The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### NAME

> socketpair - create a pair of connected sockets

#### SYNOPSIS

> ``#include <sys/socket.h>
> int socketpair(int` domain`, int` type`, int` protocol`,
> int` socket_vector`[2]);
> ``

#### DESCRIPTION

> The socketpair() function shall create an unbound pair of connected sockets in a specified domain, of a specified type, under the protocol optionally specified by the protocol argument. The two sockets shall be identical. The file descriptors used in referencing the created sockets shall be returned in socket_vector[0] and socket_vector[1]. The file descriptors shall be allocated as described in File Descriptor Allocation.

> The socketpair() function takes the following arguments:

> domain
>   Specifies the communications domain in which the sockets are to be created.
> type
>   Specifies the type of sockets to be created.
> protocol
>   Specifies a particular protocol to be used with the sockets. Specifying a protocol of 0 causes socketpair() to use an unspecified default protocol appropriate for the requested socket type.
> socket_vector
>   Specifies a 2-integer array to hold the file descriptors of the created socket pair.

> The type argument specifies the socket type, which determines the semantics of communications 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.

> 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 socketpair() function or to create some sockets.

#### RETURN VALUE

> Upon successful completion, this function shall return 0; otherwise, -1 shall be returned and errno set to indicate the error, no file descriptors shall be allocated, and the contents of socket_vector shall be left unmodified.

#### ERRORS

> The socketpair() function shall fail if:

> [EAFNOSUPPORT]
>   The implementation does not support the specified address family.
> [EMFILE]
>   All, or all but one, of the file descriptors available to the process are currently open.
> [ENFILE]
>   No more file descriptors are available for the system.
> [EOPNOTSUPP]
>   The specified protocol does not permit creation of socket pairs.
> [EPROTONOSUPPORT]
>   The protocol is not supported by the address family, or the protocol is not supported by the implementation.
> [EPROTOTYPE]
>   The socket type is not supported by the protocol.

> The socketpair() 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.

The following sections are informative.

#### 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 socketpair() function is used primarily with UNIX domain sockets and need not be supported for other domains.

#### RATIONALE

> None.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> File Descriptor Allocation, socket

> XBD <sys/socket.h>

#### CHANGE HISTORY

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

#### Issue 7

> The description of the [EMFILE] error condition is aligned with the pipe() function.

> POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0336 [835] and XSH/TC2-2008/0337 [483,835] are applied.

End of informative text.

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