Updated: 2022/Sep/29
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SOCKET(2) System Calls Manual SOCKET(2)
NAME
socket - create an endpoint for communication
LIBRARY
Standard C Library (libc, -lc)
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 local domain (previously UNIX domain) protocols
PF_INET ARPA Internet protocols
PF_INET6 IPv6 (Internet Protocol version 6) protocols
PF_NS Xerox Network Systems protocols
PF_APPLETALK AppleTalk protocols
PF_BLUETOOTH Bluetooth protocols
PF_CAN CAN bus protocols
The socket has the indicated type, which specifies the semantics of
communication. Currently defined types are:
SOCK_STREAM Provides sequenced, reliable, two-way connection
based byte streams. An out-of-band data transmission
mechanism may be supported.
SOCK_DGRAM Supports datagrams: connectionless, unreliable
messages of a fixed--typically small--maximum length.
SOCK_RAW Provides access to internal network protocols and
interfaces. Available only to the super-user. Not
described here.
SOCK_SEQPACKET
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.
SOCK_RDM Not implemented.
The following flags can be or'ed to the socket type to add conditions to
the returned file descriptor:
SOCK_CLOEXEC Set the close on exec property.
SOCK_NONBLOCK
Set non-blocking I/O.
SOCK_NOSIGPIPE
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 connected 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 options. 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.
RETURN VALUES
A -1 is returned if an error occurs, otherwise the return value is a
descriptor referencing the socket.
ERRORS
The socket() call fails if:
[EACCES] Permission to create a socket of the specified type
and/or protocol is denied.
[EAFNOSUPPORT] The address family (domain) is not supported or the
specified domain is not supported by this protocol
family.
[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.
[EPROTONOSUPPORT] The protocol family is not supported or the specified
protocol is not supported within this domain.
[EPROTOTYPE] The socket type is not supported by the protocol.
SEE ALSO
accept(2), bind(2), connect(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)
HISTORY
The socket() function call appeared in 4.2BSD.
NetBSD 10.99 June 28, 2022 NetBSD 10.99