An Advanced 4.4BSD Interprocess Communication Tutorial

An Advanced 4.4BSD Interprocess Communication Tutorial
An Advanced 4.4BSD Interprocess Communication Tutorial
Samuel J. Leffler
Robert S. Fabry
William N. Joy
Phil Lapsley
Computer Systems Research Group
Department of Electrical Engineering and Computer Science
University of California, Berkeley
Berkeley, California 94720
Steve Miller
Chris Torek
Heterogeneous Systems Laboratory
Department of Computer Science
University of Maryland, College Park
College Park, Maryland 20742
ABSTRACT
This document provides an introduction to the interprocess communication facilities included in the 4.4BSD release of the UNIX* system.
It discusses the overall model for interprocess communication and introduces the
interprocess communication primitives which have been added to the system. The majority of the document considers the use of these primitives in developing applications. The
reader is expected to be familiar with the C programming language as all examples are
written in C.
* UNIX is a trademark of UNIX System Laboratories, Inc. in the US and some other countries.
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1. INTRODUCTION
One of the most important additions to UNIX in 4.2BSD was interprocess communication. These facilities
were the result of more than two years of discussion and research. The facilities provided in 4.2BSD incorporated many of the ideas from current research, while trying to maintain the UNIX philosophy of simplicity and conciseness. The 4.3BSD release of Berkeley UNIX improved upon some of the IPC facilities
while providing an upward-compatible interface. 4.4BSD adds support for ISO protocols and IP multicasting. The BSD interprocess communication facilities have become a defacto standard for UNIX.
UNIX has previously been very weak in the area of interprocess communication. Prior to the 4BSD
facilities, the only standard mechanism which allowed two processes to communicate were pipes (the mpx
files which were part of Version 7 were experimental). Unfortunately, pipes are very restrictive in that the
two communicating processes must be related through a common ancestor. Further, the semantics of pipes
makes them almost impossible to maintain in a distributed environment.
Earlier attempts at extending the IPC facilities of UNIX have met with mixed reaction. The majority
of the problems have been related to the fact that these facilities have been tied to the UNIX file system,
either through naming or implementation. Consequently, the IPC facilities provided in 4.2BSD were
designed as a totally independent subsystem. The BSD IPC allows processes to rendezvous in many ways.
Processes may rendezvous through a UNIX file system-like name space (a space where all names are path
names) as well as through a network name space. In fact, new name spaces may be added at a future time
with only minor changes visible to users. Further, the communication facilities have been extended to
include more than the simple byte stream provided by a pipe. These extensions have resulted in a completely new part of the system which users will need time to familiarize themselves with. It is likely that as
more use is made of these facilities they will be refined; only time will tell.
This document provides a high-level description of the IPC facilities in 4.4BSD and their use. It is
designed to complement the manual pages for the IPC primitives by examples of their use. The remainder
of this document is organized in four sections. Section 2 introduces the IPC-related system calls and the
basic model of communication. Section 3 describes some of the supporting library routines users may find
useful in constructing distributed applications. Section 4 is concerned with the client/server model used in
developing applications and includes examples of the two major types of servers. Section 5 delves into
advanced topics which sophisticated users are likely to encounter when using the IPC facilities.
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2. BASICS
The basic building block for communication is the socket. A socket is an endpoint of communication
to which a name may be bound. Each socket in use has a type and one or more associated processes. Sockets exist within communication domains. A communication domain is an abstraction introduced to bundle
common properties of processes communicating through sockets. One such property is the scheme used to
name sockets. For example, in the UNIX communication domain sockets are named with UNIX path
names; e.g. a socket may be named ‘‘/dev/foo’’. Sockets normally exchange data only with sockets in the
same domain (it may be possible to cross domain boundaries, but only if some translation process is performed). The 4.4BSD IPC facilities support four separate communication domains: the UNIX domain, for
on-system communication; the Internet domain, which is used by processes which communicate using the
Internet standard communication protocols; the NS domain, which is used by processes which communicate using the Xerox standard communication protocols*; and the ISO OSI protocols, which are not documented in this tutorial. The underlying communication facilities provided by these domains have a significant influence on the internal system implementation as well as the interface to socket facilities available to
a user. An example of the latter is that a socket ‘‘operating’’ in the UNIX domain sees a subset of the error
conditions which are possible when operating in the Internet (or NS) domain.
2.1. Socket types
Sockets are typed according to the communication properties visible to a user. Processes are presumed to communicate only between sockets of the same type, although there is nothing that prevents communication between sockets of different types should the underlying communication protocols support this.
Four types of sockets currently are available to a user. A stream socket provides for the bidirectional,
reliable, sequenced, and unduplicated flow of data without record boundaries. Aside from the bidirectionality of data flow, a pair of connected stream sockets provides an interface nearly identical to that of pipes†.
A datagram socket supports bidirectional flow of data which is not promised to be sequenced, reliable, or unduplicated. That is, a process receiving messages on a datagram socket may find messages
duplicated, and, possibly, in an order different from the order in which it was sent. An important characteristic of a datagram socket is that record boundaries in data are preserved. Datagram sockets closely model
the facilities found in many contemporary packet switched networks such as the Ethernet.
A raw socket provides users access to the underlying communication protocols which support socket
abstractions. These sockets are normally datagram oriented, though their exact characteristics are dependent on the interface provided by the protocol. Raw sockets are not intended for the general user; they have
been provided mainly for those interested in developing new communication protocols, or for gaining
access to some of the more esoteric facilities of an existing protocol. The use of raw sockets is considered
in section 5.
A sequenced packet socket is similar to a stream socket, with the exception that record boundaries are
preserved. This interface is provided only as part of the NS socket abstraction, and is very important in
most serious NS applications. Sequenced-packet sockets allow the user to manipulate the SPP or IDP headers on a packet or a group of packets either by writing a prototype header along with whatever data is to be
sent, or by specifying a default header to be used with all outgoing data, and allows the user to receive the
headers on incoming packets. The use of these options is considered in section 5.
Another potential socket type which has interesting properties is the reliably delivered message
socket. The reliably delivered message socket has similar properties to a datagram socket, but with reliable
delivery. There is currently no support for this type of socket, but a reliably delivered message protocol
similar to Xerox’s Packet Exchange Protocol (PEX) may be simulated at the user level. More information
on this topic can be found in section 5.
* See Internet Transport Protocols, Xerox System Integration Standard (XSIS)028112 for more information.
This document is almost a necessity for one trying to write NS applications.
† In the UNIX domain, in fact, the semantics are identical and, as one might expect, pipes have been implemented internally as simply a pair of connected stream sockets.
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2.2. Socket creation
To create a socket the socket system call is used:
s = socket(domain, type, protocol);
This call requests that the system create a socket in the specified domain and of the specified type. A particular protocol may also be requested. If the protocol is left unspecified (a value of 0), the system will
select an appropriate protocol from those protocols which comprise the communication domain and which
may be used to support the requested socket type. The user is returned a descriptor (a small integer number) which may be used in later system calls which operate on sockets. The domain is specified as one of
the manifest constants defined in the file <sys/socket.h>. For the UNIX domain the constant is AF_UNIX*;
for the Internet domain AF_INET; and for the NS domain, AF_NS. The socket types are also defined in
this file and one of SOCK_STREAM, SOCK_DGRAM, SOCK_RAW, or SOCK_SEQPACKET must be
specified. To create a stream socket in the Internet domain the following call might be used:
s = socket(AF_INET, SOCK_STREAM, 0);
This call would result in a stream socket being created with the TCP protocol providing the underlying
communication support. To create a datagram socket for on-machine use the call might be:
s = socket(AF_UNIX, SOCK_DGRAM, 0);
The default protocol (used when the protocol argument to the socket call is 0) should be correct for
most every situation. However, it is possible to specify a protocol other than the default; this will be covered in section 5.
There are several reasons a socket call may fail. Aside from the rare occurrence of lack of memory
(ENOBUFS), a socket request may fail due to a request for an unknown protocol (EPROTONOSUPPORT),
or a request for a type of socket for which there is no supporting protocol (EPROTOTYPE).
2.3. Binding local names
A socket is created without a name. Until a name is bound to a socket, processes have no way to reference it and, consequently, no messages may be received on it. Communicating processes are bound by an
association. In the Internet and NS domains, an association is composed of local and foreign addresses,
and local and foreign ports, while in the UNIX domain, an association is composed of local and foreign
path names (the phrase ‘‘foreign pathname’’ means a pathname created by a foreign process, not a pathname on a foreign system). In most domains, associations must be unique. In the Internet domain there
may never be duplicate <protocol, local address, local port, foreign address, foreign port> tuples. UNIX
domain sockets need not always be bound to a name, but when bound there may never be duplicate <protocol, local pathname, foreign pathname> tuples. The pathnames may not refer to files already existing on
the system in 4.3; the situation may change in future releases.
The bind system call allows a process to specify half of an association, <local address, local port> (or
<local pathname>), while the connect and accept primitives are used to complete a socket’s association.
In the Internet domain, binding names to sockets can be fairly complex. Fortunately, it is usually not
necessary to specifically bind an address and port number to a socket, because the connect and send calls
will automatically bind an appropriate address if they are used with an unbound socket. The process of
binding names to NS sockets is similar in most ways to that of binding names to Internet sockets.
The bind system call is used as follows:
bind(s, name, namelen);
The bound name is a variable length byte string which is interpreted by the supporting protocol(s). Its
interpretation may vary from communication domain to communication domain (this is one of the properties which comprise the ‘‘domain’’). As mentioned, in the Internet domain names contain an Internet
address and port number. NS domain names contain an NS address and port number. In the UNIX domain,
* The manifest constants are named AF_whatever as they indicate the ‘‘address format’’ to use in interpreting
names.
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names contain a path name and a family, which is always AF_UNIX. If one wanted to bind the name
‘‘/tmp/foo’’ to a UNIX domain socket, the following code would be used*:
#include <sys/un.h>
...
struct sockaddr_un addr;
...
strcpy(addr.sun_path, "/tmp/foo");
addr.sun_family = AF_UNIX;
bind(s, (struct sockaddr *) &addr, strlen(addr.sun_path) +
sizeof (addr.sun_len) + sizeof (addr.sun_family));
Note that in determining the size of a UNIX domain address null bytes are not counted, which is why strlen
is used. In the current implementation of UNIX domain IPC, the file name referred to in addr.sun_path is
created as a socket in the system file space. The caller must, therefore, have write permission in the directory where addr.sun_path is to reside, and this file should be deleted by the caller when it is no longer
needed. Future versions of 4BSD may not create this file.
In binding an Internet address things become more complicated. The actual call is similar,
#include <sys/types.h>
#include <netinet/in.h>
...
struct sockaddr_in sin;
...
bind(s, (struct sockaddr *) &sin, sizeof (sin));
but the selection of what to place in the address sin requires some discussion. We will come back to the
problem of formulating Internet addresses in section 3 when the library routines used in name resolution
are discussed.
Binding an NS address to a socket is even more difficult, especially since the Internet library routines
do not work with NS hostnames. The actual call is again similar:
#include <sys/types.h>
#include <netns/ns.h>
...
struct sockaddr_ns sns;
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
Again, discussion of what to place in a ‘‘struct sockaddr_ns’’ will be deferred to section 3.
2.4. Connection establishment
Connection establishment is usually asymmetric, with one process a ‘‘client’’ and the other a
‘‘server’’. The server, when willing to offer its advertised services, binds a socket to a well-known address
associated with the service and then passively ‘‘listens’’ on its socket. It is then possible for an unrelated
process to rendezvous with the server. The client requests services from the server by initiating a ‘‘connection’’ to the server’s socket. On the client side the connect call is used to initiate a connection. Using the
UNIX domain, this might appear as,
struct sockaddr_un server;
...
connect(s, (struct sockaddr *)&server, strlen(server.sun_path) +
sizeof (server.sun_family));
while in the Internet domain,
* Note that, although the tendency here is to call the ‘‘addr’’ structure ‘‘sun’’, doing so would cause problems if
the code were ever ported to a Sun workstation.
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struct sockaddr_in server;
...
connect(s, (struct sockaddr *)&server, sizeof (server));
and in the NS domain,
struct sockaddr_ns server;
...
connect(s, (struct sockaddr *)&server, sizeof (server));
where server in the example above would contain either the UNIX pathname, Internet address and port
number, or NS address and port number of the server to which the client process wishes to speak. If the
client process’s socket is unbound at the time of the connect call, the system will automatically select and
bind a name to the socket if necessary; c.f. section 5.4. This is the usual way that local addresses are bound
to a socket.
An error is returned if the connection was unsuccessful (any name automatically bound by the system, however, remains). Otherwise, the socket is associated with the server and data transfer may begin.
Some of the more common errors returned when a connection attempt fails are:
ETIMEDOUT
After failing to establish a connection for a period of time, the system decided there was no point in
retrying the connection attempt any more. This usually occurs because the destination host is down,
or because problems in the network resulted in transmissions being lost.
ECONNREFUSED
The host refused service for some reason. This is usually due to a server process not being present at
the requested name.
ENETDOWN or EHOSTDOWN
These operational errors are returned based on status information delivered to the client host by the
underlying communication services.
ENETUNREACH or EHOSTUNREACH
These operational errors can occur either because the network or host is unknown (no route to the
network or host is present), or because of status information returned by intermediate gateways or
switching nodes. Many times the status returned is not sufficient to distinguish a network being
down from a host being down, in which case the system indicates the entire network is unreachable.
For the server to receive a client’s connection it must perform two steps after binding its socket. The
first is to indicate a willingness to listen for incoming connection requests:
listen(s, 5);
The second parameter to the listen call specifies the maximum number of outstanding connections which
may be queued awaiting acceptance by the server process; this number may be limited by the system.
Should a connection be requested while the queue is full, the connection will not be refused, but rather the
individual messages which comprise the request will be ignored. This gives a harried server time to make
room in its pending connection queue while the client retries the connection request. Had the connection
been returned with the ECONNREFUSED error, the client would be unable to tell if the server was up or
not. As it is now it is still possible to get the ETIMEDOUT error back, though this is unlikely. The backlog figure supplied with the listen call is currently limited by the system to a maximum of 5 pending connections on any one queue. This avoids the problem of processes hogging system resources by setting an
infinite backlog, then ignoring all connection requests.
With a socket marked as listening, a server may accept a connection:
struct sockaddr_in from;
...
fromlen = sizeof (from);
newsock = accept(s, (struct sockaddr *)&from, &fromlen);
(For the UNIX domain, from would be declared as a struct sockaddr_un, and for the NS domain, from
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would be declared as a struct sockaddr_ns, but nothing different would need to be done as far as fromlen is
concerned. In the examples which follow, only Internet routines will be discussed.) A new descriptor is
returned on receipt of a connection (along with a new socket). If the server wishes to find out who its client
is, it may supply a buffer for the client socket’s name. The value-result parameter fromlen is initialized by
the server to indicate how much space is associated with from, then modified on return to reflect the true
size of the name. If the client’s name is not of interest, the second parameter may be a null pointer.
Accept normally blocks. That is, accept will not return until a connection is available or the system
call is interrupted by a signal to the process. Further, there is no way for a process to indicate it will accept
connections from only a specific individual, or individuals. It is up to the user process to consider who the
connection is from and close down the connection if it does not wish to speak to the process. If the server
process wants to accept connections on more than one socket, or wants to avoid blocking on the accept call,
there are alternatives; they will be considered in section 5.
2.5. Data transfer
With a connection established, data may begin to flow. To send and receive data there are a number
of possible calls. With the peer entity at each end of a connection anchored, a user can send or receive a
message without specifying the peer. As one might expect, in this case, then the normal read and write system calls are usable,
write(s, buf, sizeof (buf));
read(s, buf, sizeof (buf));
In addition to read and write, the new calls send and recv may be used:
send(s, buf, sizeof (buf), flags);
recv(s, buf, sizeof (buf), flags);
While send and recv are virtually identical to read and write, the extra flags argument is important. The
flags, defined in <sys/socket.h>, may be specified as a non-zero value if one or more of the following is
required:
MSG_OOB
MSG_PEEK
MSG_DONTROUTE
send/receive out of band data
look at data without reading
send data without routing packets
Out of band data is a notion specific to stream sockets, and one which we will not immediately consider.
The option to have data sent without routing applied to the outgoing packets is currently used only by the
routing table management process, and is unlikely to be of interest to the casual user. The ability to preview data is, however, of interest. When MSG_PEEK is specified with a recv call, any data present is
returned to the user, but treated as still ‘‘unread’’. That is, the next read or recv call applied to the socket
will return the data previously previewed.
2.6. Discarding sockets
Once a socket is no longer of interest, it may be discarded by applying a close to the descriptor,
close(s);
If data is associated with a socket which promises reliable delivery (e.g. a stream socket) when a close takes
place, the system will continue to attempt to transfer the data. However, after a fairly long period of time, if
the data is still undelivered, it will be discarded. Should a user have no use for any pending data, it may
perform a shutdown on the socket prior to closing it. This call is of the form:
shutdown(s, how);
where how is 0 if the user is no longer interested in reading data, 1 if no more data will be sent, or 2 if no
data is to be sent or received.
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2.7. Connectionless sockets
To this point we have been concerned mostly with sockets which follow a connection oriented model.
However, there is also support for connectionless interactions typical of the datagram facilities found in
contemporary packet switched networks. A datagram socket provides a symmetric interface to data
exchange. While processes are still likely to be client and server, there is no requirement for connection
establishment. Instead, each message includes the destination address.
Datagram sockets are created as before. If a particular local address is needed, the bind operation
must precede the first data transmission. Otherwise, the system will set the local address and/or port when
data is first sent. To send data, the sendto primitive is used,
sendto(s, buf, buflen, flags, (struct sockaddr *)&to, tolen);
The s, buf, buflen, and flags parameters are used as before. The to and tolen values are used to indicate the
address of the intended recipient of the message. When using an unreliable datagram interface, it is
unlikely that any errors will be reported to the sender. When information is present locally to recognize a
message that can not be delivered (for instance when a network is unreachable), the call will return −1 and
the global value errno will contain an error number.
To receive messages on an unconnected datagram socket, the recvfrom primitive is provided:
recvfrom(s, buf, buflen, flags, (struct sockaddr *)&from, &fromlen);
Once again, the fromlen parameter is handled in a value-result fashion, initially containing the size of the
from buffer, and modified on return to indicate the actual size of the address from which the datagram was
received.
In addition to the two calls mentioned above, datagram sockets may also use the connect call to associate a socket with a specific destination address. In this case, any data sent on the socket will automatically be addressed to the connected peer, and only data received from that peer will be delivered to the user.
Only one connected address is permitted for each socket at one time; a second connect will change the destination address, and a connect to a null address (family AF_UNSPEC) will disconnect. Connect requests
on datagram sockets return immediately, as this simply results in the system recording the peer’s address
(as compared to a stream socket, where a connect request initiates establishment of an end to end connection). Accept and listen are not used with datagram sockets.
While a datagram socket is connected, errors from recent send calls may be returned asynchronously.
These errors may be reported on subsequent operations on the socket, or a special socket option used with
getsockopt, SO_ERROR, may be used to interrogate the error status. A select for reading or writing will
return true when an error indication has been received. The next operation will return the error, and the
error status is cleared. Other of the less important details of datagram sockets are described in section 5.
2.8. Input/Output multiplexing
One last facility often used in developing applications is the ability to multiplex i/o requests among
multiple sockets and/or files. This is done using the select call:
#include <sys/time.h>
#include <sys/types.h>
...
fd_set readmask, writemask, exceptmask;
struct timeval timeout;
...
select(nfds, &readmask, &writemask, &exceptmask, &timeout);
Select takes as arguments pointers to three sets, one for the set of file descriptors for which the caller wishes
to be able to read data on, one for those descriptors to which data is to be written, and one for which exceptional conditions are pending; out-of-band data is the only exceptional condition currently implemented by
the socket If the user is not interested in certain conditions (i.e., read, write, or exceptions), the corresponding argument to the select should be a null pointer.
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Each set is actually a structure containing an array of long integer bit masks; the size of the array is
set by the definition FD_SETSIZE. The array is be long enough to hold one bit for each of FD_SETSIZE
file descriptors.
The macros FD_SET(fd, &mask) and FD_CLR(fd, &mask) have been provided for adding and
removing file descriptor fd in the set mask. The set should be zeroed before use, and the macro
FD_ZERO(&mask) has been provided to clear the set mask. The parameter nfds in the select call specifies
the range of file descriptors (i.e. one plus the value of the largest descriptor) to be examined in a set.
A timeout value may be specified if the selection is not to last more than a predetermined period of
time. If the fields in timeout are set to 0, the selection takes the form of a poll, returning immediately. If
the last parameter is a null pointer, the selection will block indefinitely*. Select normally returns the number of file descriptors selected; if the select call returns due to the timeout expiring, then the value 0 is
returned. If the select terminates because of an error or interruption, a −1 is returned with the error number
in errno, and with the file descriptor masks unchanged.
Assuming a successful return, the three sets will indicate which file descriptors are ready to be read
from, written to, or have exceptional conditions pending. The status of a file descriptor in a select mask
may be tested with the FD_ISSET(fd, &mask) macro, which returns a non-zero value if fd is a member of
the set mask, and 0 if it is not.
To determine if there are connections waiting on a socket to be used with an accept call, select can be
used, followed by a FD_ISSET(fd, &mask) macro to check for read readiness on the appropriate socket. If
FD_ISSET returns a non-zero value, indicating permission to read, then a connection is pending on the
socket.
As an example, to read data from two sockets, s1 and s2 as it is available from each and with a onesecond timeout, the following code might be used:
* To be more specific, a return takes place only when a descriptor is selectable, or when a signal is received by
the caller, interrupting the system call.
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#include <sys/time.h>
#include <sys/types.h>
...
fd_set read_template;
struct timeval wait;
...
for (;;) {
wait.tv_sec = 1;
wait.tv_usec = 0;
/* one second */
FD_ZERO(&read_template);
FD_SET(s1, &read_template);
FD_SET(s2, &read_template);
nb = select(FD_SETSIZE, &read_template, (fd_set *) 0, (fd_set *) 0, &wait);
if (nb <= 0) {
An error occurred during the select, or
the select timed out.
}
if (FD_ISSET(s1, &read_template)) {
Socket #1 is ready to be read from.
}
if (FD_ISSET(s2, &read_template)) {
Socket #2 is ready to be read from.
}
}
In 4.2, the arguments to select were pointers to integers instead of pointers to fd_sets. This type of
call will still work as long as the number of file descriptors being examined is less than the number of bits
in an integer; however, the methods illustrated above should be used in all current programs.
Select provides a synchronous multiplexing scheme. Asynchronous notification of output completion, input availability, and exceptional conditions is possible through use of the SIGIO and SIGURG signals described in section 5.
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3. NETWORK LIBRARY ROUTINES
The discussion in section 2 indicated the possible need to locate and construct network addresses
when using the interprocess communication facilities in a distributed environment. To aid in this task a
number of routines have been added to the standard C run-time library. In this section we will consider the
new routines provided to manipulate network addresses. While the 4.4BSD networking facilities support
the Internet protocols and the Xerox NS protocols, most of the routines presented in this section do not
apply to the NS domain. Unless otherwise stated, it should be assumed that the routines presented in this
section do not apply to the NS domain.
Locating a service on a remote host requires many levels of mapping before client and server may
communicate. A service is assigned a name which is intended for human consumption; e.g. ‘‘the login
server on host monet’’. This name, and the name of the peer host, must then be translated into network
addresses which are not necessarily suitable for human consumption. Finally, the address must then used
in locating a physical location and route to the service. The specifics of these three mappings are likely to
vary between network architectures. For instance, it is desirable for a network to not require hosts to be
named in such a way that their physical location is known by the client host. Instead, underlying services
in the network may discover the actual location of the host at the time a client host wishes to communicate.
This ability to have hosts named in a location independent manner may induce overhead in connection
establishment, as a discovery process must take place, but allows a host to be physically mobile without
requiring it to notify its clientele of its current location.
Standard routines are provided for: mapping host names to network addresses, network names to network numbers, protocol names to protocol numbers, and service names to port numbers and the appropriate
protocol to use in communicating with the server process. The file <netdb.h> must be included when using
any of these routines.
3.1. Host names
An Internet host name to address mapping is represented by the hostent structure:
struct
hostent {
char
*h_name;
char
**h_aliases;
int
h_addrtype;
int
h_length;
char
**h_addr_list;
/* official name of host */
/* alias list */
/* host address type (e.g., AF_INET) */
/* length of address */
/* list of addresses, null terminated */
h_addr h_addr_list[0]
/* first address, network byte order */
};
#define
The routine gethostbyname(3N) takes an Internet host name and returns a hostent structure, while the routine gethostbyaddr(3N) maps Internet host addresses into a hostent structure.
The official name of the host and its public aliases are returned by these routines, along with the
address type (family) and a null terminated list of variable length address. This list of addresses is required
because it is possible for a host to have many addresses, all having the same name. The h_addr definition
is provided for backward compatibility, and is defined to be the first address in the list of addresses in the
hostent structure.
The database for these calls is provided either by the file /etc/hosts (hosts (5)), or by use of a nameserver, named (8). Because of the differences in these databases and their access protocols, the information
returned may differ. When using the host table version of gethostbyname, only one address will be
returned, but all listed aliases will be included. The nameserver version may return alternate addresses, but
will not provide any aliases other than one given as argument.
Unlike Internet names, NS names are always mapped into host addresses by the use of a standard NS
Clearinghouse service, a distributed name and authentication server. The algorithms for mapping NS
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names to addresses via a Clearinghouse are rather complicated, and the routines are not part of the standard
libraries. The user-contributed Courier (Xerox remote procedure call protocol) compiler contains routines
to accomplish this mapping; see the documentation and examples provided therein for more information. It
is expected that almost all software that has to communicate using NS will need to use the facilities of the
Courier compiler.
An NS host address is represented by the following:
union ns_host {
u_char
u_short
};
c_host[6];
s_host[3];
union ns_net {
u_char
u_short
};
c_net[4];
s_net[2];
struct ns_addr {
union ns_net
x_net;
union ns_host
x_host;
u_short
x_port;
};
The following code fragment inserts a known NS address into a ns_addr:
Advanced 4.4BSD IPC Tutorial
PSD:21-13
#include <sys/types.h>
#include <sys/socket.h>
#include <netns/ns.h>
...
u_long netnum;
struct sockaddr_ns dst;
...
bzero((char *)&dst, sizeof(dst));
/*
* There is no convenient way to assign a long
* integer to a ‘‘union ns_net’’ at present; in
* the future, something will hopefully be provided,
* but this is the portable way to go for now.
* The network number below is the one for the NS net
* that the desired host (gyre) is on.
*/
netnum = htonl(2266);
dst.sns_addr.x_net = *(union ns_net *) &netnum;
dst.sns_family = AF_NS;
/*
* host 2.7.1.0.2a.18 == "gyre:Computer Science:UofMaryland"
*/
dst.sns_addr.x_host.c_host[0] = 0x02;
dst.sns_addr.x_host.c_host[1] = 0x07;
dst.sns_addr.x_host.c_host[2] = 0x01;
dst.sns_addr.x_host.c_host[3] = 0x00;
dst.sns_addr.x_host.c_host[4] = 0x2a;
dst.sns_addr.x_host.c_host[5] = 0x18;
dst.sns_addr.x_port = htons(75);
3.2. Network names
As for host names, routines for mapping network names to numbers, and back, are provided. These
routines return a netent structure:
/*
* Assumption here is that a network number
* fits in 32 bits -- probably a poor one.
*/
struct netent {
char
*n_name;
/* official name of net */
char
**n_aliases;
/* alias list */
int
n_addrtype;
/* net address type */
int
n_net;
/* network number, host byte order */
};
The routines getnetbyname(3N), getnetbynumber(3N), and getnetent(3N) are the network counterparts to
the host routines described above. The routines extract their information from /etc/networks.
NS network numbers are determined either by asking your local Xerox Network Administrator (and
hardcoding the information into your code), or by querying the Clearinghouse for addresses. The internetwork router is the only process that needs to manipulate network numbers on a regular basis; if a process
wishes to communicate with a machine, it should ask the Clearinghouse for that machine’s address (which
will include the net number).
PSD:21-14
Advanced 4.4BSD IPC Tutorial
3.3. Protocol names
For protocols, which are defined in /etc/protocols, the protoent structure defines the protocol-name
mapping used with the routines getprotobyname(3N), getprotobynumber(3N), and getprotoent(3N):
struct
protoent {
char
char
int
*p_name;
**p_aliases;
p_proto;
/* official protocol name */
/* alias list */
/* protocol number */
};
In the NS domain, protocols are indicated by the "client type" field of a IDP header. No protocol
database exists; see section 5 for more information.
3.4. Service names
Information regarding services is a bit more complicated. A service is expected to reside at a specific
‘‘port’’ and employ a particular communication protocol. This view is consistent with the Internet domain,
but inconsistent with other network architectures. Further, a service may reside on multiple ports. If this
occurs, the higher level library routines will have to be bypassed or extended. Services available are contained in the file /etc/services. A service mapping is described by the servent structure,
struct
servent {
char
char
int
char
*s_name;
**s_aliases;
s_port;
*s_proto;
/* official service name */
/* alias list */
/* port number, network byte order */
/* protocol to use */
};
The routine getservbyname(3N) maps service names to a servent structure by specifying a service name
and, optionally, a qualifying protocol. Thus the call
sp = getservbyname("telnet", (char *) 0);
returns the service specification for a telnet server using any protocol, while the call
sp = getservbyname("telnet", "tcp");
returns only that telnet server which uses the TCP protocol. The routines getservbyport(3N) and getservent(3N) are also provided. The getservbyport routine has an interface similar to that provided by getservbyname; an optional protocol name may be specified to qualify lookups.
In the NS domain, services are handled by a central dispatcher provided as part of the Courier remote
procedure call facilities. Again, the reader is referred to the Courier compiler documentation and to the
Xerox standard* for further details.
3.5. Miscellaneous
With the support routines described above, an Internet application program should rarely have to deal
directly with addresses. This allows services to be developed as much as possible in a network independent
fashion. It is clear, however, that purging all network dependencies is very difficult. So long as the user is
required to supply network addresses when naming services and sockets there will always some network
dependency in a program. For example, the normal code included in client programs, such as the remote
login program, is of the form shown in Figure 1. (This example will be considered in more detail in section
4.)
If we wanted to make the remote login program independent of the Internet protocols and addressing
scheme we would be forced to add a layer of routines which masked the network dependent aspects from
the mainstream login code. For the current facilities available in the system this does not appear to be
worthwhile.
* Courier: The Remote Procedure Call Protocol, XSIS 038112.
Advanced 4.4BSD IPC Tutorial
PSD:21-15
Aside from the address-related data base routines, there are several other routines available in the
run-time library which are of interest to users. These are intended mostly to simplify manipulation of
names and addresses. Table 1 summarizes the routines for manipulating variable length byte strings and
handling byte swapping of network addresses and values.
Call
bcmp(s1, s2, n)
bcopy(s1, s2, n)
bzero(base, n)
htonl(val)
htons(val)
ntohl(val)
ntohs(val)
Synopsis
compare byte-strings; 0 if same, not 0 otherwise
copy n bytes from s1 to s2
zero-fill n bytes starting at base
convert 32-bit quantity from host to network byte order
convert 16-bit quantity from host to network byte order
convert 32-bit quantity from network to host byte order
convert 16-bit quantity from network to host byte order
Table 1. C run-time routines.
The byte swapping routines are provided because the operating system expects addresses to be supplied in network order (aka ‘‘big-endian’’ order). On ‘‘little-endian’’ architectures, such as Intel x86 and
VAX, host byte ordering is different than network byte ordering. Consequently, programs are sometimes
required to byte swap quantities. The library routines which return network addresses provide them in network order so that they may simply be copied into the structures provided to the system. This implies users
should encounter the byte swapping problem only when interpreting network addresses. For example, if an
Internet port is to be printed out the following code would be required:
printf("port number %d\n", ntohs(sp->s_port));
On machines where unneeded these routines are defined as null macros.
PSD:21-16
Advanced 4.4BSD IPC Tutorial
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <netdb.h>
...
main(argc, argv)
int argc;
char *argv[];
{
struct sockaddr_in server;
struct servent *sp;
struct hostent *hp;
int s;
...
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogin: tcp/login: unknown service\n");
exit(1);
}
hp = gethostbyname(argv[1]);
if (hp == NULL) {
fprintf(stderr, "rlogin: %s: unknown host\n", argv[1]);
exit(2);
}
bzero((char *)&server, sizeof (server));
bcopy(hp->h_addr, (char *)&server.sin_addr, hp->h_length);
server.sin_family = hp->h_addrtype;
server.sin_port = sp->s_port;
s = socket(AF_INET, SOCK_STREAM, 0);
if (s < 0) {
perror("rlogin: socket");
exit(3);
}
...
/* Connect does the bind() for us */
if (connect(s, (char *)&server, sizeof (server)) < 0) {
perror("rlogin: connect");
exit(5);
}
...
}
Figure 1. Remote login client code.
Advanced 4.4BSD IPC Tutorial
PSD:21-17
4. CLIENT/SERVER MODEL
The most commonly used paradigm in constructing distributed applications is the client/server
model. In this scheme client applications request services from a server process. This implies an asymmetry in establishing communication between the client and server which has been examined in section 2. In
this section we will look more closely at the interactions between client and server, and consider some of
the problems in developing client and server applications.
The client and server require a well known set of conventions before service may be rendered (and
accepted). This set of conventions comprises a protocol which must be implemented at both ends of a connection. Depending on the situation, the protocol may be symmetric or asymmetric. In a symmetric protocol, either side may play the master or slave roles. In an asymmetric protocol, one side is immutably recognized as the master, with the other as the slave. An example of a symmetric protocol is the TELNET protocol used in the Internet for remote terminal emulation. An example of an asymmetric protocol is the Internet file transfer protocol, FTP. No matter whether the specific protocol used in obtaining a service is symmetric or asymmetric, when accessing a service there is a ‘‘client process’’ and a ‘‘server process’’. We
will first consider the properties of server processes, then client processes.
A server process normally listens at a well known address for service requests. That is, the server
process remains dormant until a connection is requested by a client’s connection to the server’s address. At
such a time the server process ‘‘wakes up’’ and services the client, performing whatever appropriate actions
the client requests of it.
Alternative schemes which use a service server may be used to eliminate a flock of server processes
clogging the system while remaining dormant most of the time. For Internet servers in 4.4BSD, this
scheme has been implemented via inetd, the so called ‘‘internet super-server.’’ Inetd listens at a variety of
ports, determined at start-up by reading a configuration file. When a connection is requested to a port on
which inetd is listening, inetd executes the appropriate server program to handle the client. With this
method, clients are unaware that an intermediary such as inetd has played any part in the connection. Inetd
will be described in more detail in section 5.
A similar alternative scheme is used by most Xerox services. In general, the Courier dispatch process (if used) accepts connections from processes requesting services of some sort or another. The client
processes request a particular <program number, version number, procedure number> triple. If the dispatcher knows of such a program, it is started to handle the request; if not, an error is reported to the client.
In this way, only one port is required to service a large variety of different requests. Again, the Courier
facilities are not available without the use and installation of the Courier compiler. The information presented in this section applies only to NS clients and services that do not use Courier.
4.1. Servers
In 4.4BSD most servers are accessed at well known Internet addresses or UNIX domain names. For
example, the remote login server’s main loop is of the form shown in Figure 2.
The first step taken by the server is look up its service definition:
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogind: tcp/login: unknown service\n");
exit(1);
}
The result of the getservbyname call is used in later portions of the code to define the Internet port at which
it listens for service requests (indicated by a connection).
PSD:21-18
Advanced 4.4BSD IPC Tutorial
main(argc, argv)
int argc;
char *argv[];
{
int f;
struct sockaddr_in from;
struct servent *sp;
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogind: tcp/login: unknown service\n");
exit(1);
}
...
#ifndef DEBUG
/* Disassociate server from controlling terminal */
...
#endif
sin.sin_port = sp->s_port;
/* Restricted port -- see section 5 */
...
f = socket(AF_INET, SOCK_STREAM, 0);
...
if (bind(f, (struct sockaddr *) &sin, sizeof (sin)) < 0) {
...
}
...
listen(f, 5);
for (;;) {
int g, len = sizeof (from);
g = accept(f, (struct sockaddr *) &from, &len);
if (g < 0) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
if (fork() == 0) {
close(f);
doit(g, &from);
}
close(g);
}
}
Figure 2. Remote login server.
Advanced 4.4BSD IPC Tutorial
PSD:21-19
Step two is to disassociate the server from the controlling terminal of its invoker:
for (i = 0; i < 3; ++i)
close(i);
open("/", O_RDONLY);
dup2(0, 1);
dup2(0, 2);
i = open("/dev/tty", O_RDWR);
if (i >= 0) {
ioctl(i, TIOCNOTTY, 0);
close(i);
}
This step is important as the server will likely not want to receive signals delivered to the process group of
the controlling terminal. Note, however, that once a server has disassociated itself it can no longer send
reports of errors to a terminal, and must log errors via syslog.
Once a server has established a pristine environment, it creates a socket and begins accepting service
requests. The bind call is required to insure the server listens at its expected location. It should be noted
that the remote login server listens at a restricted port number, and must therefore be run with a user-id of
root. This concept of a ‘‘restricted port number’’ is 4BSD specific, and is covered in section 5.
The main body of the loop is fairly simple:
for (;;) {
int g, len = sizeof (from);
g = accept(f, (struct sockaddr *)&from, &len);
if (g < 0) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
if (fork() == 0) { /* Child */
close(f);
doit(g, &from);
}
close(g);
/* Parent */
}
An accept call blocks the server until a client requests service. This call could return a failure status if the
call is interrupted by a signal such as SIGCHLD (to be discussed in section 5). Therefore, the return value
from accept is checked to insure a connection has actually been established, and an error report is logged
via syslog if an error has occurred.
With a connection in hand, the server then forks a child process and invokes the main body of the
remote login protocol processing. Note how the socket used by the parent for queuing connection requests
is closed in the child, while the socket created as a result of the accept is closed in the parent. The address
of the client is also handed the doit routine because it requires it in authenticating clients.
4.2. Clients
The client side of the remote login service was shown earlier in Figure 1. One can see the separate,
asymmetric roles of the client and server clearly in the code. The server is a passive entity, listening for
client connections, while the client process is an active entity, initiating a connection when invoked.
Let us consider more closely the steps taken by the client remote login process. As in the server process, the first step is to locate the service definition for a remote login:
PSD:21-20
Advanced 4.4BSD IPC Tutorial
sp = getservbyname("login", "tcp");
if (sp == NULL) {
fprintf(stderr, "rlogin: tcp/login: unknown service\n");
exit(1);
}
Next the destination host is looked up with a gethostbyname call:
hp = gethostbyname(argv[1]);
if (hp == NULL) {
fprintf(stderr, "rlogin: %s: unknown host\n", argv[1]);
exit(2);
}
With this accomplished, all that is required is to establish a connection to the server at the requested host
and start up the remote login protocol. The address buffer is cleared, then filled in with the Internet address
of the foreign host and the port number at which the login process resides on the foreign host:
bzero((char *)&server, sizeof (server));
bcopy(hp->h_addr, (char *) &server.sin_addr, hp->h_length);
server.sin_family = hp->h_addrtype;
server.sin_port = sp->s_port;
A socket is created, and a connection initiated. Note that connect implicitly performs a bind call, since s is
unbound.
s = socket(hp->h_addrtype, SOCK_STREAM, 0);
if (s < 0) {
perror("rlogin: socket");
exit(3);
}
...
if (connect(s, (struct sockaddr *) &server, sizeof (server)) < 0) {
perror("rlogin: connect");
exit(4);
}
The details of the remote login protocol will not be considered here.
4.3. Connectionless servers
While connection-based services are the norm, some services are based on the use of datagram sockets. One, in particular, is the ‘‘rwho’’ service which provides users with status information for hosts connected to a local area network. This service, while predicated on the ability to broadcast information to all
hosts connected to a particular network, is of interest as an example usage of datagram sockets.
A user on any machine running the rwho server may find out the current status of a machine with the
ruptime(1) program. The output generated is illustrated in Figure 3.
Status information for each host is periodically broadcast by rwho server processes on each machine.
The same server process also receives the status information and uses it to update a database. This database
is then interpreted to generate the status information for each host. Servers operate autonomously, coupled
only by the local network and its broadcast capabilities.
Note that the use of broadcast for such a task is fairly inefficient, as all hosts must process each message, whether or not using an rwho server. Unless such a service is sufficiently universal and is frequently
used, the expense of periodic broadcasts outweighs the simplicity.
Multicasting is an alternative to broadcasting. Setting up multicast sockets is described in Section
5.10.
Advanced 4.4BSD IPC Tutorial
arpa
cad
calder
dali
degas
ear
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esvax
ingres
kim
matisse
medea
merlin
miro
monet
oz
statvax
ucbvax
up
up
up
up
up
up
down
down
down
up
up
up
down
up
up
down
up
up
PSD:21-21
9:45,
2+12:04,
10:10,
2+06:28,
25+09:48,
5+00:05,
0:24
17:04
0:26
3+09:16,
3+06:18,
3+09:39,
19+15:37
1+07:20,
1+00:43,
16:09
2+15:57,
9:34,
5 users, load
8 users, load
0 users, load
9 users, load
0 users, load
0 users, load
1.15,
4.67,
0.27,
1.04,
1.49,
1.51,
1.39,
5.13,
0.15,
1.20,
1.43,
1.54,
1.31
4.59
0.14
1.65
1.41
1.56
8 users, load
0 users, load
2 users, load
2.03,
0.03,
0.35,
2.46,
0.03,
0.37,
3.11
0.05
0.50
7 users, load
2 users, load
4.59,
0.22,
3.28,
0.09,
2.12
0.07
3 users, load
2 users, load
1.52,
6.08,
1.81,
5.16,
1.86
3.28
Figure 3. ruptime output.
The rwho server, in a simplified form, is pictured in Figure 4. There are two separate tasks performed by the server. The first task is to act as a receiver of status information broadcast by other hosts on
the network. This job is carried out in the main loop of the program. Packets received at the rwho port are
interrogated to insure they’ve been sent by another rwho server process, then are time stamped with their
arrival time and used to update a file indicating the status of the host. When a host has not been heard from
for an extended period of time, the database interpretation routines assume the host is down and indicate
such on the status reports. This algorithm is prone to error as a server may be down while a host is actually
up, but serves our current needs.
The second task performed by the server is to supply information regarding the status of its host.
This involves periodically acquiring system status information, packaging it up in a message and broadcasting it on the local network for other rwho servers to hear. The supply function is triggered by a timer and
runs off a signal. Locating the system status information is somewhat involved, but uninteresting. Deciding where to transmit the resultant packet is somewhat problematical, however.
Status information must be broadcast on the local network. For networks which do not support the
notion of broadcast another scheme must be used to simulate or replace broadcasting. One possibility is to
enumerate the known neighbors (based on the status messages received from other rwho servers). This,
unfortunately, requires some bootstrapping information, for a server will have no idea what machines are its
neighbors until it receives status messages from them. Therefore, if all machines on a net are freshly
booted, no machine will have any known neighbors and thus never receive, or send, any status information.
This is the identical problem faced by the routing table management process in propagating routing status
information. The standard solution, unsatisfactory as it may be, is to inform one or more servers of known
neighbors and request that they always communicate with these neighbors. If each server has at least one
neighbor supplied to it, status information may then propagate through a neighbor to hosts which are not
(possibly) directly neighbors. If the server is able to support networks which provide a broadcast capability, as well as those which do not, then networks with an arbitrary topology may share status information*.
It is important that software operating in a distributed environment not have any site-dependent information compiled into it. This would require a separate copy of the server at each host and make maintenance a severe headache. 4.4BSD attempts to isolate host-specific information from applications by pro* One must, however, be concerned about ‘‘loops’’. That is, if a host is connected to multiple networks, it will
receive status information from itself. This can lead to an endless, wasteful, exchange of information.
PSD:21-22
Advanced 4.4BSD IPC Tutorial
main()
{
...
sp = getservbyname("who", "udp");
net = getnetbyname("localnet");
sin.sin_addr = inet_makeaddr(INADDR_ANY, net);
sin.sin_port = sp->s_port;
...
s = socket(AF_INET, SOCK_DGRAM, 0);
...
on = 1;
if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on)) < 0) {
syslog(LOG_ERR, "setsockopt SO_BROADCAST: %m");
exit(1);
}
bind(s, (struct sockaddr *) &sin, sizeof (sin));
...
signal(SIGALRM, onalrm);
onalrm();
for (;;) {
struct whod wd;
int cc, whod, len = sizeof (from);
cc = recvfrom(s, (char *)&wd, sizeof (struct whod), 0,
(struct sockaddr *)&from, &len);
if (cc <= 0) {
if (cc < 0 && errno != EINTR)
syslog(LOG_ERR, "rwhod: recv: %m");
continue;
}
if (from.sin_port != sp->s_port) {
syslog(LOG_ERR, "rwhod: %d: bad from port",
ntohs(from.sin_port));
continue;
}
...
if (!verify(wd.wd_hostname)) {
syslog(LOG_ERR, "rwhod: malformed host name from %x",
ntohl(from.sin_addr.s_addr));
continue;
}
(void) sprintf(path, "%s/whod.%s", RWHODIR, wd.wd_hostname);
whod = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
...
(void) time(&wd.wd_recvtime);
(void) write(whod, (char *)&wd, cc);
(void) close(whod);
}
}
Figure 4. rwho server.
viding system calls which return the necessary information*. A mechanism exists, in the form of an ioctl
* An example of such a system call is the gethostname(2) call which returns the host’s ‘‘official’’ name.
Advanced 4.4BSD IPC Tutorial
PSD:21-23
call, for finding the collection of networks to which a host is directly connected. Further, a local network
broadcasting mechanism has been implemented at the socket level. Combining these two features allows a
process to broadcast on any directly connected local network which supports the notion of broadcasting in a
site independent manner. This allows 4.4BSD to solve the problem of deciding how to propagate status
information in the case of rwho, or more generally in broadcasting: Such status information is broadcast to
connected networks at the socket level, where the connected networks have been obtained via the appropriate ioctl calls. The specifics of such broadcastings are complex, however, and will be covered in section 5.
PSD:21-24
Advanced 4.4BSD IPC Tutorial
5. ADVANCED TOPICS
A number of facilities have yet to be discussed. For most users of the IPC the mechanisms already
described will suffice in constructing distributed applications. However, others will find the need to utilize
some of the features which we consider in this section.
5.1. Out of band data
The stream socket abstraction includes the notion of ‘‘out of band’’ data. Out of band data is a logically independent transmission channel associated with each pair of connected stream sockets. Out of band
data is delivered to the user independently of normal data. The abstraction defines that the out of band data
facilities must support the reliable delivery of at least one out of band message at a time. This message may
contain at least one byte of data, and at least one message may be pending delivery to the user at any one
time. For communications protocols which support only in-band signaling (i.e. the urgent data is delivered
in sequence with the normal data), the system normally extracts the data from the normal data stream and
stores it separately. This allows users to choose between receiving the urgent data in order and receiving it
out of sequence without having to buffer all the intervening data. It is possible to ‘‘peek’’ (via
MSG_PEEK) at out of band data. If the socket has a process group, a SIGURG signal is generated when
the protocol is notified of its existence. A process can set the process group or process id to be informed by
the SIGURG signal via the appropriate fcntl call, as described below for SIGIO. If multiple sockets may
have out of band data awaiting delivery, a select call for exceptional conditions may be used to determine
those sockets with such data pending. Neither the signal nor the select indicate the actual arrival of the outof-band data, but only notification that it is pending.
In addition to the information passed, a logical mark is placed in the data stream to indicate the point
at which the out of band data was sent. The remote login and remote shell applications use this facility to
propagate signals between client and server processes. When a signal flushs any pending output from the
remote process(es), all data up to the mark in the data stream is discarded.
To send an out of band message the MSG_OOB flag is supplied to a send or sendto calls, while to
receive out of band data MSG_OOB should be indicated when performing a recvfrom or recv call. To find
out if the read pointer is currently pointing at the mark in the data stream, the SIOCATMARK ioctl is provided:
ioctl(s, SIOCATMARK, &yes);
If yes is a 1 on return, the next read will return data after the mark. Otherwise (assuming out of band data
has arrived), the next read will provide data sent by the client prior to transmission of the out of band signal. The routine used in the remote login process to flush output on receipt of an interrupt or quit signal is
shown in Figure 5. It reads the normal data up to the mark (to discard it), then reads the out-of-band byte.
A process may also read or peek at the out-of-band data without first reading up to the mark. This is
more difficult when the underlying protocol delivers the urgent data in-band with the normal data, and only
sends notification of its presence ahead of time (e.g., the TCP protocol used to implement streams in the
Internet domain). With such protocols, the out-of-band byte may not yet have arrived when a recv is done
with the MSG_OOB flag. In that case, the call will return an error of EWOULDBLOCK. Worse, there
may be enough in-band data in the input buffer that normal flow control prevents the peer from sending the
urgent data until the buffer is cleared. The process must then read enough of the queued data that the
urgent data may be delivered.
Certain programs that use multiple bytes of urgent data and must handle multiple urgent signals (e.g.,
telnet (1C)) need to retain the position of urgent data within the stream. This treatment is available as a
socket-level option, SO_OOBINLINE; see setsockopt (2) for usage. With this option, the position of urgent
data (the ‘‘mark’’) is retained, but the urgent data immediately follows the mark within the normal data
stream returned without the MSG_OOB flag. Reception of multiple urgent indications causes the mark to
move, but no out-of-band data are lost.
Advanced 4.4BSD IPC Tutorial
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#include <sys/ioctl.h>
#include <sys/file.h>
...
oob()
{
int out = FWRITE, mark;
char waste[BUFSIZ];
/* flush local terminal output */
ioctl(1, TIOCFLUSH, (char *)&out);
for (;;) {
if (ioctl(rem, SIOCATMARK, &mark) < 0) {
perror("ioctl");
break;
}
if (mark)
break;
(void) read(rem, waste, sizeof (waste));
}
if (recv(rem, &mark, 1, MSG_OOB) < 0) {
perror("recv");
...
}
...
}
Figure 5. Flushing terminal I/O on receipt of out of band data.
5.2. Non-Blocking Sockets
It is occasionally convenient to make use of sockets which do not block; that is, I/O requests which
cannot complete immediately and would therefore cause the process to be suspended awaiting completion
are not executed, and an error code is returned. Once a socket has been created via the socket call, it may
be marked as non-blocking by fcntl as follows:
#include <fcntl.h>
...
int s;
...
s = socket(AF_INET, SOCK_STREAM, 0);
...
if (fcntl(s, F_SETFL, FNDELAY) < 0)
perror("fcntl F_SETFL, FNDELAY");
exit(1);
}
...
When performing non-blocking I/O on sockets, one must be careful to check for the error EWOULDBLOCK (stored in the global variable errno), which occurs when an operation would normally block, but
the socket it was performed on is marked as non-blocking. In particular, accept, connect, send, recv, read,
and write can all return EWOULDBLOCK, and processes should be prepared to deal with such return
codes. If an operation such as a send cannot be done in its entirety, but partial writes are sensible (for
example, when using a stream socket), the data that can be sent immediately will be processed, and the
return value will indicate the amount actually sent.
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Advanced 4.4BSD IPC Tutorial
5.3. Interrupt driven socket I/O
The SIGIO signal allows a process to be notified via a signal when a socket (or more generally, a file
descriptor) has data waiting to be read. Use of the SIGIO facility requires three steps: First, the process
must set up a SIGIO signal handler by use of the signal or sigvec calls. Second, it must set the process id or
process group id which is to receive notification of pending input to its own process id, or the process group
id of its process group (note that the default process group of a socket is group zero). This is accomplished
by use of an fcntl call. Third, it must enable asynchronous notification of pending I/O requests with another
fcntl call. Sample code to allow a given process to receive information on pending I/O requests as they
occur for a socket s is given in Figure 6. With the addition of a handler for SIGURG, this code can also be
used to prepare for receipt of SIGURG signals.
#include <fcntl.h>
...
int io_handler();
...
signal(SIGIO, io_handler);
/* Set the process receiving SIGIO/SIGURG signals to us */
if (fcntl(s, F_SETOWN, getpid()) < 0) {
perror("fcntl F_SETOWN");
exit(1);
}
/* Allow receipt of asynchronous I/O signals */
if (fcntl(s, F_SETFL, FASYNC) < 0) {
perror("fcntl F_SETFL, FASYNC");
exit(1);
}
Figure 6. Use of asynchronous notification of I/O requests.
5.4. Signals and process groups
Due to the existence of the SIGURG and SIGIO signals each socket has an associated process number, just as is done for terminals. This value is initialized to zero, b
Advanced 4.4BSD IPC Tutorial
PSD:21-27
int reaper();
...
signal(SIGCHLD, reaper);
listen(f, 5);
for (;;) {
int g, len = sizeof (from);
g = accept(f, (struct sockaddr *)&from, &len,);
if (g < 0) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
...
}
...
#include <wait.h>
reaper()
{
union wait status;
while (wait3(&status, WNOHANG, 0) > 0)
;
}
Figure 7. Use of the SIGCHLD signal.
and slave, which allow a process to serve as an active agent in communication between processes and users.
Data written on the slave side of a pseudo-terminal is supplied as input to a process reading from the master
side, while data written on the master side are processed as terminal input for the slave. In this way, the
process manipulating the master side of the pseudo-terminal has control over the information read and written on the slave side as if it were manipulating the keyboard and reading the screen on a real terminal. The
purpose of this abstraction is to preserve terminal semantics over a network connection— that is, the slave
side appears as a normal terminal to any process reading from or writing to it.
For example, the remote login server uses pseudo-terminals for remote login sessions. A user logging in to a machine across the network is provided a shell with a slave pseudo-terminal as standard input,
output, and error. The server process then handles the communication between the programs invoked by
the remote shell and the user’s local client process. When a user sends a character that generates an interrupt on the remote machine that flushes terminal output, the pseudo-terminal generates a control message
for the server process. The server then sends an out of band message to the client process to signal a flush
of data at the real terminal and on the intervening data buffered in the network.
Under 4.4BSD, the name of the slave side of a pseudo-terminal is of the form /dev/ttyxy, where x is a
single letter starting at ‘p’ and continuing to ‘t’. y is a hexadecimal digit (i.e., a single character in the
range 0 through 9 or ‘a’ through ‘f’). The master side of a pseudo-terminal is /dev/ptyxy, where x and y
correspond to the slave side of the pseudo-terminal.
In general, the method of obtaining a pair of master and slave pseudo-terminals is to find a pseudoterminal which is not currently in use. The master half of a pseudo-terminal is a single-open device; thus,
each master may be opened in turn until an open succeeds. The slave side of the pseudo-terminal is then
opened, and is set to the proper terminal modes if necessary. The process then forks; the child closes the
master side of the pseudo-terminal, and execs the appropriate program. Meanwhile, the parent closes the
slave side of the pseudo-terminal and begins reading and writing from the master side. Sample code making use of pseudo-terminals is given in Figure 8; this code assumes that a connection on a socket s exists,
connected to a peer who wants a service of some kind, and that the process has disassociated itself from any
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Advanced 4.4BSD IPC Tutorial
previous controlling terminal.
gotpty = 0;
for (c = ’p’; !gotpty && c <= ’s’; c++) {
line = "/dev/ptyXX";
line[sizeof("/dev/pty")-1] = c;
line[sizeof("/dev/ptyp")-1] = ’0’;
if (stat(line, &statbuf) < 0)
break;
for (i = 0; i < 16; i++) {
line[sizeof("/dev/ptyp")-1] = "0123456789abcdef"[i];
master = open(line, O_RDWR);
if (master > 0) {
gotpty = 1;
break;
}
}
}
if (!gotpty) {
syslog(LOG_ERR, "All network ports in use");
exit(1);
}
line[sizeof("/dev/")-1] = ’t’;
slave = open(line, O_RDWR); /* slave is now slave side */
if (slave < 0) {
syslog(LOG_ERR, "Cannot open slave pty %s", line);
exit(1);
}
ioctl(slave, TIOCGETP, &b); /* Set slave tty modes */
b.sg_flags = CRMOD|XTABS|ANYP;
ioctl(slave, TIOCSETP, &b);
i = fork();
if (i < 0) {
syslog(LOG_ERR, "fork: %m");
exit(1);
} else if (i) {
/* Parent */
close(slave);
...
} else {
/* Child */
(void) close(s);
(void) close(master);
dup2(slave, 0);
dup2(slave, 1);
dup2(slave, 2);
if (slave > 2)
(void) close(slave);
...
}
Figure 8. Creation and use of a pseudo terminal
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5.6. Selecting specific protocols
If the third argument to the socket call is 0, socket will select a default protocol to use with the
returned socket of the type requested. The default protocol is usually correct, and alternate choices are not
usually available. However, when using ‘‘raw’’ sockets to communicate directly with lower-level protocols
or hardware interfaces, the protocol argument may be important for setting up demultiplexing. For example, raw sockets in the Internet family may be used to implement a new protocol above IP, and the socket
will receive packets only for the protocol specified. To obtain a particular protocol one determines the protocol number as defined within the communication domain. For the Internet domain one may use one of
the library routines discussed in section 3, such as getprotobyname:
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
...
pp = getprotobyname("newtcp");
s = socket(AF_INET, SOCK_STREAM, pp->p_proto);
This would result in a socket s using a stream based connection, but with protocol type of ‘‘newtcp’’ instead
of the default ‘‘tcp.’’
In the NS domain, the available socket protocols are defined in <netns/ns.h>. To create a raw socket
for Xerox Error Protocol messages, one might use:
#include <sys/types.h>
#include <sys/socket.h>
#include <netns/ns.h>
...
s = socket(AF_NS, SOCK_RAW, NSPROTO_ERROR);
5.7. Address binding
As was mentioned in section 2, binding addresses to sockets in the Internet and NS domains can be
fairly complex. As a brief reminder, these associations are composed of local and foreign addresses, and
local and foreign ports. Port numbers are allocated out of separate spaces, one for each system and one for
each domain on that system. Through the bind system call, a process may specify half of an association,
the <local address, local port> part, while the connect and accept primitives are used to complete a socket’s
association by specifying the <foreign address, foreign port> part. Since the association is created in two
steps the association uniqueness requirement indicated previously could be violated unless care is taken.
Further, it is unrealistic to expect user programs to always know proper values to use for the local address
and local port since a host may reside on multiple networks and the set of allocated port numbers is not
directly accessible to a user.
To simplify local address binding in the Internet domain the notion of a ‘‘wildcard’’ address has been
provided. When an address is specified as INADDR_ANY (a manifest constant defined in <netinet/in.h>),
the system interprets the address as ‘‘any valid address’’. For example, to bind a specific port number to a
socket, but leave the local address unspecified, the following code might be used:
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Advanced 4.4BSD IPC Tutorial
#include <sys/types.h>
#include <netinet/in.h>
...
struct sockaddr_in sin;
...
s = socket(AF_INET, SOCK_STREAM, 0);
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sin, sizeof (sin));
Sockets with wildcarded local addresses may receive messages directed to the specified port number, and
sent to any of the possible addresses assigned to a host. For example, if a host has addresses 128.32.0.4 and
10.0.0.78, and a socket is bound as above, the process will be able to accept connection requests which are
addressed to 128.32.0.4 or 10.0.0.78. If a server process wished to only allow hosts on a given network
connect to it, it would bind the address of the host on the appropriate network.
In a similar fashion, a local port may be left unspecified (specified as zero), in which case the system
will select an appropriate port number for it. This shortcut will work both in the Internet and NS domains.
For example, to bind a specific local address to a socket, but to leave the local port number unspecified:
hp = gethostbyname(hostname);
if (hp == NULL) {
...
}
bcopy(hp->h_addr, (char *) sin.sin_addr, hp->h_length);
sin.sin_port = htons(0);
bind(s, (struct sockaddr *) &sin, sizeof (sin));
The system selects the local port number based on two criteria. The first is that on 4BSD systems, Internet
ports below IPPORT_RESERVED (1024) (for the Xerox domain, 0 through 3000) are reserved for privileged users (i.e., the super user); Internet ports above IPPORT_USERRESERVED (50000) are reserved for
non-privileged servers. The second is that the port number is not currently bound to some other socket. In
order to find a free Internet port number in the privileged range the rresvport library routine may be used as
follows to return a stream socket in with a privileged port number:
int lport = IPPORT_RESERVED − 1;
int s;
...
s = rresvport(&lport);
if (s < 0) {
if (errno == EAGAIN)
fprintf(stderr, "socket: all ports in use\n");
else
perror("rresvport: socket");
...
}
The restriction on allocating ports was done to allow processes executing in a ‘‘secure’’ environment to perform authentication based on the originating address and port number. For example, the rlogin(1) command allows users to log in across a network without being asked for a password, if two conditions hold:
First, the name of the system the user is logging in from is in the file /etc/hosts.equiv on the system he is
logging in to (or the system name and the user name are in the user’s .rhosts file in the user’s home directory), and second, that the user’s rlogin process is coming from a privileged port on the machine from
which he is logging. The port number and network address of the machine from which the user is logging
in can be determined either by the from result of the accept call, or from the getpeername call.
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In certain cases the algorithm used by the system in selecting port numbers is unsuitable for an application. This is because associations are created in a two step process. For example, the Internet file transfer protocol, FTP, specifies that data connections must always originate from the same local port. However,
duplicate associations are avoided by connecting to different foreign ports. In this situation the system
would disallow binding the same local address and port number to a socket if a previous data connection’s
socket still existed. To override the default port selection algorithm, an option call must be performed prior
to address binding:
...
int on = 1;
...
setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));
bind(s, (struct sockaddr *) &sin, sizeof (sin));
With the above call, local addresses may be bound which are already in use. This does not violate the
uniqueness requirement as the system still checks at connect time to be sure any other sockets with the
same local address and port do not have the same foreign address and port. If the association already exists,
the error EADDRINUSE is returned. A related socket option, SO_REUSEPORT, which allows completely
duplicate bindings, is described in the IP multicasting section.
5.8. Socket Options
It is possible to set and get a number of options on sockets via the setsockopt and getsockopt system
calls. These options include such things as marking a socket for broadcasting, not to route, to linger on
close, etc. In addition, there are protocol-specific options for IP and TCP, as described in ip(4), tcp(4), and
in the section on multicasting below.
The general forms of the calls are:
setsockopt(s, level, optname, optval, optlen);
and
getsockopt(s, level, optname, optval, optlen);
The parameters to the calls are as follows: s is the socket on which the option is to be applied. Level
specifies the protocol layer on which the option is to be applied; in most cases this is the ‘‘socket level’’,
indicated by the symbolic constant SOL_SOCKET, defined in <sys/socket.h>. The actual option is specified in optname, and is a symbolic constant also defined in <sys/socket.h>. Optval and Optlen point to the
value of the option (in most cases, whether the option is to be turned on or off), and the length of the value
of the option, respectively. For getsockopt, optlen is a value-result parameter, initially set to the size of the
storage area pointed to by optval, and modified upon return to indicate the actual amount of storage used.
An example should help clarify things. It is sometimes useful to determine the type (e.g., stream,
datagram, etc.) of an existing socket; programs under inetd (described below) may need to perform this
task. This can be accomplished as follows via the SO_TYPE socket option and the getsockopt call:
#include <sys/types.h>
#include <sys/socket.h>
int type, size;
size = sizeof (int);
if (getsockopt(s, SOL_SOCKET, SO_TYPE, (char *) &type, &size) < 0) {
...
}
After the getsockopt call, type will be set to the value of the socket type, as defined in <sys/socket.h>. If,
for example, the socket were a datagram socket, type would have the value corresponding to
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SOCK_DGRAM.
5.9. Broadcasting and determining network configuration
By using a datagram socket, it is possible to send broadcast packets on many networks supported by
the system. The network itself must support broadcast; the system provides no simulation of broadcast in
software. Broadcast messages can place a high load on a network since they force every host on the network to service them. Consequently, the ability to send broadcast packets has been limited to sockets
which are explicitly marked as allowing broadcasting. Broadcast is typically used for one of two reasons: it
is desired to find a resource on a local network without prior knowledge of its address, or important functions such as routing require that information be sent to all accessible neighbors.
Multicasting is an alternative to broadcasting. Setting up IP multicast sockets is described in the next
section.
To send a broadcast message, a datagram socket should be created:
s = socket(AF_INET, SOCK_DGRAM, 0);
or
s = socket(AF_NS, SOCK_DGRAM, 0);
The socket is marked as allowing broadcasting,
int
on = 1;
setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof (on));
and at least a port number should be bound to the socket:
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sin, sizeof (sin));
or, for the NS domain,
sns.sns_family = AF_NS;
netnum = htonl(net);
sns.sns_addr.x_net = *(union ns_net *) &netnum; /* insert net number */
sns.sns_addr.x_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sns, sizeof (sns));
The destination address of the message to be broadcast depends on the network(s) on which the message is
to be broadcast. The Internet domain supports a shorthand notation for broadcast on the local network, the
address INADDR_BROADCAST (defined in <netinet/in.h>. To determine the list of addresses for all
reachable neighbors requires knowledge of the networks to which the host is connected. Since this information should be obtained in a host-independent fashion and may be impossible to derive, 4.4BSD provides
a method of retrieving this information from the system data structures. The SIOCGIFCONF ioctl call
returns the interface configuration of a host in the form of a single ifconf structure; this structure contains a
‘‘data area’’ which is made up of an array of of ifreq structures, one for each network interface to which the
host is connected. These structures are defined in <net/if.h> as follows:
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struct ifconf {
int
ifc_len;
union {
caddr_t ifcu_buf;
struct ifreq *ifcu_req;
} ifc_ifcu;
};
/* size of associated buffer */
#define ifc_buf ifc_ifcu.ifcu_buf
#define ifc_req ifc_ifcu.ifcu_req
#define IFNAMSIZ
/* buffer address */
/* array of structures returned */
16
struct ifreq {
char
ifr_name[IFNAMSIZ];
union {
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
struct sockaddr ifru_broadaddr;
short
ifru_flags;
caddr_t ifru_data;
} ifr_ifru;
};
#define
#define
#define
#define
#define
ifr_addr
ifr_dstaddr
ifr_broadaddr
ifr_flags
ifr_data
ifr_ifru.ifru_addr
ifr_ifru.ifru_dstaddr
ifr_ifru.ifru_broadaddr
ifr_ifru.ifru_flags
ifr_ifru.ifru_data
/* if name, e.g. "en0" */
/* address */
/* other end of p-to-p link */
/* broadcast address */
/* flags */
/* for use by interface */
The actual call which obtains the interface configuration is
struct ifconf ifc;
char buf[BUFSIZ];
ifc.ifc_len = sizeof (buf);
ifc.ifc_buf = buf;
if (ioctl(s, SIOCGIFCONF, (char *) &ifc) < 0) {
...
}
After this call buf will contain one ifreq structure for each network to which the host is connected, and
ifc.ifc_len will have been modified to reflect the number of bytes used by the ifreq structures.
For each structure there exists a set of ‘‘interface flags’’ which tell whether the network corresponding to that interface is up or down, point to point or broadcast, etc. The SIOCGIFFLAGS ioctl retrieves
these flags for an interface specified by an ifreq structure as follows:
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struct ifreq *ifr;
ifr = ifc.ifc_req;
for (n = ifc.ifc_len / sizeof (struct ifreq); --n >= 0; ifr++) {
/*
* We must be careful that we don’t use an interface
* devoted to an address family other than those intended;
* if we were interested in NS interfaces, the
* AF_INET would be AF_NS.
*/
if (ifr->ifr_addr.sa_family != AF_INET)
continue;
if (ioctl(s, SIOCGIFFLAGS, (char *) ifr) < 0) {
...
}
/*
* Skip boring cases.
*/
if ((ifr->ifr_flags & IFF_UP) == 0 ||
(ifr->ifr_flags & IFF_LOOPBACK) ||
(ifr->ifr_flags & (IFF_BROADCAST | IFF_POINTTOPOINT)) == 0)
continue;
Once the flags have been obtained, the broadcast address must be obtained. In the case of broadcast
networks this is done via the SIOCGIFBRDADDR ioctl, while for point-to-point networks the address of
the destination host is obtained with SIOCGIFDSTADDR.
struct sockaddr dst;
if (ifr->ifr_flags & IFF_POINTTOPOINT) {
if (ioctl(s, SIOCGIFDSTADDR, (char *) ifr) < 0) {
...
}
bcopy((char *) ifr->ifr_dstaddr, (char *) &dst, sizeof (ifr->ifr_dstaddr));
} else if (ifr->ifr_flags & IFF_BROADCAST) {
if (ioctl(s, SIOCGIFBRDADDR, (char *) ifr) < 0) {
...
}
bcopy((char *) ifr->ifr_broadaddr, (char *) &dst, sizeof (ifr->ifr_broadaddr));
}
After the appropriate ioctl’s have obtained the broadcast or destination address (now in dst), the
sendto call may be used:
sendto(s, buf, buflen, 0, (struct sockaddr *)&dst, sizeof (dst));
}
In the above loop one sendto occurs for every interface to which the host is connected that supports the
notion of broadcast or point-to-point addressing. If a process only wished to send broadcast messages on a
given network, code similar to that outlined above would be used, but the loop would need to find the correct destination address.
Received broadcast messages contain the senders address and port, as datagram sockets are bound
before a message is allowed to go out.
Advanced 4.4BSD IPC Tutorial
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5.10. IP Multicasting
IP multicasting is the transmission of an IP datagram to a "host group", a set of zero or more hosts
identified by a single IP destination address. A multicast datagram is delivered to all members of its destination host group with the same "best-efforts" reliability as regular unicast IP datagrams, i.e., the datagram
is not guaranteed to arrive intact at all members of the destination group or in the same order relative to
other datagrams.
The membership of a host group is dynamic; that is, hosts may join and leave groups at any time.
There is no restriction on the location or number of members in a host group. A host may be a member of
more than one group at a time. A host need not be a member of a group to send datagrams to it.
A host group may be permanent or transient. A permanent group has a well-known, administratively
assigned IP address. It is the address, not the membership of the group, that is permanent; at any time a
permanent group may have any number of members, even zero. Those IP multicast addresses that are not
reserved for permanent groups are available for dynamic assignment to transient groups which exist only as
long as they have members.
In general, a host cannot assume that datagrams sent to any host group address will reach only the
intended hosts, or that datagrams received as a member of a transient host group are intended for the recipient. Misdelivery must be detected at a level above IP, using higher-level identifiers or authentication
tokens. Information transmitted to a host group address should be encrypted or governed by administrative
routing controls if the sender is concerned about unwanted listeners.
IP multicasting is currently supported only on AF_INET sockets of type SOCK_DGRAM and
SOCK_RAW, and only on subnetworks for which the interface driver has been modified to support multicasting.
The next subsections describe how to send and receive multicast datagrams.
5.10.1. Sending IP Multicast Datagrams
To send a multicast datagram, specify an IP multicast address in the range 224.0.0.0 to
239.255.255.255 as the destination address in a sendto(2) call.
The definitions required for the multicast-related socket options are found in <netinet/in.h>. All IP
addresses are passed in network byte-order.
By default, IP multicast datagrams are sent with a time-to-live (TTL) of 1, which prevents them from
being forwarded beyond a single subnetwork. A new socket option allows the TTL for subsequent multicast datagrams to be set to any value from 0 to 255, in order to control the scope of the multicasts:
u_char ttl;
setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl));
Multicast datagrams with a TTL of 0 will not be transmitted on any subnet, but may be delivered locally if
the sending host belongs to the destination group and if multicast loopback has not been disabled on the
sending socket (see below). Multicast datagrams with TTL greater than one may be delivered to more than
one subnet if there are one or more multicast routers attached to the first-hop subnet. To provide meaningful scope control, the multicast routers support the notion of TTL "thresholds", which prevent datagrams
with less than a certain TTL from traversing certain subnets. The thresholds enforce the following convention:
Scope
restricted to the same host
restricted to the same subnet
restricted to the same site
restricted to the same region
restricted to the same continent
unrestricted
Initial TTL
0
1
32
64
128
255
"Sites" and "regions" are not strictly defined, and sites may be further subdivided into smaller
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administrative units, as a local matter.
An application may choose an initial TTL other than the ones listed above. For example, an application might perform an "expanding-ring search" for a network resource by sending a multicast query, first
with a TTL of 0, and then with larger and larger TTLs, until a reply is received, perhaps using the TTL
sequence 0, 1, 2, 4, 8, 16, 32.
The multicast router mrouted(8), refuses to forward any multicast datagram with a destination
address between 224.0.0.0 and 224.0.0.255, inclusive, regardless of its TTL. This range of addresses is
reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols,
such as gateway discovery and group membership reporting.
The address 224.0.0.0 is guaranteed not to be assigned to any group, and 224.0.0.1 is assigned to the
permanent group of all IP hosts (including gateways). This is used to address all multicast hosts on the
directly connected network. There is no multicast address (or any other IP address) for all hosts on the total
Internet. The addresses of other well-known, permanent groups are published in the "Assigned Numbers"
RFC, which is available from the InterNIC.
Each multicast transmission is sent from a single network interface, even if the host has more than
one multicast-capable interface. (If the host is also serving as a multicast router, a multicast may be forwarded to interfaces other than originating interface, provided that the TTL is greater than 1.) The default
interface to be used for multicasting is the primary network interface on the system. A socket option is
available to override the default for subsequent transmissions from a given socket:
struct in_addr addr;
setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF, &addr, sizeof(addr));
where "addr" is the local IP address of the desired outgoing interface. An address of INADDR_ANY may
be used to revert to the default interface. The local IP address of an interface can be obtained via the
SIOCGIFCONF ioctl. To determine if an interface supports multicasting, fetch the interface flags via the
SIOCGIFFLAGS ioctl and see if the IFF_MULTICAST flag is set. (Normal applications should not need
to use this option; it is intended primarily for multicast routers and other system services specifically concerned with internet topology.) The SIOCGIFCONF and SIOCGIFFLAGS ioctls are described in the previous section.
If a multicast datagram is sent to a group to which the sending host itself belongs (on the outgoing
interface), a copy of the datagram is, by default, looped back by the IP layer for local delivery. Another
socket option gives the sender explicit control over whether or not subsequent datagrams are looped back:
u_char loop;
setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop));
where loop is set to 0 to disable loopback, and set to 1 to enable loopback. This option improves performance for applications that may have no more than one instance on a single host (such as a router demon),
by eliminating the overhead of receiving their own transmissions. It should generally not be used by applications for which there may be more than one instance on a single host (such as a conferencing program) or
for which the sender does not belong to the destination group (such as a time querying program).
A multicast datagram sent with an initial TTL greater than 1 may be delivered to the sending host on
a different interface from that on which it was sent, if the host belongs to the destination group on that other
interface. The loopback control option has no effect on such delivery.
5.10.2. Receiving IP Multicast Datagrams
Before a host can receive IP multicast datagrams, it must become a member of one or more IP multicast groups. A process can ask the host to join a multicast group by using the following socket option:
struct ip_mreq mreq;
setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, sizeof(mreq))
where "mreq" is the following structure:
Advanced 4.4BSD IPC Tutorial
PSD:21-37
struct ip_mreq {
struct in_addr imr_multiaddr; /* multicast group to join */
struct in_addr imr_interface; /* interface to join on */
}
Every membership is associated with a single interface, and it is possible to join the same group on more
than one interface. "imr_interface" should be INADDR_ANY to choose the default multicast interface, or
one of the host’s local addresses to choose a particular (multicast-capable) interface. Up to
IP_MAX_MEMBERSHIPS (currently 20) memberships may be added on a single socket.
To drop a membership, use:
struct ip_mreq mreq;
setsockopt(sock, IPPROTO_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq));
where "mreq" contains the same values as used to add the membership. The memberships associated with
a socket are also dropped when the socket is closed or the process holding the socket is killed. However,
more than one socket may claim a membership in a particular group, and the host will remain a member of
that group until the last claim is dropped.
The memberships associated with a socket do not necessarily determine which datagrams are
received on that socket. Incoming multicast packets are accepted by the kernel IP layer if any socket has
claimed a membership in the destination group of the datagram; however, delivery of a multicast datagram
to a particular socket is based on the destination port (or protocol type, for raw sockets), just as with unicast
datagrams. To receive multicast datagrams sent to a particular port, it is necessary to bind to that local port,
leaving the local address unspecified (i.e., INADDR_ANY). To receive multicast datagrams sent to a particular group and port, bind to the local port, with the local address set to the multicast group address. Once
bound to a multicast address, the socket cannot be used for sending data.
More than one process may bind to the same SOCK_DGRAM UDP port or the same multicast group
and port if the bind call is preceded by:
int on = 1;
setsockopt(sock, SOL_SOCKET, SO_REUSEPORT, &on, sizeof(on));
All processes sharing the port must enable this option. Every incoming multicast or broadcast UDP datagram destined to the shared port is delivered to all sockets bound to the port. For backwards compatibility
reasons, this does not apply to incoming unicast datagrams. Unicast datagrams are never delivered to more
than one socket, regardless of how many sockets are bound to the datagram’s destination port.
A final multicast-related extension is independent of IP: two new ioctls, SIOCADDMULTI and
SIOCDELMULTI, are available to add or delete link-level (e.g., Ethernet) multicast addresses accepted by
a particular interface. The address to be added or deleted is passed as a sockaddr structure of family
AF_UNSPEC, within the standard ifreq structure.
These ioctls are for the use of protocols other than IP, and require superuser privileges. A link-level
multicast address added via SIOCADDMULTI is not automatically deleted when the socket used to add it
goes away; it must be explicitly deleted. It is inadvisable to delete a link-level address that may be in use
by IP.
5.10.3. Sample Multicast Program
The following program sends or receives multicast packets. If invoked with one argument, it sends a
packet containing the current time to an arbitrarily-chosen multicast group and UDP port. If invoked with
no arguments, it receives and prints these packets. Start it as a sender on just one host and as a receiver on
all the other hosts.
PSD:21-38
Advanced 4.4BSD IPC Tutorial
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include <stdio.h>
#define EXAMPLE_PORT 60123
#define EXAMPLE_GROUP "224.0.0.250"
main(argc)
int argc;
{
struct sockaddr_in addr;
int addrlen, fd, cnt;
struct ip_mreq mreq;
char message[50];
fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0) {
perror("socket");
exit(1);
}
bzero(&addr, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = htonl(INADDR_ANY);
addr.sin_port = htons(EXAMPLE_PORT);
addrlen = sizeof(addr);
if (argc > 1) { /* Send */
addr.sin_addr.s_addr = inet_addr(EXAMPLE_GROUP);
while (1) {
time_t t = time(0);
sprintf(message, "time is %-24.24s", ctime(&t));
cnt = sendto(fd, message, sizeof(message), 0,
(struct sockaddr *)&addr, addrlen);
if (cnt < 0) {
perror("sendto");
exit(1);
}
sleep(5);
}
} else {
/* Receive */
if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
perror("bind");
exit(1);
}
mreq.imr_multiaddr.s_addr = inet_addr(EXAMPLE_GROUP);
mreq.imr_interface.s_addr = htonl(INADDR_ANY);
if (setsockopt(fd, IPPROTO_IP, IP_ADD_MEMBERSHIP,
&mreq, sizeof(mreq)) < 0) {
perror("setsockopt mreq");
Advanced 4.4BSD IPC Tutorial
PSD:21-39
exit(1);
}
while (1) {
cnt = recvfrom(fd, message, sizeof(message), 0,
(struct sockaddr *)&addr, &addrlen);
if (cnt <= 0) {
if (cnt == 0) {
break;
}
perror("recvfrom");
exit(1);
}
printf("%s: message = \"%s\"\n",
inet_ntoa(addr.sin_addr), message);
}
}
}
5.11. NS Packet Sequences
The semantics of NS connections demand that the user both be able to look inside the network header
associated with any incoming packet and be able to specify what should go in certain fields of an outgoing
packet. Using different calls to setsockopt, it is possible to indicate whether prototype headers will be associated by the user with each outgoing packet (SO_HEADERS_ON_OUTPUT), to indicate whether the
headers received by the system should be delivered to the user (SO_HEADERS_ON_INPUT), or to indicate default information that should be associated with all outgoing packets on a given socket
(SO_DEFAULT_HEADERS).
The contents of a SPP header (minus the IDP header) are:
struct sphdr {
u_char
#define SP_SP
#define SP_SA
#define SP_OB
#define SP_EM
u_char
u_short
u_short
u_short
u_short
u_short
};
sp_cc;
0x80
0x40
0x20
0x10
sp_dt;
sp_sid;
sp_did;
sp_seq;
sp_ack;
sp_alo;
/* connection control */
/* system packet */
/* send acknowledgement */
/* attention (out of band data) */
/* end of message */
/* datastream type */
/* source connection identifier */
/* destination connection identifier */
/* sequence number */
/* acknowledge number */
/* allocation number */
Here, the items of interest are the datastream type and the connection control fields. The semantics of the
datastream type are defined by the application(s) in question; the value of this field is, by default, zero, but
it can be used to indicate things such as Xerox’s Bulk Data Transfer Protocol (in which case it is set to one).
The connection control field is a mask of the flags defined just below it. The user may set or clear the endof-message bit to indicate that a given message is the last of a given substream type, or may set/clear the
attention bit as an alternate way to indicate that a packet should be sent out-of-band. As an example, to
associate prototype headers with outgoing SPP packets, consider:
PSD:21-40
Advanced 4.4BSD IPC Tutorial
#include <sys/types.h>
#include <sys/socket.h>
#include <netns/ns.h>
#include <netns/sp.h>
...
struct sockaddr_ns sns, to;
int s, on = 1;
struct databuf {
struct sphdr proto_spp; /* prototype header */
char buf[534];
/* max. possible data by Xerox std. */
} buf;
...
s = socket(AF_NS, SOCK_SEQPACKET, 0);
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_OUTPUT, &on, sizeof(on));
...
buf.proto_spp.sp_dt = 1; /* bulk data */
buf.proto_spp.sp_cc = SP_EM;
/* end-of-message */
strcpy(buf.buf, "hello world\n");
sendto(s, (char *) &buf, sizeof(struct sphdr) + strlen("hello world\n"),
(struct sockaddr *) &to, sizeof(to));
...
Note that one must be careful when writing headers; if the prototype header is not written with the data
with which it is to be associated, the kernel will treat the first few bytes of the data as the header, with
unpredictable results. To turn off the above association, and to indicate that packet headers received by the
system should be passed up to the user, one might use:
#include <sys/types.h>
#include <sys/socket.h>
#include <netns/ns.h>
#include <netns/sp.h>
...
struct sockaddr sns;
int s, on = 1, off = 0;
...
s = socket(AF_NS, SOCK_SEQPACKET, 0);
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_OUTPUT, &off, sizeof(off));
setsockopt(s, NSPROTO_SPP, SO_HEADERS_ON_INPUT, &on, sizeof(on));
...
Output is handled somewhat differently in the IDP world. The header of an IDP-level packet looks
like:
struct idp {
u_short
u_short
u_char
u_char
struct ns_addr
struct ns_addr
};
idp_sum;
idp_len;
idp_tc;
idp_pt;
idp_dna;
idp_sna;
/* Checksum */
/* Length, in bytes, including header */
/* Transport Control (i.e., hop count) */
/* Packet Type (i.e., level 2 protocol) */
/* Destination Network Address */
/* Source Network Address */
The primary field of interest in an IDP header is the packet type field. The standard values for this field are
Advanced 4.4BSD IPC Tutorial
PSD:21-41
(as defined in <netns/ns.h>):
#define NSPROTO_RI
#define NSPROTO_ECHO
#define NSPROTO_ERROR
#define NSPROTO_PE
#define NSPROTO_SPP
1
2
3
4
5
/* Routing Information */
/* Echo Protocol */
/* Error Protocol */
/* Packet Exchange */
/* Sequenced Packet */
For SPP connections, the contents of this field are automatically set to NSPROTO_SPP; for IDP packets,
this value defaults to zero, which means ‘‘unknown’’.
Setting the value of that field with SO_DEFAULT_HEADERS is easy:
#include <sys/types.h>
#include <sys/socket.h>
#include <netns/ns.h>
#include <netns/idp.h>
...
struct sockaddr sns;
struct idp proto_idp;
/* prototype header */
int s, on = 1;
...
s = socket(AF_NS, SOCK_DGRAM, 0);
...
bind(s, (struct sockaddr *) &sns, sizeof (sns));
proto_idp.idp_pt = NSPROTO_PE; /* packet exchange */
setsockopt(s, NSPROTO_IDP, SO_DEFAULT_HEADERS, (char *) &proto_idp,
sizeof(proto_idp));
...
Using SO_HEADERS_ON_OUTPUT is somewhat more difficult. When SO_HEADERS_ON_OUTPUT is turned on for an IDP socket, the socket becomes (for all intents and purposes) a raw
socket. In this case, all the fields of the prototype header (except the length and checksum fields, which are
computed by the kernel) must be filled in correctly in order for the socket to send and receive data in a sensible manner. To be more specific, the source address must be set to that of the host sending the data; the
destination address must be set to that of the host for whom the data is intended; the packet type must be set
to whatever value is desired; and the hopcount must be set to some reasonable value (almost always zero).
It should also be noted that simply sending data using write will not work unless a connect or sendto call is
used, in spite of the fact that it is the destination address in the prototype header that is used, not the one
given in either of those calls. For almost all IDP applications , using SO_DEFAULT_HEADERS is easier
and more desirable than writing headers.
5.12. Three-way Handshake
The semantics of SPP connections indicates that a three-way handshake, involving changes in the
datastream type, should — but is not absolutely required to — take place before a SPP connection is closed.
Almost all SPP connections are ‘‘well-behaved’’ in this manner; when communicating with any process, it
is best to assume that the three-way handshake is required unless it is known for certain that it is not
required. In a three-way close, the closing process indicates that it wishes to close the connection by sending a zero-length packet with end-of-message set and with datastream type 254. The other side of the connection indicates that it is OK to close by sending a zero-length packet with end-of-message set and datastream type 255. Finally, the closing process replies with a zero-length packet with substream type 255; at
this point, the connection is considered closed. The following code fragments are simplified examples of
how one might handle this three-way handshake at the user level; in the future, support for this type of close
will probably be provided as part of the C library or as part of the kernel. The first code fragment below
illustrates how a process might handle three-way handshake if it sees that the process it is communicating
with wants to close the connection:
PSD:21-42
Advanced 4.4BSD IPC Tutorial
#include <sys/types.h>
#include <sys/socket.h>
#include <netns/ns.h>
#include <netns/sp.h>
...
#ifndef SPPSST_END
#define SPPSST_END 254
#define SPPSST_ENDREPLY 255
#endif
struct sphdr proto_sp;
int s;
...
read(s, buf, BUFSIZE);
if (((struct sphdr *)buf)->sp_dt == SPPSST_END) {
/*
* SPPSST_END indicates that the other side wants to
* close.
*/
proto_sp.sp_dt = SPPSST_ENDREPLY;
proto_sp.sp_cc = SP_EM;
setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
sizeof(proto_sp));
write(s, buf, 0);
/*
* Write a zero-length packet with datastream type = SPPSST_ENDREPLY
* to indicate that the close is OK with us. The packet that we
* don’t see (because we don’t look for it) is another packet
* from the other side of the connection, with SPPSST_ENDREPLY
* on it it, too. Once that packet is sent, the connection is
* considered closed; note that we really ought to retransmit
* the close for some time if we do not get a reply.
*/
close(s);
}
...
To indicate to another process that we would like to close the connection, the following code would suffice:
Advanced 4.4BSD IPC Tutorial
PSD:21-43
#include <sys/types.h>
#include <sys/socket.h>
#include <netns/ns.h>
#include <netns/sp.h>
...
#ifndef SPPSST_END
#define SPPSST_END 254
#define SPPSST_ENDREPLY 255
#endif
struct sphdr proto_sp;
int s;
...
proto_sp.sp_dt = SPPSST_END;
proto_sp.sp_cc = SP_EM;
setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
sizeof(proto_sp));
write(s, buf, 0); /* send the end request */
proto_sp.sp_dt = SPPSST_ENDREPLY;
setsockopt(s, NSPROTO_SPP, SO_DEFAULT_HEADERS, (char *)&proto_sp,
sizeof(proto_sp));
/*
* We assume (perhaps unwisely)
* that the other side will send the
* ENDREPLY, so we’ll just send our final ENDREPLY
* as if we’d seen theirs already.
*/
write(s, buf, 0);
close(s);
...
5.13. Packet Exchange
The Xerox standard protocols include a protocol that is both reliable and datagram-oriented. This
protocol is known as Packet Exchange (PEX or PE) and, like SPP, is layered on top of IDP. PEX is important for a number of things: Courier remote procedure calls may be expedited through the use of PEX, and
many Xerox servers are located by doing a PEX ‘‘BroadcastForServers’’ operation. Although there is no
implementation of PEX in the kernel, it may be simulated at the user level with some clever coding and the
use of one peculiar getsockopt. A PEX packet looks like:
/*
* The packet-exchange header shown here is not defined
* as part of any of the system include files.
*/
struct pex {
struct idp p_idp;
/* idp header */
u_short ph_id[2];
/* unique transaction ID for pex */
u_short ph_client;
/* client type field for pex */
};
The ph_id field is used to hold a ‘‘unique id’’ that is used in duplicate suppression; the ph_client field indicates the PEX client type (similar to the packet type field in the IDP header). PEX reliability stems from
the fact that it is an idempotent (‘‘I send a packet to you, you send a packet to me’’) protocol. Processes on
each side of the connection may use the unique id to determine if they have seen a given packet before (the
unique id field differs on each packet sent) so that duplicates may be detected, and to indicate which message a given packet is in response to. If a packet with a given unique id is sent and no response is received
in a given amount of time, the packet is retransmitted until it is decided that no response will ever be
PSD:21-44
Advanced 4.4BSD IPC Tutorial
received. To simulate PEX, one must be able to generate unique ids -- something that is hard to do at the
user level with any real guarantee that the id is really unique. Therefore, a means (via getsockopt) has been
provided for getting unique ids from the kernel. The following code fragment indicates how to get a unique
id:
long uniqueid;
int s, idsize = sizeof(uniqueid);
...
s = socket(AF_NS, SOCK_DGRAM, 0);
...
/* get id from the kernel -- only on IDP sockets */
getsockopt(s, NSPROTO_PE, SO_SEQNO, (char *)&uniqueid, &idsize);
...
The retransmission and duplicate suppression code required to simulate PEX fully is left as an exercise for
the reader.
5.14. Inetd
One of the daemons provided with 4.4BSD is inetd, the so called ‘‘internet super-server.’’ Having
one daemon listen for requests for many daemons instead of having each daemon listen for its own requests
reduces the number of idle daemons and simplies their implementation. Inetd handles two types of services: standard and TCPMUX. A standard service has a well-known port assigned to it and is listed in
/etc/services (see services(5)); it may be a service that implements an official Internet standard or is a BSDspecific service. TCPMUX services are nonstandard and do not have a well-known port assigned to them.
They are invoked from inetd when a program connects to the "tcpmux" well-known port and specifies the
service name. This is useful for adding locally-developed servers.
Inetd is invoked at boot time, and determines from the file /etc/inetd.conf the servers for which it is to
listen. Once this information has been read and a pristine environment created, inetd proceeds to create one
socket for each service it is to listen for, binding the appropriate port number to each socket.
Inetd then performs a select on all these sockets for read availability, waiting for somebody wishing a
connection to the service corresponding to that socket. Inetd then performs an accept on the socket in question, forks, dups the new socket to file descriptors 0 and 1 (stdin and stdout), closes other open file descriptors, and execs the appropriate server.
Servers making use of inetd are considerably simplified, as inetd takes care of the majority of the IPC
work required in establishing a connection. The server invoked by inetd expects the socket connected to its
client on file descriptors 0 and 1, and may immediately perform any operations such as read, write, send, or
recv. Indeed, servers may use buffered I/O as provided by the ‘‘stdio’’ conventions, as long as as they
remember to use fflush when appropriate.
One call which may be of interest to individuals writing servers under inetd is the getpeername call,
which returns the address of the peer (process) connected on the other end of the socket. For example, to
log the Internet address in ‘‘dot notation’’ (e.g., ‘‘128.32.0.4’’) of a client connected to a server under inetd,
the following code might be used:
struct sockaddr_in name;
int namelen = sizeof (name);
...
if (getpeername(0, (struct sockaddr *)&name, &namelen) < 0) {
syslog(LOG_ERR, "getpeername: %m");
exit(1);
} else
syslog(LOG_INFO, "Connection from %s", inet_ntoa(name.sin_addr));
...
While the getpeername call is especially useful when writing programs to run with inetd, it can be used
under other circumstances. Be warned, however, that getpeername will fail on UNIX domain sockets.
Advanced 4.4BSD IPC Tutorial
PSD:21-45
Standard TCP services are assigned unique well-known port numbers in the range of 0 to 1023 by the
Internet Assigned Numbers Authority ([email protected]). The limited number of ports in this range are
assigned to official Internet protocols. The TCPMUX service allows you to add locally-developed protocols without needing an official TCP port assignment. The TCPMUX protocol described in RFC-1078 is
simple:
‘‘A TCP client connects to a foreign host on TCP port 1. It sends the service name followed by
a carriage-return line-feed <CRLF>. The service name is never case sensitive. The server
replies with a single character indicating positive ("+") or negative ("−") acknowledgment,
immediately followed by an optional message of explanation, terminated with a <CRLF>. If
the reply was positive, the selected protocol begins; otherwise the connection is closed.’’
In 4.4BSD, the TCPMUX service is built into inetd, that is, inetd listens on TCP port 1 for requests for
TCPMUX services listed in inetd.conf. inetd(8) describes the format of TCPMUX entries for inetd.conf.
The following is an example TCPMUX server and its inetd.conf entry. More sophisticated servers
may want to do additional processing before returning the positive or negative acknowledgement.
#include <sys/types.h>
#include <stdio.h>
main()
{
time_t t;
printf("+Go\r\n");
fflush(stdout);
time(&t);
printf("%d = %s", t, ctime(&t));
fflush(stdout);
}
The inetd.conf entry is:
tcpmux/current_time stream tcp nowait nobody /d/curtime curtime
Here’s the portion of the client code that handles the TCPMUX handshake:
PSD:21-46
char line[BUFSIZ];
FILE *fp;
...
/* Use stdio for reading data from the server */
fp = fdopen(sock, "r");
if (fp == NULL) {
fprintf(stderr, "Can’t create file pointer\n");
exit(1);
}
/* Send service request */
sprintf(line, "%s\r\n", "current_time");
if (write(sock, line, strlen(line)) < 0) {
perror("write");
exit(1);
}
/* Get ACK/NAK response from the server */
if (fgets(line, sizeof(line), fp) == NULL) {
if (feof(fp)) {
die();
} else {
fprintf(stderr, "Error reading response\n");
exit(1);
}
}
/* Delete <CR> */ ’)) != NULL) {
if ((lp = index(line, ’
*lp = ’ ’;
}
switch (line[0]) {
case ’+’:
printf("Got ACK: %s\n", &line[1]);
break;
case ’-’:
printf("Got NAK: %s\n", &line[1]);
exit(0);
default:
printf("Got unknown response: %s\n", line);
exit(1);
}
/* Get rest of data from the server */
while ((fgets(line, sizeof(line), fp)) != NULL) {
fputs(line, stdout);
}
Advanced 4.4BSD IPC Tutorial
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