SOCKET(2) Linux Programmer’s Manual SOCKET(2)
NAME
socket - create an endpoint for communication
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
int socket(int domain, int type, int protocol);
DESCRIPTION
socket() creates an endpoint for communication and returns a descriptor.
The domain parameter specifies a communication domain; this selects the protocol family which will be used for
communication. These families are defined in <sys/socket.h>. The currently understood formats include:
Name Purpose Man page
PF_UNIX, PF_LOCAL Local communication unix(7)
PF_INET IPv4 Internet protocols ip(7)
PF_INET6 IPv6 Internet protocols
PF_IPX IPX - Novell protocols
PF_NETLINK Kernel user interface device netlink(7)
PF_X25 ITU-T X.25 / ISO-8208 protocol x25(7)
PF_AX25 Amateur radio AX.25 protocol
PF_ATMPVC Access to raw ATM PVCs
PF_APPLETALK Appletalk ddp(7)
PF_PACKET Low level packet interface packet(7)
The socket has the indicated type, which specifies the communication semantics. Currently defined types are:
SOCK_STREAM
Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission
mechanism may be supported.
SOCK_DGRAM
Supports datagrams (connectionless, unreliable messages of a fixed maximum length).
SOCK_SEQPACKET
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 read system call.
SOCK_RAW
Provides raw network protocol access.
SOCK_RDM
Provides a reliable datagram layer that does not guarantee ordering.
SOCK_PACKET
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, SOCK_SEQPACKET is not imple-
mented for AF_INET.
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 speci-
fied 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 commu-
nication is to take place; see protocols(5). See getprotoent(3) on how to map protocol name strings to proto-
col numbers.
Sockets of type SOCK_STREAM are full-duplex byte streams, similar to pipes. They do not preserve record bound-
aries. A stream socket must be in a connected state before any data may be sent or received on it. A connec-
tion 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 SOCK_STREAM 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 reason-
able length of time, then the connection is considered to be dead. When SO_KEEPALIVE is enabled on the socket
the protocol checks in a protocol-specific manner if the other end is still alive. A SIGPIPE 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. SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM 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.
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in sendto(2) calls. Data-
grams are generally received with recvfrom(2), which returns the next datagram along with the address of its
sender.
SOCK_PACKET is an obsolete socket type to receive raw packets directly from the device driver. Use packet(7)
instead.
An fcntl(2) F_SETOWN 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 SOCK_STREAM connection breaks unexpectedly. This
operation may also be used to set the process or process group that receives the I/O and asynchronous notifica-
tion of I/O events via SIGIO. Using F_SETOWN 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 infor-
mation about the error; see IP_RECVERR in ip(7).
The operation of sockets is controlled by socket level options. These options are defined in <sys/socket.h>.
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 protocol is denied.
EAFNOSUPPORT
The implementation does not support the specified address family.
EINVAL Unknown protocol, or protocol family not available.
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. 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).
NOTE
The manifest constants used under 4.x BSD for protocol families are PF_UNIX, PF_INET, etc., while AF_UNIX etc.
are used for address families. However, already the BSD man page promises: "The protocol family generally is
the same as the address family", and subsequent standards use AF_* everywhere.
BUGS
SOCK_UUCP is not implemented yet.
SEE ALSO
accept(2), bind(2), connect(2), fcntl(2), getpeername(2), getsockname(2), getsockopt(2), ioctl(2), listen(2),
read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2), getprotoent(3), ip(7), socket(7),
tcp(7), udp(7), unix(7)
“An Introductory 4.3BSD Interprocess Communication Tutorial” is reprinted in UNIX Programmer?[m?[4ms Supplementary
Documents Volume 1.
“BSD Interprocess Communication Tutorial” is reprinted in UNIX Programmer?[m?[4ms Supplementary Documents Volume 1.
Linux 2.6.7 2004-06-17 SOCKET(2)