SOCKET(2)

NAME
socket - create an endpoint for communication

SYNOPSIS
#include <sys/socket.h>
int
socket(int domain, int type, int protocol);

DESCRIPTION
socket() creates an endpoint for communication and returns a descriptor.

The domain parameter specifies a communications domain within which communication will take place; this selects the protocol family which should be used. These families are defined in the include file <sys/socket.h>. The currently understood formats are

    PF_LOCAL	Host-internal protocols, formerly called PF_UNIX,
    PF_UNIX		Host-internal protocols, deprecated, use PF_LOCAL,
    PF_INET		Internet version 4 protocols,
    PF_ROUTE	Internal Routing protocol,
    PF_KEY		Internal key-management function,
    PF_INET6	Internet version 6 protocols,
    PF_SYSTEM	System domain,
    PF_NDRV		Raw access to network device,
    PF_VSOCK	VM Sockets protocols

The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:

    SOCK_STREAM
    SOCK_DGRAM
    SOCK_RAW

A SOCK_STREAM type provides sequenced, reliable, two-way connection based byte streams. An out-of-band data transmission mechanism may be supported. A SOCK_DGRAM socket supports datagrams (connectionless, unreliable messages of a fixed (typically small) maximum length). SOCK_RAW sockets provide access to internal network protocols and interfaces. The type SOCK_RAW, which is available only to the super-user.

The protocol specifies a particular protocol to be used with the socket. Normally only a single protocol exists to support a particular socket type within a given protocol family. However, it is possible that many protocols may exist, in which case a particular protocol must be specified in this manner. The protocol number to use is particular to the “communication domain” in which communication is to take place; see protocols(5).

Sockets of type SOCK_STREAM are full-duplex byte streams, similar to pipes. A stream socket must be in a connected state before any data may be sent or received on it. A connection to another socket is created with a connect(2) or connectx(2) call. Once connected, data may be transferred using read(2) and write(2) calls or some variant of the send(2) and recv(2) calls. When a session has been completed a close(2) may be performed. Out-of-band data may also be transmitted as described in send(2) and received as described in recv(2).

The communications protocols used to implement a SOCK_STREAM insure that data is not lost or duplicated. If a piece of data for which the peer protocol has buffer space cannot be successfully transmitted within a reasonable length of time, then the connection is considered broken and calls will indicate an error with -1 returns and with ETIMEDOUT as the specific code in the global variable errno. The protocols optionally keep sockets “warm” by forcing transmissions roughly every minute in the absence of other activity. An error is then indicated if no response can be elicited on an otherwise idle connection for a extended period (e.g. 5 minutes). A SIGPIPE signal is raised if a process sends on a broken stream; this causes naive processes, which do not handle the signal, to exit.

SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspondents named in send(2) calls. Datagrams are generally received with recvfrom(2), which returns the next datagram with its return address.

An fcntl(2) call can be used to specify a process group to receive a SIGURG signal when the out-of-band data arrives. It may also enable non-blocking I/O and asynchronous notification of I/O events via SIGIO.

The operation of sockets is controlled by socket level options. These options are defined in the file <sys/socket.h>. Setsockopt(2) and getsockopt(2) are used to set and get options, respectively.

RETURN VALUES
A -1 is returned if an error occurs, otherwise the return value is a descriptor referencing the socket.

ERRORS
The socket() system call fails if:

[EACCES]
    Permission to create a socket of the specified type and/or protocol is denied.

[EAFNOSUPPORT]
    The specified address family is not supported.

[EMFILE]
    The per-process descriptor table is full.

[ENFILE]
    The system file table is full.

[ENOBUFS]
    Insufficient buffer space is available. The socket cannot be created until sufficient resources are freed.

[ENOMEM]
    Insufficient memory was available to fulfill the request.

[EPROTONOSUPPORT]
    The protocol type or the specified protocol is not supported within this domain.

[EPROTOTYPE]
    The socket type is not supported by the protocol.

If a new protocol family is defined, the socreate process is free to return any desired error code. The socket() system call will pass this error code along (even if it is undefined).

LEGACY SYNOPSIS
#include <sys/types.h> #include <sys/socket.h>

The include file <sys/types.h> is necessary.

SEE ALSO
accept(2), bind(2), connect(2), connectx(2), disconnectx(2), getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2), getprotoent(3), inet(4), inet6(4), unix(4), compat(5) An Introductory 4.3 BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's Supplementary Documents Volume 1. BSD Interprocess Communication Tutorial, reprinted in UNIX Programmer's Supplementary Documents Volume 1.

HISTORY
The socket() function call appeared in 4.2BSD.