Known Causes of Trouble with GCC. GCC GCC

Chapter 11: Known Causes of Trouble with GCC 665

11 Known Causes of Trouble with GCC

This section describes known problems that affect users of GCC. Most of these are not

GCC bugs per se—if they were, we would fix them. But the result for a user may be like the result of a bug.

Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people’s opinions differ as to what is best.

11.1 Actual Bugs We Haven’t Fixed Yet

• The fixincludes script interacts badly with automounters; if the directory of system header files is automounted, it tends to be unmounted while fixincludes is running.

This would seem to be a bug in the automounter. We don’t know any good way to work around it.

11.2 Cross-Compiler Problems

You may run into problems with cross compilation on certain machines, for several reasons.

• At present, the program ‘mips-tfile’ which adds debug support to object files on

Tru64 UNIX systems does not work in a cross compile environment.

11.3 Interoperation

This section lists various difficulties encountered in using GCC together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems.

• On many platforms, GCC supports a different ABI for C++ than do other compilers, so the object files compiled by GCC cannot be used with object files generated by another

C++ compiler.

An area where the difference is most apparent is name mangling. The use of different name mangling is intentional, to protect you from more subtle problems. Compilers differ as to many internal details of C++ implementation, including: how class instances are laid out, how multiple inheritance is implemented, and how virtual function calls are handled. If the name encoding were made the same, your programs would link against libraries provided from other compilers—but the programs would then crash when run. Incompatible libraries are then detected at link time, rather than at run time.

• On some BSD systems, including some versions of Ultrix, use of profiling causes static variable destructors (currently used only in C++) not to be run.

• On some SGI systems, when you use ‘-lgl_s’ as an option, it gets translated magically to ‘-lgl_s -lX11_s -lc_s’. Naturally, this does not happen when you use GCC. You must specify all three options explicitly.

• On a SPARC, GCC aligns all values of type double on an 8-byte boundary, and it expects every double to be so aligned. The Sun compiler usually gives double values

8-byte alignment, with one exception: function arguments of type double may not be aligned.

666 Using the GNU Compiler Collection (GCC)

As a result, if a function compiled with Sun CC takes the address of an argument of type double and passes this pointer of type double * to a function compiled with

GCC, dereferencing the pointer may cause a fatal signal.

One way to solve this problem is to compile your entire program with GCC. Another solution is to modify the function that is compiled with Sun CC to copy the argument into a local variable; local variables are always properly aligned. A third solution is to modify the function that uses the pointer to dereference it via the following function access_double instead of directly with ‘*’: inline double access_double (double *unaligned_ptr)

{ union d2i { double d; int i[2]; }; union d2i *p = (union d2i *) unaligned_ptr; union d2i u; u.i[0] = p->i[0]; u.i[1] = p->i[1]; return u.d;

}

Storing into the pointer can be done likewise with the same union.

• On Solaris, the malloc function in the ‘libmalloc.a’ library may allocate memory that is only 4 byte aligned. Since GCC on the SPARC assumes that doubles are 8 byte aligned, this may result in a fatal signal if doubles are stored in memory allocated by the ‘libmalloc.a’ library.

The solution is to not use the ‘libmalloc.a’ library. Use instead malloc and related functions from ‘libc.a’; they do not have this problem.

• On the HP PA machine, ADB sometimes fails to work on functions compiled with

GCC. Specifically, it fails to work on functions that use alloca or variable-size arrays.

This is because GCC doesn’t generate HP-UX unwind descriptors for such functions.

It may even be impossible to generate them.

• Debugging (‘-g’) is not supported on the HP PA machine, unless you use the preliminary GNU tools.

• Taking the address of a label may generate errors from the HP-UX PA assembler. GAS for the PA does not have this problem.

• Using floating point parameters for indirect calls to static functions will not work when using the HP assembler. There simply is no way for GCC to specify what registers hold arguments for static functions when using the HP assembler. GAS for the PA does not have this problem.

• In extremely rare cases involving some very large functions you may receive errors from the HP linker complaining about an out of bounds unconditional branch offset. This used to occur more often in previous versions of GCC, but is now exceptionally rare.

If you should run into it, you can work around by making your function smaller.

• GCC compiled code sometimes emits warnings from the HP-UX assembler of the form:

(warning) Use of GR3 when frame >= 8192 may cause conflict.

Chapter 11: Known Causes of Trouble with GCC 667

These warnings are harmless and can be safely ignored.

• In extremely rare cases involving some very large functions you may receive errors from the AIX Assembler complaining about a displacement that is too large. If you should run into it, you can work around by making your function smaller.

• The ‘libstdc++.a’ library in GCC relies on the SVR4 dynamic linker semantics which merges global symbols between libraries and applications, especially necessary for C++ streams functionality. This is not the default behavior of AIX shared libraries and dynamic linking. ‘libstdc++.a’ is built on AIX with “runtime-linking” enabled so that symbol merging can occur. To utilize this feature, the application linked with

‘libstdc++.a’ must include the ‘-Wl,-brtl’ flag on the link line. G++ cannot impose this because this option may interfere with the semantics of the user program and users may not always use ‘g++’ to link his or her application. Applications are not required to use the ‘-Wl,-brtl’ flag on the link line—the rest of the ‘libstdc++.a’ library which is not dependent on the symbol merging semantics will continue to function correctly.

• An application can interpose its own definition of functions for functions invoked by

‘libstdc++.a’ with “runtime-linking” enabled on AIX. To accomplish this the application must be linked with “runtime-linking” option and the functions explicitly must be exported by the application (‘-Wl,-brtl,-bE:exportfile’).

• AIX on the RS/6000 provides support (NLS) for environments outside of the United

States. Compilers and assemblers use NLS to support locale-specific representations of various objects including floating-point numbers (‘.’ vs ‘,’ for separating decimal fractions). There have been problems reported where the library linked with GCC does not produce the same floating-point formats that the assembler accepts. If you have this problem, set the LANG environment variable to ‘C’ or ‘En_US’.

• Even if you specify ‘-fdollars-in-identifiers’, you cannot successfully use ‘$’ in identifiers on the RS/6000 due to a restriction in the IBM assembler. GAS supports these identifiers.

11.4 Incompatibilities of GCC

There are several noteworthy incompatibilities between GNU C and K&R (non-ISO) versions of C.

• GCC normally makes string constants read-only. If several identical-looking string constants are used, GCC stores only one copy of the string.

One consequence is that you cannot call mktemp with a string constant argument. The function mktemp always alters the string its argument points to.

Another consequence is that sscanf does not work on some very old systems when passed a string constant as its format control string or input. This is because sscanf incorrectly tries to write into the string constant. Likewise fscanf and scanf.

The solution to these problems is to change the program to use char-array variables with initialization strings for these purposes instead of string constants.

• -2147483648 is positive.

This is because 2147483648 cannot fit in the type int, so (following the ISO C rules) its data type is unsigned long int. Negating this value yields 2147483648 again.

668 Using the GNU Compiler Collection (GCC)

• GCC does not substitute macro arguments when they appear inside of string constants.

For example, the following macro in GCC

#define foo(a) "a" will produce output "a" regardless of what the argument a is.

• When you use setjmp and longjmp, the only automatic variables guaranteed to remain valid are those declared volatile. This is a consequence of automatic register allocation. Consider this function: jmp_buf j; foo ()

{ int a, b; a = fun1 (); if (setjmp (j)) return a; a = fun2 ();

/* longjmp (j) may occur in fun3. */ return a + fun3 ();

}

Here a may or may not be restored to its first value when the longjmp occurs. If a is allocated in a register, then its first value is restored; otherwise, it keeps the last value stored in it.

If you use the ‘-W’ option with the ‘-O’ option, you will get a warning when GCC thinks such a problem might be possible.

• Programs that use preprocessing directives in the middle of macro arguments do not work with GCC. For example, a program like this will not work: foobar (

#define luser hack)

ISO C does not permit such a construct.

• K&R compilers allow comments to cross over an inclusion boundary (i.e. started in an include file and ended in the including file).

• Declarations of external variables and functions within a block apply only to the block containing the declaration. In other words, they have the same scope as any other declaration in the same place.

In some other C compilers, an extern declaration affects all the rest of the file even if it happens within a block.

• In traditional C, you can combine long, etc., with a typedef name, as shown here: typedef int foo; typedef long foo bar;

In ISO C, this is not allowed: long and other type modifiers require an explicit int.

• PCC allows typedef names to be used as function parameters.

• Traditional C allows the following erroneous pair of declarations to appear together in a given scope: typedef int foo; typedef foo foo;

Chapter 11: Known Causes of Trouble with GCC 669

• GCC treats all characters of identifiers as significant. According to K&R-1 (2.2), “No more than the first eight characters are significant, although more may be used.”. Also according to K&R-1 (2.2), “An identifier is a sequence of letters and digits; the first character must be a letter. The underscore counts as a letter.”, but GCC also allows dollar signs in identifiers.

• PCC allows whitespace in the middle of compound assignment operators such as ‘+=’.

GCC, following the ISO standard, does not allow this.

• GCC complains about unterminated character constants inside of preprocessing conditionals that fail. Some programs have English comments enclosed in conditionals that are guaranteed to fail; if these comments contain apostrophes, GCC will probably report an error. For example, this code would produce an error:

#if 0

You can’t expect this to work.

#endif

The best solution to such a problem is to put the text into an actual C comment delimited by ‘/*...*/’.

• Many user programs contain the declaration ‘long time ();’. In the past, the system header files on many systems did not actually declare time, so it did not matter what type your program declared it to return. But in systems with ISO C headers, time is declared to return time_t, and if that is not the same as long, then ‘long time ();’ is erroneous.

The solution is to change your program to use appropriate system headers (<time.h> on systems with ISO C headers) and not to declare time if the system header files declare it, or failing that to use time_t as the return type of time.

• When compiling functions that return float, PCC converts it to a double. GCC actually returns a float. If you are concerned with PCC compatibility, you should declare your functions to return double; you might as well say what you mean.

• When compiling functions that return structures or unions, GCC output code normally uses a method different from that used on most versions of Unix. As a result, code compiled with GCC cannot call a structure-returning function compiled with PCC, and vice versa.

The method used by GCC is as follows: a structure or union which is 1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union with any other size is stored into an address supplied by the caller (usually in a special, fixed register, but on some machines it is passed on the stack). The target hook TARGET_STRUCT_VALUE_RTX tells

GCC where to pass this address.

By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. GCC does not use this method because it is slower and nonreentrant.

On some newer machines, PCC uses a reentrant convention for all structure and union returning. GCC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers.

670 Using the GNU Compiler Collection (GCC)

You can tell GCC to use a compatible convention for all structure and union returning with the option ‘-fpcc-struct-return’.

• GCC complains about program fragments such as ‘0x74ae-0x4000’ which appear to be two hexadecimal constants separated by the minus operator. Actually, this string is a single preprocessing token. Each such token must correspond to one token in C. Since this does not, GCC prints an error message. Although it may appear obvious that what is meant is an operator and two values, the ISO C standard specifically requires that this be treated as erroneous.

A preprocessing token is a preprocessing number if it begins with a digit and is followed by letters, underscores, digits, periods and ‘e+’, ‘e-’, ‘E+’, ‘E-’, ‘p+’, ‘p-’, ‘P+’, or ‘P-’ character sequences. (In strict C90 mode, the sequences ‘p+’, ‘p-’, ‘P+’ and ‘P-’ cannot appear in preprocessing numbers.)

To make the above program fragment valid, place whitespace in front of the minus sign. This whitespace will end the preprocessing number.

11.5 Fixed Header Files

GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won’t work with GCC unless they are changed.

Some have bugs, some are incompatible with ISO C, and some depend on special features of other compilers.

Installing GCC automatically creates and installs the fixed header files, by running a program called fixincludes. Normally, you don’t need to pay attention to this. But there are cases where it doesn’t do the right thing automatically.

• If you update the system’s header files, such as by installing a new system version, the fixed header files of GCC are not automatically updated.

They can be updated using the mkheaders script installed in ‘libexecdir /gcc/target /version /install-tools/’.

• On some systems, header file directories contain machine-specific symbolic links in certain places. This makes it possible to share most of the header files among hosts running the same version of the system on different machine models.

The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it.

It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you’ll have to do this by hand.

11.6 Standard Libraries

GCC by itself attempts to be a conforming freestanding implementation. See

Chapter 2

[Language Standards Supported by GCC], page 5

, for details of what this means. Beyond the library facilities required of such an implementation, the rest of the C library is supplied by the vendor of the operating system. If that C library doesn’t conform to the C standards, then your programs might get warnings (especially when using ‘-Wall’) that you don’t expect.

Chapter 11: Known Causes of Trouble with GCC 671

For example, the sprintf function on SunOS 4.1.3 returns char * while the C standard says that sprintf returns an int. The fixincludes program could make the prototype for this function match the Standard, but that would be wrong, since the function will still return char *.

If you need a Standard compliant library, then you need to find one, as GCC does not provide one. The GNU C library (called glibc) provides ISO C, POSIX, BSD, SystemV and

X/Open compatibility for GNU/Linux and HURD-based GNU systems; no recent version of it supports other systems, though some very old versions did. Version 2.2 of the GNU

C library includes nearly complete C99 support. You could also ask your operating system vendor if newer libraries are available.

11.7 Disappointments and Misunderstandings

These problems are perhaps regrettable, but we don’t know any practical way around them.

• Certain local variables aren’t recognized by debuggers when you compile with optimization.

This occurs because sometimes GCC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable “would have had”, and it is not clear that would be desirable anyway. So GCC simply does not mention the eliminated variable when it writes debugging information.

You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization.

• Users often think it is a bug when GCC reports an error for code like this: int foo (struct mumble *); struct mumble { ... }; int foo (struct mumble *x)

{ ... }

This code really is erroneous, because the scope of struct mumble in the prototype is limited to the argument list containing it. It does not refer to the struct mumble defined with file scope immediately below—they are two unrelated types with similar names in different scopes.

But in the definition of foo, the file-scope type is used because that is available to be inherited. Thus, the definition and the prototype do not match, and you get an error.

This behavior may seem silly, but it’s what the ISO standard specifies. It is easy enough for you to make your code work by moving the definition of struct mumble above the prototype. It’s not worth being incompatible with ISO C just to avoid an error for the example shown above.

• Accesses to bit-fields even in volatile objects works by accessing larger objects, such as a byte or a word. You cannot rely on what size of object is accessed in order to read or write the bit-field; it may even vary for a given bit-field according to the precise usage.

If you care about controlling the amount of memory that is accessed, use volatile but do not use bit-fields.

• GCC comes with shell scripts to fix certain known problems in system header files.

They install corrected copies of various header files in a special directory where only

672 Using the GNU Compiler Collection (GCC)

GCC will normally look for them. The scripts adapt to various systems by searching all the system header files for the problem cases that we know about.

If new system header files are installed, nothing automatically arranges to update the corrected header files. They can be updated using the mkheaders script installed in

‘libexecdir /gcc/target /version /install-tools/’.

• On 68000 and x86 systems, for instance, you can get paradoxical results if you test the precise values of floating point numbers. For example, you can find that a floating point value which is not a NaN is not equal to itself. This results from the fact that the floating point registers hold a few more bits of precision than fit in a double in memory. Compiled code moves values between memory and floating point registers at its convenience, and moving them into memory truncates them.

You can partially avoid this problem by using the ‘-ffloat-store’ option (see

Section 3.10 [Optimize Options], page 92

).

• On AIX and other platforms without weak symbol support, templates need to be instantiated explicitly and symbols for static members of templates will not be generated.

• On AIX, GCC scans object files and library archives for static constructors and destructors when linking an application before the linker prunes unreferenced symbols.

This is necessary to prevent the AIX linker from mistakenly assuming that static constructor or destructor are unused and removing them before the scanning can occur.

All static constructors and destructors found will be referenced even though the modules in which they occur may not be used by the program. This may lead to both increased executable size and unexpected symbol references.

11.8 Common Misunderstandings with GNU C++

C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort.

11.8.1 Declare and Define Static Members

When a class has static data members, it is not enough to declare the static member; you must also define it. For example: class Foo

{

...

void method();

}; static int bar;

This declaration only establishes that the class Foo has an int named Foo::bar, and a member function named Foo::method. But you still need to define both method and bar elsewhere. According to the ISO standard, you must supply an initializer in one (and only one) source file, such as: int Foo::bar = 0;

Other C++ compilers may not correctly implement the standard behavior. As a result, when you switch to g++ from one of these compilers, you may discover that a program

Chapter 11: Known Causes of Trouble with GCC 673 that appeared to work correctly in fact does not conform to the standard: g++ reports as undefined symbols any static data members that lack definitions.

11.8.2 Name lookup, templates, and accessing members of base classes

The C++ standard prescribes that all names that are not dependent on template parameters are bound to their present definitions when parsing a template function or class.

1

Only names that are dependent are looked up at the point of instantiation. For example, consider void foo(double); struct A { template <typename T> void f () { foo (1); int i = N;

T t;

// 1

// 2 t.bar(); foo (t);

// 3

// 4

}

}; static const int N;

Here, the names foo and N appear in a context that does not depend on the type of T.

The compiler will thus require that they are defined in the context of use in the template, not only before the point of instantiation, and will here use ::foo(double) and A::N, respectively. In particular, it will convert the integer value to a double when passing it to

::foo(double).

Conversely, bar and the call to foo in the fourth marked line are used in contexts that do depend on the type of T, so they are only looked up at the point of instantiation, and you can provide declarations for them after declaring the template, but before instantiating it.

In particular, if you instantiate A::f<int>, the last line will call an overloaded ::foo(int) if one was provided, even if after the declaration of struct A.

This distinction between lookup of dependent and non-dependent names is called twostage (or dependent) name lookup. G++ implements it since version 3.4.

Two-stage name lookup sometimes leads to situations with behavior different from nontemplate codes. The most common is probably this: template <typename T> struct Base { int i;

}; template <typename T> struct Derived : public Base<T> {

}; int get_i() { return i; }

In get_i(), i is not used in a dependent context, so the compiler will look for a name declared at the enclosing namespace scope (which is the global scope here). It will not look into the base class, since that is dependent and you may declare specializations of Base

1

The C++ standard just uses the term “dependent” for names that depend on the type or value of template parameters. This shorter term will also be used in the rest of this section.

674 Using the GNU Compiler Collection (GCC) even after declaring Derived, so the compiler can’t really know what i would refer to. If there is no global variable i, then you will get an error message.

In order to make it clear that you want the member of the base class, you need to defer lookup until instantiation time, at which the base class is known. For this, you need to access i in a dependent context, by either using this->i (remember that this is of type

Derived<T>*, so is obviously dependent), or using Base<T>::i. Alternatively, Base<T>::i might be brought into scope by a using-declaration.

Another, similar example involves calling member functions of a base class: template <typename T> struct Base { int f();

}; template <typename T> struct Derived : Base<T> { int g() { return f(); };

};

Again, the call to f() is not dependent on template arguments (there are no arguments that depend on the type T, and it is also not otherwise specified that the call should be in a dependent context). Thus a global declaration of such a function must be available, since the one in the base class is not visible until instantiation time. The compiler will consequently produce the following error message: x.cc: In member function ‘int Derived<T>::g()’: x.cc:6: error: there are no arguments to ‘f’ that depend on a template parameter, so a declaration of ‘f’ must be available x.cc:6: error: (if you use ‘-fpermissive’, G++ will accept your code, but allowing the use of an undeclared name is deprecated)

To make the code valid either use this->f(), or Base<T>::f().

Using the

‘-fpermissive’ flag will also let the compiler accept the code, by marking all function calls for which no declaration is visible at the time of definition of the template for later lookup at instantiation time, as if it were a dependent call. We do not recommend using

‘-fpermissive’ to work around invalid code, and it will also only catch cases where functions in base classes are called, not where variables in base classes are used (as in the example above).

Note that some compilers (including G++ versions prior to 3.4) get these examples wrong and accept above code without an error. Those compilers do not implement two-stage name lookup correctly.

11.8.3 Temporaries May Vanish Before You Expect

It is dangerous to use pointers or references to portions of a temporary object. The compiler may very well delete the object before you expect it to, leaving a pointer to garbage. The most common place where this problem crops up is in classes like string classes, especially ones that define a conversion function to type char * or const char *—which is one reason why the standard string class requires you to call the c_str member function. However, any class that returns a pointer to some internal structure is potentially subject to this problem.

For example, a program may use a function strfunc that returns string objects, and another function charfunc that operates on pointers to char: string strfunc ();

Chapter 11: Known Causes of Trouble with GCC 675 void charfunc (const char *); void f ()

{ const char *p = strfunc().c_str();

...

charfunc (p);

...

charfunc (p);

}

In this situation, it may seem reasonable to save a pointer to the C string returned by the c_str member function and use that rather than call c_str repeatedly. However, the temporary string created by the call to strfunc is destroyed after p is initialized, at which point p is left pointing to freed memory.

Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable.

The safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example: const string& tmp = strfunc (); charfunc (tmp.c_str ());

11.8.4 Implicit Copy-Assignment for Virtual Bases

When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example: struct Base{ char *name;

Base(char *n) : name(strdup(n)){}

Base& operator= (const Base& other){ free (name);

};

} name = strdup (other.name); struct A:virtual Base{ int val;

};

A():Base("A"){} struct B:virtual Base{ int bval;

};

B():Base("B"){} struct Derived:public A, public B{

Derived():Base("Derived"){}

}; void func(Derived &d1, Derived &d2)

676 Using the GNU Compiler Collection (GCC)

{ d1 = d2;

}

The C++ standard specifies that ‘Base::Base’ is only called once when constructing or copy-constructing a Derived object. It is unspecified whether ‘Base::operator=’ is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside ‘func’ in the example).

G++ implements the “intuitive” algorithm for copy-assignment: assign all direct bases, then assign all members. In that algorithm, the virtual base subobject can be encountered more than once. In the example, copying proceeds in the following order: ‘val’, ‘name’ (via strdup), ‘bval’, and ‘name’ again.

If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned.

11.9 Certain Changes We Don’t Want to Make

This section lists changes that people frequently request, but which we do not make because we think GCC is better without them.

• Checking the number and type of arguments to a function which has an old-fashioned definition and no prototype.

Such a feature would work only occasionally—only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile.

• Warning about using an expression whose type is signed as a shift count.

Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good.

• Warning about assigning a signed value to an unsigned variable.

Such assignments must be very common; warning about them would cause more annoyance than good.

• Warning when a non-void function value is ignored.

C contains many standard functions that return a value that most programs choose to ignore. One obvious example is printf. Warning about this practice only leads the defensive programmer to clutter programs with dozens of casts to void. Such casts are required so frequently that they become visual noise. Writing those casts becomes so automatic that they no longer convey useful information about the intentions of the programmer. For functions where the return value should never be ignored, use the warn_unused_result function attribute (see

Section 6.30 [Function Attributes], page 334

).

• Making ‘-fshort-enums’ the default.

This would cause storage layout to be incompatible with most other C compilers. And it doesn’t seem very important, given that you can get the same result in other ways.

The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly.

Chapter 11: Known Causes of Trouble with GCC 677

• Making bit-fields unsigned by default on particular machines where “the ABI standard” says to do so.

The ISO C standard leaves it up to the implementation whether a bit-field declared plain int is signed or not. This in effect creates two alternative dialects of C.

The GNU C compiler supports both dialects; you can specify the signed dialect with

‘-fsigned-bitfields’ and the unsigned dialect with ‘-funsigned-bitfields’. However, this leaves open the question of which dialect to use by default.

Currently, the preferred dialect makes plain bit-fields signed, because this is simplest.

Since int is the same as signed int in every other context, it is cleanest for them to be the same in bit-fields as well.

Some computer manufacturers have published Application Binary Interface standards which specify that plain bit-fields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bit-fields distinguishes two dialects of C. Both dialects are meaningful on every type of machine.

Whether a particular object file was compiled using signed bit-fields or unsigned is of no concern to other object files, even if they access the same bit-fields in the same data structures.

A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect.

It is unlikely to work at all if compiled with the wrong dialect.

Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bit-fields differently on certain machines.

Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility.

This is why GCC does and will treat plain bit-fields in the same fashion on all types of machines (by default).

There are some arguments for making bit-fields unsigned by default on all machines.

If, for example, this becomes a universal de facto standard, it would make sense for

GCC to go along with it. This is something to be considered in the future.

(Of course, users strongly concerned about portability should indicate explicitly in each bit-field whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.)

• Undefining __STDC__ when ‘-ansi’ is not used.

Currently, GCC defines __STDC__ unconditionally. This provides good results in practice.

Programmers normally use conditionals on __STDC__ to ask whether it is safe to use certain features of ISO C, such as function prototypes or ISO token concatenation.

Since plain gcc supports all the features of ISO C, the correct answer to these questions is “yes”.

678 Using the GNU Compiler Collection (GCC)

Some users try to use __STDC__ to check for the availability of certain library facilities.

This is actually incorrect usage in an ISO C program, because the ISO C standard says that a conforming freestanding implementation should define __STDC__ even though it does not have the library facilities. ‘gcc -ansi -pedantic’ is a conforming freestanding implementation, and it is therefore required to define __STDC__, even though it does not come with an ISO C library.

Sometimes people say that defining __STDC__ in a compiler that does not completely conform to the ISO C standard somehow violates the standard. This is illogical. The standard is a standard for compilers that claim to support ISO C, such as ‘gcc -ansi’— not for other compilers such as plain gcc.

Whatever the ISO C standard says is relevant to the design of plain gcc without ‘-ansi’ only for pragmatic reasons, not as a requirement.

GCC normally defines __STDC__ to be 1, and in addition defines __STRICT_ANSI__ if you specify the ‘-ansi’ option, or a ‘-std’ option for strict conformance to some version of ISO C. On some hosts, system include files use a different convention, where __STDC_

_ is normally 0, but is 1 if the user specifies strict conformance to the C Standard. GCC follows the host convention when processing system include files, but when processing user files it follows the usual GNU C convention.

• Undefining __STDC__ in C++.

Programs written to compile with C++-to-C translators get the value of __STDC__ that goes with the C compiler that is subsequently used. These programs must test __STDC_

_ to determine what kind of C preprocessor that compiler uses: whether they should concatenate tokens in the ISO C fashion or in the traditional fashion.

These programs work properly with GNU C++ if __STDC__ is defined. They would not work otherwise.

In addition, many header files are written to provide prototypes in ISO C but not in traditional C. Many of these header files can work without change in C++ provided

__STDC__ is defined. If __STDC__ is not defined, they will all fail, and will all need to be changed to test explicitly for C++ as well.

• Deleting “empty” loops.

Historically, GCC has not deleted “empty” loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster.

However, the rationale here is that optimization of a nonempty loop cannot produce an empty one. This held for carefully written C compiled with less powerful optimizers but is not always the case for carefully written C++ or with more powerful optimizers. Thus

GCC will remove operations from loops whenever it can determine those operations are not externally visible (apart from the time taken to execute them, of course). In case the loop can be proved to be finite, GCC will also remove the loop itself.

Be aware of this when performing timing tests, for instance the following loop can be completely removed, provided some_expression can provably not change any global state.

{ int sum = 0; int ix;

Chapter 11: Known Causes of Trouble with GCC 679 for (ix = 0; ix != 10000; ix++) sum += some_expression;

}

Even though sum is accumulated in the loop, no use is made of that summation, so the accumulation can be removed.

• Making side effects happen in the same order as in some other compiler.

It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another: void func (int, int); int i = 2; func (i++, i++);

There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first. func might get the arguments ‘2, 3’, or it might get ‘3, 2’, or even ‘2, 2’.

• Making certain warnings into errors by default.

Some ISO C testsuites report failure when the compiler does not produce an error message for a certain program.

ISO C requires a “diagnostic” message for certain kinds of invalid programs, but a warning is defined by GCC to count as a diagnostic. If GCC produces a warning but not an error, that is correct ISO C support. If testsuites call this “failure”, they should be run with the GCC option ‘-pedantic-errors’, which will turn these warnings into errors.

11.10 Warning Messages and Error Messages

The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:

Errors report problems that make it impossible to compile your program. GCC reports errors with the source file name and line number where the problem is apparent.

Warnings report other unusual conditions in your code that may indicate a problem, although compilation can (and does) proceed. Warning messages also report the source file name and line number, but include the text ‘warning:’ to distinguish them from error messages.

Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the ‘-W’ options (for instance, ‘-Wall’ requests a variety of useful warnings).

GCC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The ‘-pedantic’ option tells GCC to issue warnings in such cases; ‘-pedantic-errors’ says to make them errors instead. This does not mean that all non-ISO constructs get warnings or errors.

680 Using the GNU Compiler Collection (GCC)

See

Section 3.8 [Options to Request or Suppress Warnings], page 49

, for more detail on these and related command-line options.