SOCKET(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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 Section 2.14, 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.

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 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 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.

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

RATIONALE
None.

FUTURE DIRECTIONS
None.

SEE ALSO
Section 2.14, File Descriptor Allocation, accept(), bind(), connect(), getsockname(), getsockopt(), listen(), recv(), recvfrom(), recvmsg(), send(), sendmsg(), setsockopt(), shutdown(), socketpair()

The Base Definitions volume of POSIX.1-2017, <netinet_in.h>, <sys_socket.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .