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 socket() function takes the following arguments:
Specifies the communications domain in which a socket is to be created.
Specifies the type of socket to be created.
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
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
Provides sequenced, reliable, bidirectional, connection-mode byte
streams, and may provide a transmission mechanism for
Provides datagrams, which are connectionless-mode, unreliable messages
of fixed maximum length.
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
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.
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.
The socket() function shall fail if:
The implementation does not support the specified address family.
No more file descriptors are available for this process.
No more file descriptors are available for the system.
The protocol is not supported by the address family, or the protocol
is not supported by the implementation.
The socket type is not supported by the protocol.
The socket() function may fail if:
The process does not have appropriate privileges.
Insufficient resources were available in the system to perform the
Insufficient memory was available to fulfill the request.
The following sections are informative.
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.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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