NAME
socket - create an endpoint for communication SYNOPSIS
#include
/* See NOTES */ #include
int socket(int domain, int type, int protocol); DESCRIPTION socket() creates an endpoint for communication and returns a descrip‐ tor. The domain argument specifies a communication domain; this selects the protocol family which will be used for communication. These families are defined in . The currently understood formats include: Name Purpose Man page AFUNIX, AFLOCAL Local communication unix(7) AFINET IPv4 Internet protocols ip(7) AFINET6 IPv6 Internet protocols ipv6(7) AFIPX IPX - Novell protocols AFNETLINK Kernel user interface device netlink(7)
AFX25 ITU-T X.25 / ISO-8208 protocol x25(7) AFAX25 Amateur radio AX.25 protocol AFATMPVC Access to raw ATM PVCs AFAPPLETALK Appletalk ddp(7) AFPACKET Low level packet interface packet(7) The socket has the indicated type, which specifies the communication semantics. Currently defined types are:
SOCKSTREAM Provides sequenced, reliable, two-way, connection-based
byte streams. An out-of-band data transmission mecha‐ nism may be supported. SOCKDGRAM Supports datagrams (connectionless, unreliable messages of a fixed maximum length).
SOCKSEQPACKET Provides a sequenced, reliable, two-way connection- based data transmission path for datagrams of fixed maximum length; a consumer is required to read an entire packet with each input system call. SOCKRAW Provides raw network protocol access. SOCKRDM Provides a reliable datagram layer that does not guar‐ antee ordering. SOCKPACKET Obsolete and should not be used in new programs; see packet(7). Some socket types may not be implemented by all protocol families; for example, SOCKSEQPACKET is not implemented for AFINET. Since Linux 2.6.27, the type argument serves a second purpose: in addi‐ tion to specifying a socket type, it may include the bitwise OR of any of the following values, to modify the behavior of socket(): SOCKNONBLOCK Set the ONONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(2) to achieve the same result.
SOCKCLOEXEC Set the close-on-exec (FDCLOEXEC) flag on the new file descriptor. See the description of the OCLOEXEC flag in open(2) for reasons why this may be useful. The protocol specifies a particular protocol to be used with the socket. Normally only a single protocol exists to support a particular socket type within a given protocol family, in which case protocol can be specified as 0. However, it is possible that many protocols may exist, in which case a particular protocol must be specified in this manner. The protocol number to use is specific to the “communication domain” in which communication is to take place; see protocols(5). See getprotoent(3) on how to map protocol name strings to protocol numbers.
Sockets of type SOCKSTREAM are full-duplex byte streams, similar to pipes. They do not preserve record boundaries. A stream socket must be in a connected state before any data may be sent or received on it. A connection to another socket is created with a connect(2) call. Once connected, data may be transferred using read(2) and write(2) calls or some variant of the send(2) and recv(2) calls. When a session has been
completed a close(2) may be performed. Out-of-band data may also be transmitted as described in send(2) and received as described in recv(2). The communications protocols which implement a SOCKSTREAM ensure that data is not lost or duplicated. If a piece of data for which the peer protocol has buffer space cannot be successfully transmitted within a reasonable length of time, then the connection is considered to be dead. When SOKEEPALIVE is enabled on the socket the protocol checks
in a protocol-specific manner if the other end is still alive. A SIG‐ PIPE signal is raised if a process sends or receives on a broken stream; this causes naive processes, which do not handle the signal, to exit. SOCKSEQPACKET sockets employ the same system calls as SOCKSTREAM sockets. The only difference is that read(2) calls will return only the amount of data requested, and any data remaining in the arriving packet will be discarded. Also all message boundaries in incoming datagrams are preserved. SOCKDGRAM and SOCKRAW sockets allow sending of datagrams to corre‐ spondents named in sendto(2) calls. Datagrams are generally received with recvfrom(2), which returns the next datagram along with the address of its sender. SOCKPACKET is an obsolete socket type to receive raw packets directly from the device driver. Use packet(7) instead. An fcntl(2) FSETOWN operation can be used to specify a process or
process group to receive a SIGURG signal when the out-of-band data arrives or SIGPIPE signal when a SOCKSTREAM connection breaks unex‐ pectedly. This operation may also be used to set the process or process group that receives the I/O and asynchronous notification of I/O events via SIGIO. Using FSETOWN is equivalent to an ioctl(2) call with the FIOSETOWN or SIOCSPGRP argument. When the network signals an error condition to the protocol module (e.g., using a ICMP message for IP) the pending error flag is set for the socket. The next operation on this socket will return the error code of the pending error. For some protocols it is possible to enable
a per-socket error queue to retrieve detailed information about the error; see IPRECVERR in ip(7). The operation of sockets is controlled by socket level options. These options are defined in
. The functions setsockopt(2) and getsockopt(2) are used to set and get options, respectively. RETURN VALUE On success, a file descriptor for the new socket is returned. On error, -1 is returned, and errno is set appropriately. ERRORS EACCES Permission to create a socket of the specified type and/or pro‐ tocol is denied. EAFNOSUPPORT The implementation does not support the specified address fam‐ ily. EINVAL Unknown protocol, or protocol family not available. EINVAL Invalid flags in type. EMFILE Process file table overflow. ENFILE The system limit on the total number of open files has been reached. ENOBUFS or ENOMEM Insufficient memory is available. The socket cannot be created until sufficient resources are freed. EPROTONOSUPPORT The protocol type or the specified protocol is not supported within this domain. Other errors may be generated by the underlying protocol modules. CONFORMING TO
4.4BSD, POSIX.1-2001.
The SOCKNONBLOCK and SOCKCLOEXEC flags are Linux-specific.
socket() appeared in 4.2BSD. It is generally portable to/from non-BSD systems supporting clones of the BSD socket layer (including System V variants). NOTES
POSIX.1-2001 does not require the inclusion of
, and this header file is not required on Linux. However, some historical (BSD) implementations required this header file, and portable applications are probably wise to include it. The manifest constants used under 4.x BSD for protocol families are PFUNIX, PFINET, and so on, while AFUNIX, AFINET, and so on are used for address families. However, already the BSD man page promises: "The protocol family generally is the same as the address family", and sub‐ sequent standards use AF* everywhere. EXAMPLE An example of the use of socket() is shown in getaddrinfo(3). SEE ALSO accept(2), bind(2), connect(2), fcntl(2), getpeername(2), getsock‐ name(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2), getpro‐ toent(3), ip(7), socket(7), tcp(7), udp(7), unix(7) “An Introductory 4.3BSD Interprocess Communication Tutorial” and “BSD Interprocess Communication Tutorial”, reprinted in UNIX Programmer's Supplementary Documents Volume 1. COLOPHON This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2009-01-19 SOCKET(2)