socket(2)socket(2)NAMEsocket() - create an endpoint for communication
The system call creates an endpoint for communication and returns a
descriptor. The socket descriptor returned is used in all subsequent
socket-related system calls.
The af parameter specifies an address family to be used to interpret
addresses in later operations that specify the socket. These address
families are defined in the include files and The only currently sup‐
ported address families are:
AF_INET (DARPA Internet addresses)
AF_INET6 (Internet Protocol version 6)
AF_ROUTE (routing messages followed by IPv4 or IPv6
AF_UNIX (path names on a local node)
AF_CCITT (CCITT X.25 addresses)
AF_VME_LINK (backplane communications on VMEbus)
The type specifies the semantics of communication for the socket. Cur‐
rently defined types are:
SOCK_STREAM Sequenced, reliable, two-way-connection-
based byte streams.
SOCK_DGRAM Datagrams (connectionless, unreliable mes‐
sages of a fixed, typically small, maximum
protocol specifies a particular protocol to be used with the socket.
Normally, only a single protocol exists to support a particular socket
type using a given address family. However, many protocols may exist,
in which case a particular protocol must be specified. The protocol
number to use depends on the communication domain in which communica‐
tion is to take place (see services(4) and protocols(4)). protocol can
be specified as zero, which causes the system to choose a protocol type
Sockets of type SOCK_STREAM are byte streams similar to pipes, except
that they are full-duplex instead of half-duplex. A stream socket must
be in a connected state before any data can be sent or received on it.
A connection to another socket is created with a or call. Once con‐
nected, data can be transferred using some variant of the and or the
and calls. When a session is complete, use or calls to terminate the
TCP, the communications protocol used to implement SOCK_STREAM for
AF_INET or AF_INET6 sockets, ensures that data is not lost or dupli‐
cated. If a peer has buffer space for data and the data cannot be suc‐
cessfully transmitted within a reasonable length of time, the connec‐
tion is considered broken and the next call indicates an error with set
to If is set and the connection has been idle for two hours, the TCP
protocol sends "keepalive" packets every 75 seconds to determine
whether the connection is active. These transmissions are not visible
to users and cannot be read by a call. If the remote system does not
respond within 10 minutes (after 8 "keepalive" packets have been sent),
the next socket call returns an error with set to A signal is raised if
a process sends on a broken stream. This causes naive processes that
do not handle the signal to exit. An end-of-file condition (zero bytes
read) is returned if a process tries to read on a broken stream.
SOCK_DGRAM sockets allow sending of messages to correspondents named in
calls. It is also possible to receive messages at such a socket with
The operation of sockets is controlled by socket level options set by
the system call described by the getsockopt(2) manual entry. These
options are defined in the file and explained in the getsockopt(2) man‐
Socket endpoints for communication over an X.25/9000 link can be in
either address family, AF_INET or AF_CCITT. If the socket is in the
AF_INET family, the connection behaves as described above. TCP is used
if the socket type is SOCK_STREAM. UDP is used if the socket type is
SOCK_DGRAM. In both cases, Internet protocol (IP) and the X.25-to-IP
interface module are used.
If the socket is in the AF_CCITT address family, only the SOCK_STREAM
socket type is supported. Refer to the topic "Comparing X.25 Level 3
Access to IP" in the for more details on the difference between pro‐
grammatic access to X.25 via IP and X.25 Level 3.
If the socket is in the AF_CCITT family, the connection and all other
operations pass data directly from the application to the X.25 Packet
Level (level 3) without passing through a TCP or UDP protocol. Connec‐
tions of the AF_CCITT family cannot use most of the socket level
options described in getsockopt(2). However, AF_CCITT connections can
use many X.25-specific calls.
X/Open Sockets Compilation Environment
AF_CCITT and AF_VME_LINK
Only the SOCK_STREAM type is supported.
Stream Control Transmission Protocol (SCTP) provides two styles of
interfaces for applications that want to communicate using SCTP as the
transport protocol: one-to-one and one-to-many. The one-to-one style
is similar to TCP mode of operation whereas the one-to-many style is
similar to the UDP mode of operation.
Applications can create a one-to-one style socket using the following
To create a one-to-many style socket, the syntax is:
The first form for both styles creates an endpoint which can use only
IPv4 addresses. The second form creates an endpoint which can use both
IPv6 and IPv4 addresses. For more details, see sctp(7).
Note: SCTP is only available when the SCTP optional bundle is
returns the following values:
n is a valid file descriptor referring to the socket.
is set to indicate the error.
If fails, is set to one of the following values.
The specified address family
is not supported in this version of the
The networking subsystem is not up.
SOCK_DGRAM sockets are currently not supported for the AF_UNIX
or AF_VME_LINK address families.
The per-process descriptor table is full.
The system's table of open files is temporarily full and no more
calls can be accepted.
No buffer space is available.
The socket cannot be created.
No memory is available. The socket cannot be created.
The specified protocol is not supported.
The type of socket and protocol do not match.
The specified socket type is not supported in this address fam‐
Connection timed out.
Not all possible values are documented in each socket related manpage
due to dependencies from the underlying protocol modules. Refer to the
errno(2) manpage for a complete list of error codes.
Linking binary objects compiled to specification and binary objects
compiled to specification to the same executable may result in unex‐
pected behavior, including application abnormal termination and unex‐
pected socket errors. See xopen_networking(7) for details and remedy.
Currently, the default behavior is the however, it might be changed to
in a future release. At that time, any behavior that is incompatible
with might be obsoleted. Applications that conform to the X/Open spec‐
ification now will avoid migration problems (see xopen_networking(7)).
was developed by HP and the University of California, Berkeley.
SEE ALSOaccept(2), bind(2), connect(2), getsockname(2), getsockopt(2),
ioctl(2), listen(2), recv(2), select(2), send(2), shutdown(2), privi‐
leges(5), thread_safety(5), route(7P), socket(7), sctp(7), TCP(7P),
UDP(7P), UNIX(7P), xopen_networking(7).