14
defining pattern aliases . . . . . . . . . . . . . . . . . . . . . . . . . 6
Definitions, in flex input . . . . . . . . . . . . . . . . . . . . . . . . 6
deleting lines from input . . . . . . . . . . . . . . . . . . . . . . . 15
discarding C comments . . . . . . . . . . . . . . . . . . . . . . . . 17
distributing flex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Indices
128
E
K
ECHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
ECHO, and yyout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
embedding C code in flex input . . . . . . . . . . . . . . . . . 6
end of file, in patterns . . . . . . . . . . . . . . . . . . . . . . . . . 11
end of line, in negated character classes . . . . . . . . 12
end of line, in patterns . . . . . . . . . . . . . . . . . . . . . . . . . 10
end-of-file, and yyrestart() . . . . . . . . . . . . . . . . . . . . . 19
EOF and yyrestart(). . . . . . . . . . . . . . . . . . . . . . . . . . . 19
EOF in patterns, syntax of . . . . . . . . . . . . . . . . . . . . 11
EOF, example using multiple input buffers . . . . . 28
EOF, explanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
EOF, pushing back . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
EOL, in negated character classes . . . . . . . . . . . . . . 12
EOL, in patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
error messages, end of buffer missed . . . . . . . . . . . . 61
error reporting, diagnostic messages . . . . . . . . . . . . 72
error reporting, in C++. . . . . . . . . . . . . . . . . . . . . . . . . 51
error rules, to eliminate backing up . . . . . . . . . . . . 46
escape sequences in patterns, syntax of . . . . . . . . . . 9
exiting with yyterminate() . . . . . . . . . . . . . . . . . . . . . 18
experimental form of c++ scanner class . . . . . . . . . 50
extended scope of start conditions . . . . . . . . . . . . . 25
keywords, for performance . . . . . . . . . . . . . . . . . . . . . 48
F
file format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
file format, serialized tables . . . . . . . . . . . . . . . . . . . . 68
flushing an input buffer . . . . . . . . . . . . . . . . . . . . . . . . 27
flushing the internal buffer . . . . . . . . . . . . . . . . . . . . . 18
format of flex input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
format of input file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
freeing tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
L
lex (traditional) and POSIX . . . . . . . . . . . . . . . . . . . 61
LexerInput, overriding . . . . . . . . . . . . . . . . . . . . . . . . . 85
LexerOutput, overriding . . . . . . . . . . . . . . . . . . . . . . . 85
limitations of flex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
literal text in patterns, syntax of . . . . . . . . . . . . . . . . 9
loading tables at runtime . . . . . . . . . . . . . . . . . . . . . . 67
M
m4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
Makefile, example of implicit rules . . . . . . . . . . . . 121
Makefile, explicit example . . . . . . . . . . . . . . . . . . . . 121
Makefile, syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
matching C-style double-quoted strings . . . . . . . . 24
matching, and trailing context . . . . . . . . . . . . . . . . . 14
matching, length of . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
matching, multiple matches . . . . . . . . . . . . . . . . . . . . 14
member functions, C++ . . . . . . . . . . . . . . . . . . . . . . . . 50
memory management . . . . . . . . . . . . . . . . . . . . . . . . . . 64
memory, allocating input buffers . . . . . . . . . . . . . . . 27
memory, considerations for reentrant scanners . . 56
memory, deleting input buffers . . . . . . . . . . . . . . . . . 27
memory, for start condition stacks . . . . . . . . . . . . . 26
memory, serialized tables . . . . . . . . . . . . . . . . . . . . . . 67
methods, c++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
minimal scanner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
multiple input streams . . . . . . . . . . . . . . . . . . . . . . . . . 27
N
G
getting current start state with YY START . . . . 24
H
halting with yyterminate() . . . . . . . . . . . . . . . . . . . . . 18
handling include files with multiple input buffers
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
header files, with C++ . . . . . . . . . . . . . . . . . . . . . . . . . . 53
I
include files, with C++ . . . . . . . . . . . . . . . . . . . . . . . . . 53
input file, Definitions section . . . . . . . . . . . . . . . . . . . . 6
input file, Rules Section . . . . . . . . . . . . . . . . . . . . . . . . . 7
input file, user code Section . . . . . . . . . . . . . . . . . . . . . 7
input() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
input(), and C++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
input, format of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
input, matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
name definitions, not POSIX. . . . . . . . . . . . . . . . . . . 62
negating ranges in patterns . . . . . . . . . . . . . . . . . . . . . 9
newline, matching in patterns . . . . . . . . . . . . . . . . . . 10
non-POSIX features of flex . . . . . . . . . . . . . . . . . . . . 63
noyywrap, %option . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
NULL character in patterns, syntax of . . . . . . . . . . 9
O
octal characters in patterns . . . . . . . . . . . . . . . . . . . . . 9
options, command-line . . . . . . . . . . . . . . . . . . . . . . . . . 35
overriding LexerInput . . . . . . . . . . . . . . . . . . . . . . . . . . 85
overriding LexerOutput . . . . . . . . . . . . . . . . . . . . . . . . 85
overriding the memory routines . . . . . . . . . . . . . . . . 65
P
Pascal-like language . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pattern aliases, defining . . . . . . . . . . . . . . . . . . . . . . . . .
pattern aliases, expansion of . . . . . . . . . . . . . . . . . . . .
pattern aliases, how to define . . . . . . . . . . . . . . . . . . .
pattern aliases, use of . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
6
9
6
6
Indices
patterns and actions on different lines . . . . . . . . . . 62
patterns, character class equivalence . . . . . . . . . . . 11
patterns, common . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
patterns, end of line . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
patterns, grouping and precedence . . . . . . . . . . . . . 11
patterns, in rules section . . . . . . . . . . . . . . . . . . . . . . . . 9
patterns, invalid trailing context . . . . . . . . . . . . . . . 13
patterns, matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
patterns, precedence of operators . . . . . . . . . . . . . . 11
patterns, repetitions with grouping . . . . . . . . . . . . . 11
patterns, special characters treated as non-special
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
patterns, syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
patterns, tuning for performance . . . . . . . . . . . . . . . 45
patterns, valid character classes . . . . . . . . . . . . . . . . 11
performance optimization, matching longer tokens
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
performance optimization, recognizing keywords
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
performance, backing up . . . . . . . . . . . . . . . . . . . . . . . 45
performance, considerations . . . . . . . . . . . . . . . . . . . . 45
performance, using keywords . . . . . . . . . . . . . . . . . . . 48
popping an input buffer. . . . . . . . . . . . . . . . . . . . . . . . 27
POSIX and lex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
POSIX comp;compliance. . . . . . . . . . . . . . . . . . . . . . . 63
POSIX, character classes in patterns, syntax of . . 9
preprocessor macros, for use in actions . . . . . . . . . 15
pushing an input buffer . . . . . . . . . . . . . . . . . . . . . . . . 27
pushing back characters with unput . . . . . . . . . . . . 17
pushing back characters with unput() . . . . . . . . . . 17
pushing back characters with yyless . . . . . . . . . . . . 16
pushing back EOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
R
ranges in patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
ranges in patterns, negating . . . . . . . . . . . . . . . . . . . . . 9
recognizing C comments . . . . . . . . . . . . . . . . . . . . . . . 23
reentrant scanners, multiple interleaved scanners
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
reentrant scanners, recursive invocation . . . . . . . . 54
reentrant, accessing flex variables . . . . . . . . . . . . . . 56
reentrant, accessor functions . . . . . . . . . . . . . . . . . . . 57
reentrant, API explanation . . . . . . . . . . . . . . . . . . . . 55
reentrant, calling functions . . . . . . . . . . . . . . . . . . . . . 56
reentrant, example of . . . . . . . . . . . . . . . . . . . . . . . . . . 55
reentrant, explanation . . . . . . . . . . . . . . . . . . . . . . . . . 54
reentrant, extra data . . . . . . . . . . . . . . . . . . . . . . . . . . 58
reentrant, initialization . . . . . . . . . . . . . . . . . . . . . . . . 56
regular expressions, in patterns . . . . . . . . . . . . . . . . . 9
REJECT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
REJECT, calling multiple times . . . . . . . . . . . . . . . 16
REJECT, performance costs . . . . . . . . . . . . . . . . . . . 45
reporting bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
restarting the scanner. . . . . . . . . . . . . . . . . . . . . . . . . . 61
RETURN, within actions . . . . . . . . . . . . . . . . . . . . . . 19
rules, default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
rules, in flex input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
129
S
scanner, definition of . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
sections of flex input . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
serialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
serialization of tables . . . . . . . . . . . . . . . . . . . . . . . . . . 67
serialized tables, multiple scanners . . . . . . . . . . . . . 67
stack, input buffer pop. . . . . . . . . . . . . . . . . . . . . . . . . 27
stack, input buffer push . . . . . . . . . . . . . . . . . . . . . . . . 27
stacks, routines for manipulating . . . . . . . . . . . . . . . 26
start condition, applying to multiple patterns . . 25
start conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
start conditions, behavior of default rule . . . . . . . 22
start conditions, exclusive . . . . . . . . . . . . . . . . . . . . . . 21
start conditions, for different interpretations of
same input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
start conditions, in patterns . . . . . . . . . . . . . . . . . . . 10
start conditions, inclusive . . . . . . . . . . . . . . . . . . . . . . 21
start conditions, inclusive v.s. exclusive . . . . . . . . 21
start conditions, integer values . . . . . . . . . . . . . . . . . 23
start conditions, multiple . . . . . . . . . . . . . . . . . . . . . . 21
start conditions, special wildcard condition . . . . . 21
start conditions, use of a stack . . . . . . . . . . . . . . . . . 26
start conditions, use of wildcard condition (<*>)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
start conditions, using BEGIN . . . . . . . . . . . . . . . . . 22
stdin, default for yyin. . . . . . . . . . . . . . . . . . . . . . . . . . 19
stdout, as default for yyout . . . . . . . . . . . . . . . . . . . . 20
strings, scanning strings instead of files. . . . . . . . . 30
T
tables, creating serialized . . . . . . . . . . . . . . . . . . . . . .
tables, file format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
tables, freeing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
tables, loading and unloading . . . . . . . . . . . . . . . . . .
terminating with yyterminate() . . . . . . . . . . . . . . . .
token . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
trailing context, in patterns . . . . . . . . . . . . . . . . . . . .
trailing context, limits of . . . . . . . . . . . . . . . . . . . . . .
trailing context, matching . . . . . . . . . . . . . . . . . . . . .
trailing context, performance costs . . . . . . . . . . . . .
trailing context, variable length . . . . . . . . . . . . . . . .
67
68
67
67
18
14
10
12
14
45
47
U
unput() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
unput(), and %pointer . . . . . . . . . . . . . . . . . . . . . . . . . 17
unput(), pushing back characters . . . . . . . . . . . . . . 17
user code, in flex input. . . . . . . . . . . . . . . . . . . . . . . . . . 7
username expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
using integer values of start condition names . . . 23
V
verbatim text in patterns, syntax of . . . . . . . . . . . . . 9
Indices
130
W
warning, dangerous trailing context . . . . . . . . . . . .
warning, rule cannot be matched . . . . . . . . . . . . . . .
warnings, diagnostic messages. . . . . . . . . . . . . . . . . .
whitespace, compressing . . . . . . . . . . . . . . . . . . . . . . .
74
72
72
15
Y
yacc interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
yacc, interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
YY CURRENT BUFFER, and multiple buffers
Finally, the macro . . . . . . . . . . . . . . . . . . . . . . . . . 28
YY EXTRA TYPE, defining your own type . . . 58
YY FLUSH BUFFER . . . . . . . . . . . . . . . . . . . . . . . . . 18
YY INPUT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
YY INPUT, overriding . . . . . . . . . . . . . . . . . . . . . . . . 20
YY START, example . . . . . . . . . . . . . . . . . . . . . . . . . . 24
YY USER ACTION to track each time a rule is
matched . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
yyalloc, overriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
yyfree, overriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
yyin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
yyinput() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
yyleng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
yyleng, modification of. . . . . . . . . . . . . . . . . . . . . . . . .
yyless(). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yyless(), pushing back characters. . . . . . . . . . . . . . .
yylex(), in generated scanner . . . . . . . . . . . . . . . . . .
yylex(), overriding . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yylex, overriding the prototype of . . . . . . . . . . . . . .
yylineno, in a reentrant scanner . . . . . . . . . . . . . . . .
yylineno, performance costs . . . . . . . . . . . . . . . . . . . .
yymore() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yymore() to append token to previous token . . .
yymore(), mega-kludge . . . . . . . . . . . . . . . . . . . . . . . .
yymore, and yyleng . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yymore, performance penalty of . . . . . . . . . . . . . . . .
yyout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yyrealloc, overriding . . . . . . . . . . . . . . . . . . . . . . . . . . .
yyrestart() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yyterminate() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yytext . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yytext, default array size . . . . . . . . . . . . . . . . . . . . . .
yytext, memory considerations . . . . . . . . . . . . . . . . .
yytext, modification of . . . . . . . . . . . . . . . . . . . . . . . . .
yytext, two types of . . . . . . . . . . . . . . . . . . . . . . . . . . .
yywrap() . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yywrap, default for . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
16
16
19
19
19
60
45
16
16
16
15
16
20
65
19
18
14
33
66
15
14
20
20
Indices
131
Index of Functions and Macros
This is an index of functions and preprocessor macros that look like functions. For macros
that expand to variables or constants, see [Index of Variables], page 131.
B
BEGIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
D
debug (C++ only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
L
LexerError (C++ only) . . . . . . . . . . . . . . . . . . . . . . . .
LexerInput (C++ only) . . . . . . . . . . . . . . . . . . . . . . . .
LexerOutput (C++ only). . . . . . . . . . . . . . . . . . . . . . .
lineno (C++ only) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
51
51
50
S
set_debug (C++ only) . . . . . . . . . . . . . . . . . . . . . . . . . 50
switch_streams (C++ only) . . . . . . . . . . . . . . . . . . . 51
Y
YY_AT_BOL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yy_create_buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yy_delete_buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yy_flush_buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yy_new_buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
YY_NEW_FILE (now obsolete) . . . . . . . . . . . . . . . . . .
yy_pop_state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yy_push_state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
yy_scan_buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
27
27
27
28
31
26
26
30
yy_scan_bytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
yy_scan_string . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
yy_set_bol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
yy_set_interactive . . . . . . . . . . . . . . . . . . . . . . . . . . 32
yy_switch_to_buffer . . . . . . . . . . . . . . . . . . . . . . . . . 27
yy_top_state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
yyFlexLexer constructor (C++ only) . . . . . . . . . 50
yyget_debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyget_extra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 59
yyget_in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyget_leng. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyget_lineno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyget_out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyget_text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
YYLeng (C++ only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
yylex (C++ version) . . . . . . . . . . . . . . . . . . . . . . . . . . 50
yylex (reentrant version) . . . . . . . . . . . . . . . . . . 122
yylex_destroy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
yylex_init. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
yypop_buffer_state . . . . . . . . . . . . . . . . . . . . . . . . . . 27
yypush_buffer_state . . . . . . . . . . . . . . . . . . . . . . . . . 27
yyrestart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
yyset_debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyset_extra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58, 59
yyset_in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyset_lineno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yyset_out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
yytables_destroy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
yytables_fload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
YYText (C++ only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Index of Variables
This is an index of variables, constants, and preprocessor macros that expand to variables
or constants.
I
INITIAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Y
YY_CURRENT_BUFFER. . . . . . . . . . . . . . . . . . . . . . . . . . . .
YY_END_OF_BUFFER_CHAR . . . . . . . . . . . . . . . . . . . . . . .
YY_NUM_RULES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
YY_START . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24,
33
30
32
33
yyextra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
yyin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
yyleng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
yylloc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
YYLMAX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
yylval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
yylval, with yacc . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
yyout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
yyscanner (reentrant only) . . . . . . . . . . . . . . . . . . 56
yytext. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 33
Indices
132
Index of Data Types
F
FlexLexer (C++ only) . . . . . . . . . . . . . . . . . . . . . . . . . 50
Y
YY_BUFFER_STATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
YY_EXTRA_TYPE (reentrant only) . . . . . . . . . . . . . 58
yy_size_t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
yyFlexLexer (C++ only). . . . . . . . . . . . . . . . . . . . . . . 50
YYLTYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
yyscan_t (reentrant only) . . . . . . . . . . . . . . . . . . . 59
YYSTYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
Index of Hooks
This is an index of "hooks" that the user may define. These hooks typically correspond to
specific locations in the generated scanner, and may be used to insert arbitrary code.
YY BREAK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
YY USER ACTION . . . . . . . . . . . . . . . . . . . . . . . . . . 32
YY USER INIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Index of Scanner Options
-+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—7bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—8bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—align . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—always-interactive . . . . . . . . . . . . . . . . . . . . . . . . . . .
—array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—backup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—batch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—bison-bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—bison-locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—c++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—case-insensitive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—ecs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—fast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—full . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—header-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—interactive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—lex-compat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—main . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—meta-ecs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—never-interactive . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—nodefault. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—noline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—nounistd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—nowarn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—option-ansi-definitions . . . . . . . . . . . . . . . . . . . . . . .
—option-ansi-prototypes . . . . . . . . . . . . . . . . . . . . . . .
—outfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—perf-report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—pointer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
39
37
37
41
38
39
43
36
39
39
39
36
43
38
41
42
42
35
44
37
36
40
41
38
43
39
40
43
39
39
36
43
39
—posix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—prefix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—read. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—reentrant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—skel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—stdinit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—stdout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—tables-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—tables-verify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—trace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—verbose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—warn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—yyclass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—yylineno. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
—yywrap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-c . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-Ca. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-Ce . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-Cf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-CF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-Cm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-Cr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-F . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
40
41
39
36
38
38
36
36
36
43
44
44
44
40
38
38
37
37
43
36
44
41
41
41
41
41
41
41
43
42
42
44
36
37
Indices
-l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-p . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-v . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-w . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
-X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
133
36
39
44
36
43
40
39
43
36
43
44
44
43
38
7bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8
8bit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
A
41
38
39
39
39
B
backup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
batch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
bison-bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
bison-locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
header-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
I
interactive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
L
lex-compat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
M
main . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
meta-ecs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
7
align . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
always-interactive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ansi-definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ansi-prototypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
H
43
36
39
39
C
c++ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
case-insensitive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
D
debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
default . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
E
ecs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
F
fast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
full . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
N
nodefault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
noline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
nounistd. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
nowarn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
noyyalloc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
39
40
43
65
O
outfile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
P
perf-report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
pointer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
posix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
prefix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
39
38
40
R
read . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
reentrant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
S
stack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
stdinit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
stdout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
T
tables-file . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
tables-verify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
trace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
V
verbose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Indices
134
W
Y
warn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
yyclass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
yylineno . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
yywrap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising