SOCKET(2) BSD System Calls Manual SOCKET(2)NAMEsocket — create an endpoint for communication
socket(int domain, int type, int protocol);
DESCRIPTIONSocket() creates an endpoint for communication and returns a descriptor.
The domain parameter specifies a communications domain within which com‐
munication 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
AF_UNIX (UNIX internal protocols),
AF_INET (ARPA Internet protocols),
AF_ISO (ISO protocols),
AF_NS (Xerox Network Systems protocols), and
AF_IMPLINK (IMP host at IMP link layer).
The socket has the indicated type, which specifies the semantics of com‐
munication. Currently defined types are:
A SOCK_STREAM type provides sequenced, reliable, two-way connection based
byte streams. An out-of-band data transmission mechanism may be sup‐
ported. A SOCK_DGRAM socket supports datagrams (connectionless, unreli‐
able messages of a fixed (typically small) maximum length). A
SOCK_SEQPACKET socket may provide a sequenced, reliable, two-way connec‐
tion-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. This facility is protocol specific, and presently implemented only
for PF_NS. SOCK_RAW sockets provide access to internal network protocols
and interfaces. The types SOCK_RAW, which is available only to the
super-user, and SOCK_RDM, which is planned, but not yet implemented, are
not described here.
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 speci‐
fied in this manner. The protocol number to use is particular to the
communication domain in which communication is to take place; see
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) 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
SOCK_SEQPACKET sockets employ the same system calls as SOCK_STREAM sock‐
ets. The only difference is that read(2) calls will return only the
amount of data requested, and any remaining in the arriving packet will
SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to correspon‐
dents 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.
A -1 is returned if an error occurs, otherwise the return value is a
descriptor referencing the socket.
The socket() call fails if:
[EPROTONOSUPPORT] The protocol type or the specified protocol is not
supported within this domain.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[EACCESS] Permission to create a socket of the specified type
and/or protocol is denied.
[ENOBUFS] Insufficient buffer space is available. The socket
cannot be created until sufficient resources are
SEE ALSOaccept(2), bind(2), connect(2), getprotoent(3), getsockname(2),
getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2),
shutdown(2), socketpair(2), write(2)
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.
The socket function call appeared in 4.2BSD.
4.2 Berkeley Distribution June 4, 1993 4.2 Berkeley Distribution