SOCKET(3P) POSIX Programmer's Manual SOCKET(3P)
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.
socket — create an endpoint for communication
int socket(int domain, int type, int protocol);
The socket() function shall create an unbound socket in a communica‐
tions 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:
domain Specifies the communications domain in which a socket is to
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
The domain argument specifies the address family used in the communica‐
tions domain. The address families supported by the system are imple‐
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, unreli‐
able 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 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.
Upon successful completion, socket() shall return a non-negative inte‐
ger, 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 fam‐
EMFILE All file descriptors available to the process are currently
ENFILE No more file descriptors are available for the system.
The protocol is not supported by the address family, or the pro‐
tocol is not supported by the implementation.
The socket type is not supported by the protocol.
The socket() function may fail if:
EACCES The process does not have appropriate privileges.
Insufficient resources were available in the system to perform
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.
The documentation for specific address families specifies which proto‐
cols each address family supports. The documentation for specific pro‐
tocols 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.
accept(), bind(), connect(), getsockname(), getsockopt(), listen(),
recv(), recvfrom(), recvmsg(), send(), sendmsg(), setsockopt(), shut‐
The Base Definitions volume of POSIX.1‐2008, <netinet_in.h>,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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.ker‐
IEEE/The Open Group 2013 SOCKET(3P)