socket (3p) - Linux Manuals

socket: create an endpoint for communication


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


#include <sys/socket.h>

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


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 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:

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


accept(), bind(), connect(), getsockname(), getsockopt(), listen(), recv(), recvfrom(), recvmsg(), send(), sendmsg(), setsockopt(), shutdown(), socketpair(), the Base Definitions volume of IEEE Std 1003.1-2001, <netinet/in.h>, <sys/socket.h>