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_INET6: (Internet Protocol version 6)
AF_ROUTE: (routing messages followed by IPv4 or IPv6 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, maximum length).

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 communication 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 or AF_INET6 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 (after 8 "keepalive" packets have been sent), the next socket call (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 signal to 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.

If the socket is in the AF_CCITT address family, only the SOCK_STREAM socket type is supported. Refer to the topic "Comparing X.25 Level 3 Access to IP" in the X.25 Programmer's Guide for more details on the difference between programmatic access to X.25 via IP and X.25 Level 3.

If the socket is in the AF_CCITT family, the connection and all other operations pass data directly from the application to the X.25 Packet Level (level 3) without passing through a TCP or UDP protocol. Connections of the AF_CCITT family cannot use most of the socket level options described in getsockopt(2). However, AF_CCITT connections can use many X.25-specific ioctl() calls.

X/Open Sockets Compilation Environment

See xopen_networking(7).

DEPENDENCIES

AF_CCITT and AF_VME_LINK

Only the SOCK_STREAM type is supported.

RETURN VALUE

socket() returns the following values:

n: Successful completion. n is a valid file descriptor referring to the socket.
-1: Failure. errno is set to indicate the error.

ERRORS

If socket() fails, errno is set to one of the following values.

EAFNOSUPPORT: The specified address family is not supported in this version of the system.
EHOSTDOWN: The networking subsystem is not up.
EINVAL: SOCK_DGRAM sockets are currently not supported for the AF_UNIX or AF_VME_LINK address families.
EMFILE: The per-process descriptor table is full.
ENFILE: The system's table of open files is temporarily full and no more socket() calls can be accepted.
ENOBUFS: No buffer space is available. The socket cannot be created.
ENOMEM: No memory is available. The socket cannot be created.
EPROTONOSUPPORT: The specified protocol is not supported.
EPROTOTYPE: The type of socket and protocol do not match.
ESOCKTNOSUPPORT: The specified socket type is not supported in this address family.
ETIMEDOUT: Connection timed out.

WARNINGS

Not all possible errno values are documented in each socket related manpage due to dependencies from the underlying protocol modules. Refer to the errno(2) manpage for a complete list of error codes.

Linking binary objects compiled to X/Open Sockets specification and binary objects compiled to HP-UX BSD Sockets specification to the same executable may result in unexpected behavior, including application abnormal termination and unexpected socket errors. See xopen_networking(7) for details and remedy.

FUTURE DIRECTION

Currently, the default behavior is the HP-UX BSD Sockets; however, it might be changed to X/Open Sockets in a future release. At that time, any HP-UX BSD Sockets behavior that is incompatible with X/Open Sockets might be obsoleted. Applications that conform to the X/Open specification now will avoid migration problems (see xopen_networking(7)).

AUTHOR

socket() was developed by HP and the University of California, Berkeley.

STANDARDS CONFORMANCE

socket(): XPG4, UNIX 95, UNIX 03