The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


socket - create an endpoint for communication


#include <sys/socket.h>

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


The socket() function creates an unbound socket in a communications domain, and returns a file descriptor that can be used in later function calls that operate on sockets.

The 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-dependent.

The <sys/socket.h> header defines at least the following values for the domain argument:

File system pathnames.
Internet address.

The type argument specifies the socket type, which determines the semantics of communication over the socket. The socket types supported by the system are implementation-dependent. Possible socket types include:

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 path 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 must specify a protocol that is supported by the address family. The protocols supported by the system are implementation-dependent.

The process may need to have appropriate privileges to use the socket() function or to create some sockets.


Upon successful completion, socket() returns a nonnegative integer, the socket file descriptor. Otherwise a value of -1 is returned and errno is set to indicate the error.


The socket() function will 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.
There were insufficient STREAMS resources available for the operation to complete.


The documentation for specific address families specify which protocols each address family supports. The documentation for specific protocols specify which socket types each protocol supports.

The application can determine if 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(), shutdown(), socketpair(), <netinet/in.h>, <sys/socket.h>.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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