HP-UX Reference (11i v1 00/12) - 2 System Calls (vol 5)

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socket(2) socket(2)

NAME

socket() - create an endpoint for communication

SYNOPSIS

#include <sys/socket.h>

AF_CCITT Only

#include <x25/x25ccittproto.h>

int socket(int af, int type, int protocol);

DESCRIPTION

The socket() system call creates an endpoint for communication and returns a descriptor. The socket

descriptor returned is used in all subsequent socket-related system calls.

The af parameter speciﬁes an address family to be used to interpret addresses in later operations that

specify the socket. These address families are deﬁned in the include ﬁles <sys/socket.h> and

<x25/ccittproto.h>. The only currently supported address families are:

AF_INET (DARPA Internet addresses)

AF_UNIX (path names on a local node)

AF_CCITT (CCITT X.25 addresses)

AF_VME_LINK (backplane communications on VMEbus)

The type speciﬁes the semantics of communication for the socket. Currently deﬁned types are:

SOCK_STREAM Sequenced, reliable, two-way-connection-based byte streams.

SOCK_DGRAM Datagrams (connectionless, unreliable messages of a ﬁxed, typically small, max-

imum length; for AF_INET only).

protocol speciﬁes a particular protocol to be used with the socket. Normally, only a single protocol exists to

support a particular socket type using a given address family. However, many protocols may exist, in

which case a particular protocol must be speciﬁed. The protocol number to use depends on the communi-

cation domain in which communication is to take place (see services(4) and protocols(4)). protocol can be

speciﬁed as zero, which causes the system to choose a protocol type to use.

Sockets of type SOCK_STREAM are byte streams similar to pipes, except that they are full-duplex instead

of half-duplex. A stream socket must be in a connected state before any data can be sent or received on it.

A connection to another socket is created with a

connect() or accept() call. Once connected, data

can be transferred using some variant of the send() and recv() or the read() and write() calls.

When a session is complete, use

close() or shutdown() calls to terminate the connection.

TCP, the communications protocol used to implement SOCK_STREAM for AF_INET sockets, ensures that

data is not lost or duplicated. If a peer has buffer space for data and the data cannot be successfully

transmitted within a reasonable length of time, the connection is considered broken and the next recv()

call indicates an error with errno set to [ETIMEDOUT]. If SO_KEEPALIVE is set and the connection

has been idle for two hours, the TCP protocol sends "keepalive" packets every 75 seconds to determine

whether the connection is active. These transmissions are not visible to users and cannot be read by a

recv() call. If the remote system does not respond within 10 minutes (i.e., after 8 "keepalive" packets

have been sent), the next socket call (e.g., recv()) returns an error with errno set to [ETIMEDOUT]. A

SIGPIPE signal is raised if a process sends on a broken stream. This causes naive processes that do not

handle the signalto exit. An end-of-ﬁle condition (zero bytes read) is returned if a process tries to read on a

broken stream.

SOCK_DGRAM sockets allow sending of messages to correspondents named in

send() calls. It is also

possible to receive messages at such a socket with

recv().

The operation of sockets is controlled by socket level options set by the setsockopt() system call

described by the getsockopt(2) manual entry. These options are deﬁned in the ﬁle <sys/socket.h> and

explained in the getsockopt(2) manual entry.

X.25 Only

Socket endpoints for communication over an X.25/9000 link can be in either address family, AF_INET or

AF_CCITT. If the socket is in the AF_INET family, the connection behaves as described above. TCP is

used if the socket type is SOCK_STREAM. UDP is used if the socket type is SOCK_DGRAM. In both

cases, Internet protocol (IP) and the X.25-to-IP interface module are used.

HP-UX Release 11i: December 2000 − 1 − Section 2−−375

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