SOCKET(2) System Calls Manual SOCKET(2)

socket — create an endpoint for communication

Standard C Library (libc, -lc)

#include <sys/socket.h>

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

() 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:

local domain (previously UNIX domain) protocols
ARPA Internet protocols
IPv6 (Internet Protocol version 6) protocols
Xerox Network Systems protocols
AppleTalk protocols
Bluetooth protocols
CAN bus protocols

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

Provides sequenced, reliable, two-way connection based byte streams. An out-of-band data transmission mechanism may be supported.
Supports datagrams: connectionless, unreliable messages of a fixed—typically small—maximum length.
Provides access to internal network protocols and interfaces. Available only to the super-user. Not described here.
Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length. A consumer may be required to read an entire packet with each read system call.
Not implemented.

The following flags can be or'ed to the socket type to add conditions to the returned file descriptor:

Set the “close on exec” property.
Set the “close on fork” property.
Set non-blocking I/O.
Return EPIPE instead of raising SIGPIPE.

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. A stream socket must be in a state before any data may be sent or received on it. A connection to another socket is created with a connect(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 ensure 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 an 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_SEQPACKET sockets employ the same system calls as SOCK_STREAM sockets. The only difference is that read(2) calls will return only the amount of data requested, and any remaining in the arriving packet will be discarded.

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 . These options are defined in the file ⟨sys/socket.h⟩. The setsockopt(2) and getsockopt(2) system calls are used to set and get options, respectively.

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

The socket() call fails if:

[]
Permission to create a socket of the specified type and/or protocol is denied.
[]
The address family (domain) is not supported or the specified domain is not supported by this protocol family.
[]
The per-process descriptor table is full.
[]
The system file table is full.
[]
Insufficient buffer space is available. The socket cannot be created until sufficient resources are freed.
[]
The protocol family is not supported or the specified protocol is not supported within this domain.
[]
The socket type is not supported by the protocol.

accept(2), bind(2), connect(2), fcntl(2), getsockname(2), getsockopt(2), ioctl(2), listen(2), poll(2), read(2), recv(2), select(2), send(2), setsockopt(2), shutdown(2), socketpair(2), write(2), getprotoent(3)

Stuart Sechrest, An Introductory 4.4BSD Interprocess Communication Tutorial. (see /usr/share/doc/reference/ref3/sockets)

Samuel J. Leffler, Robert S. Fabry, William N. Joy, Phil Lapsley, Steve Miller, and Chris Torek, Advanced 4.4BSD IPC Tutorial. (see /usr/share/doc/reference/ref3/sockets-advanced)

The socket() call conforms to IEEE Std 1003.1-2001 (“POSIX.1”). Including the SOCK_CLOEXEC, SOCK_CLOFORK, and SOCK_NONBLOCK flags in the type conforms to IEEE Std 1003.1-2024 (“POSIX.1”). Using SOCK_NOSIGPIPE is an extension to the standard.

The socket() function call appeared in 4.2BSD.

The SOCK_CLOFORK flag appeared in FreeBSD 15.0, DragonFly 6.5, and NetBSD 11.0.

July 17, 2025 NetBSD 11.0