NAME

socket — create an endpoint for communication

SYNOPSIS

#include <sys/socket.h>

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


DESCRIPTION

The socket() function shall create an unbound socket in a communications domain, and return a file descriptor that can be used in later function calls that operate on sockets. The file descriptor shall be allocated as described in 2.6 File Descriptor Allocation.

The socket() function takes the following arguments:

domain

Specifies the communications domain in which a socket is to be created.

type

Specifies the type of socket to be created.

protocol

Specifies a particular protocol to be used with the socket. Specifying a protocol of 0 causes socket() to use an unspecified default protocol appropriate for the requested socket type.

The domain argument specifies the address family used in the communications domain. The address families supported by the system are implementation-defined.

Symbolic constants that can be used for the domain argument are defined in the <sys/socket.h> header.

The type argument specifies the socket type, which determines the semantics of communication over the socket. The following socket types are defined; implementations may specify additional socket types:

SOCK_STREAM

Provides sequenced, reliable, bidirectional, connection-mode byte streams, and may provide a transmission mechanism for out-of-band data.

SOCK_DGRAM

Provides datagrams, which are connectionless-mode, unreliable messages of fixed maximum length.

SOCK_SEQPACKET

Provides sequenced, reliable, bidirectional, connection-mode transmission paths for records. A record can be sent using one or more output operations and received using one or more input operations, but a single operation never transfers part of more than one record. Record boundaries are visible to the receiver via the MSG_EOR flag.

Additionally, the type argument can contain the bitwise-inclusive OR of flags from the following list:

SOCK_CLOEXEC

Atomically set the FD_CLOEXEC flag on the new file descriptor.

SOCK_CLOFORK

Atomically set the FD_CLOFORK flag on the new file descriptor.

SOCK_NONBLOCK

Set the O_NONBLOCK file status flag on the new file description.

Implementations may define additional flags.

If the protocol argument is non-zero, it shall specify a protocol that is supported by the address family. If the protocol argument is zero, the default protocol for this address family and type shall be used. The protocols supported by the system are implementation-defined.

The process may need to have appropriate privileges to use the socket() function or to create some sockets.

RETURN VALUE

Upon successful completion, socket() shall return a non-negative integer, the socket file descriptor. Otherwise, a value of -1 shall be returned and errno set to indicate the error.

ERRORS

The socket() function shall fail if:

[EAFNOSUPPORT]

The implementation does not support the specified address family.

[EMFILE]

All file descriptors available to the process are currently open.

[ENFILE]

No more file descriptors are available for the system.

[EPROTONOSUPPORT]

The value of protocol is non-zero and either the protocol is not supported by the address family or the protocol is not supported by the implementation.

[EPROTOTYPE]

The value of protocol is non-zero and the socket type is not supported by the protocol.

[ESOCKTNOSUPPORT] ^[OB] or [EPROTONOSUPPORT] or [EPROTOTYPE]

The socket type is not supported by the address family, or the socket type is not supported by the implementation.

The socket() function may fail if:

[EACCES]

The process does not have appropriate privileges.

[ENOBUFS]

Insufficient resources were available in the system to perform the operation.

[ENOMEM]

Insufficient memory was available to fulfill the request.

EXAMPLES

None.

APPLICATION USAGE

The documentation for specific address families specifies which protocols each address family supports. The documentation for specific protocols specifies which socket types each protocol supports.

The application can determine whether an address family is supported by trying to create a socket with domain set to the protocol in question.

RATIONALE

The use of the SOCK_CLOEXEC and SOCK_CLOFORK flags in the type argument of socket() is necessary to avoid a data race in multi-threaded applications. Without SOCK_CLOFORK, a file descriptor is leaked into a child process created by one thread in the window between another thread calling socket() and using fcntl() to set the FD_CLOFORK flag. Without SOCK_CLOEXEC, a file descriptor intentionally inherited by child processes is similarly leaked into an executed program if FD_CLOEXEC is not set atomically. The SOCK_NONBLOCK flag is for convenience in avoiding additional fcntl() calls.

Historically the standard did not specify the errno value to be used when the socket type is not supported, and there were differences between implementations. Some reused the existing standard [EPROTONOSUPPORT] or [EPROTOTYPE] values while others set errno to a (then) non-standard value of [ESOCKTNOSUPPORT]. All three values are permitted in this version of the standard, but the use of [EPROTONOSUPPORT] or [EPROTOTYPE] is considered to be misleading when no protocol is specified (that is, the value of protocol is zero) and consequently those alternatives have been marked obsolescent. If protocol is non-zero, since there is no precedence between error conditions, all three values will still be permitted even after the obsolescent alternatives for the [ESOCKTNOSUPPORT] condition have been removed.

FUTURE DIRECTIONS

A future version of this standard may disallow setting errno to [EPROTONOSUPPORT] or [EPROTOTYPE] when the socket type is not supported and protocol is zero.

SEE ALSO

2.6 File Descriptor Allocation, accept, bind, close, connect, getsockname, getsockopt, listen, recv, recvfrom, recvmsg, send, sendmsg, setsockopt, shutdown, socketpair

XBD <netinet/in.h>, <sys/socket.h>

CHANGE HISTORY

First released in Issue 6. Derived from the XNS, Issue 5.2 specification.

Issue 7

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0335 [835] is applied.

Issue 8

Austin Group Defects 411 and 1318 are applied, adding SOCK_CLOEXEC, SOCK_CLOFORK, and SOCK_NONBLOCK.

Austin Group Defect 1067 is applied, adding the [ESOCKTNOSUPPORT] error and changing the [EPROTONOSUPPORT] and [EPROTOTYPE] errors.

Austin Group Defect 1475 is applied, adding close() to the SEE ALSO section.