SOCKET(2)

NAME
socket - create an endpoint for communication

SYNOPSIS
#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 communications domain within which communication will take place; this selects the protocol family which should be used. These families are defined in the include file <sys/socket.h>. The currently understood formats are:

AF_UNIX
    UNIX internal protocols
AF_INET
    Internet Protocol version 4 (IPv4) protocol family
AF_INET6
    Internet Protocol version 6 (IPv6) protocol family

The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:

SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
SOCK_SEQPACKET

A SOCK_STREAM type provides sequenced, reliable, two-way connection based byte streams. An out-of-band data transmission mechanism may be supported. A SOCK_DGRAM socket supports datagrams (connectionless, unreliable messages of a fixed (typically small) maximum length). A SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer may be required to read an entire packet with each read system call. This facility is protocol specific, and presently implemented only for AF_UNIX. SOCK_RAW sockets provide access to internal network protocols and interfaces, and are available only to the superuser.

Any combination of the following flags may additionally be used in the type argument:

SOCK_CLOEXEC
    Set close-on-exec flag on the new descriptor.
SOCK_CLOFORK
    Set close-on-fork flag on the new descriptor.
SOCK_NONBLOCK
    Set non-blocking I/O mode on the new socket.
SOCK_DNS
    For domains AF_INET or AF_INET6, only allow connect(2), sendto(2), or sendmsg(2) to the DNS port (typically 53).

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. 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 particular to the “communication domain” in which communication is to take place; see protocols(5). A value of 0 for protocol will let the system select an appropriate protocol for the requested socket type.

Sockets of type SOCK_STREAM are full-duplex byte streams. 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 used to 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 reasonable length of time, then the connection is considered broken and calls will indicate an error with -1 returns and with ETIMEDOUT as the specific code in the global variable errno. The protocols optionally keep sockets “warm” by forcing transmissions roughly every minute in the absence of other activity. An error is then indicated if no response can be elicited on an otherwise idle connection for an extended period (e.g., 5 minutes). A SIGPIPE signal is raised if a process sends 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 remaining in the arriving packet will be discarded.

SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in send(2) calls. Datagrams are generally received with recvfrom(2), which returns the next datagram with its return address.

An fcntl(2) call can be used to specify a process group to receive a SIGURG signal when the out-of-band data arrives. It may also enable non-blocking I/O and asynchronous notification of I/O events via SIGIO.

The operation of sockets is controlled by socket level options. These options are defined in the file <sys/socket.h>. setsockopt(2) and getsockopt(2) are used to set and get options, respectively.

RETURN VALUES
If successful, socket() returns a non-negative integer, the socket file descriptor. Otherwise, a value of -1 is returned and errno is set to indicate the error.

ERRORS
The socket() call fails if:

[EAFNOSUPPORT]
    The specified address family is not supported on this machine.

[EPROTONOSUPPORT]
    The protocol type or the specified protocol is not supported within this domain.

[EPROTOTYPE]
    The combination of the specified protocol and type is not supported.

[EMFILE]
    The per-process descriptor table is full.

[ENFILE]
    The system file table is full.

[ENOBUFS]
    Insufficient resources were available in the system to perform the operation.

[EACCES]
    Permission to create a socket of the specified type and/or protocol is denied.

SEE ALSO
accept(2), bind(2), connect(2), getsockname(2), getsockopt(2), ioctl(2), listen(2), poll(2), read(2), recv(2), select(2), send(2), setsockopt(2), shutdown(2), socketpair(2), write(2), getprotoent(3), inet(4), inet6(4), netintro(4), unix(4) An Introductory 4.3 BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's Supplementary Documents Volume 1. BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's Supplementary Documents Volume 1.

STANDARDS
The socket() function conforms to p1003.1-2024.

The SOCK_DNS flag is an OpenBSD extension.

HISTORY
The socket() system call first appeared in 4.1cBSD. Support for the SOCK_CLOEXEC and SOCK_NONBLOCK flags appeared in OpenBSD 5.7. Support for the SOCK_DNS flag appeared in OpenBSD 5.9.