|SOCKET(2)||System Calls Manual||SOCKET(2)|
socket — create an
endpoint for communication
Standard C Library (libc, -lc)
domain, int type,
system call creates an endpoint for communication and returns a
The domain argument 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
currently understood formats are:
PF_LOCAL Host-internal protocols (alias for PF_UNIX), PF_UNIX Host-internal protocols, PF_INET Internet version 4 protocols, PF_INET6 Internet version 6 protocols, PF_ROUTE Internal routing protocol, PF_LINK Link layer interface, PF_KEY Internal key-management function, PF_NATM Asynchronous transfer mode protocols, PF_NETGRAPH Netgraph sockets, PF_IEEE80211 IEEE 802.11 wireless link-layer protocols (WiFi), PF_BLUETOOTH Bluetooth protocols, PF_INET_SDP OFED socket direct protocol (IPv4), PF_INET6_SDP OFED socket direct protocol (IPv6)
Each protocol family is connected to an address family, which has
the same name except that the prefix is
AF_” in place of
PF_”. Other protocol families may be
also defined, beginning with “
with corresponding address families.
The socket has the indicated type, which specifies the semantics of communication. Currently defined types are:
SOCK_STREAM Stream socket, SOCK_DGRAM Datagram socket, SOCK_RAW Raw-protocol interface, SOCK_RDM Reliably-delivered packet, SOCK_SEQPACKET Sequenced packet stream
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
SOCK_SEQPACKET socket may provide 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. This facility may have
provide access to internal network protocols and interfaces. The types
SOCK_RAW, which is available only to the super-user,
SOCK_RDM, which is planned, but not yet
implemented, are not described here.
Additionally, the following flags are allowed in the type argument:
SOCK_CLOEXEC Set close-on-exec on the new descriptor, SOCK_NONBLOCK Set non-blocking mode on the new socket
The protocol argument 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).
The protocol argument may be set to zero (0) to request the default implementation of a socket type for the protocol, if any.
Sockets of type
are full-duplex byte streams, similar to pipes. A stream socket must be in a
before any data may be sent or received on it. A connection to another
socket is created with a connect(2) system call. Once
connected, data may be transferred using read(2) and
write(2) calls or some variant of the
send(2) and recv(2) functions. (Some
protocol families, such as the Internet family, support the notion of an
“implied connect”, which permits data to be sent piggybacked
onto a connect operation by using the sendto(2) system
call.) 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
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). By default, a
signal is raised if a process sends on a broken stream, but this behavior
may be inhibited via setsockopt(2).
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
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) system 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
The operation of sockets is controlled by socket
These options are defined in the file
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.
socket() system 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.
- User has insufficient privileges to carry out the requested operation.
- The protocol type 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), getpeername(2), getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2), CMSG_DATA(3), getprotoent(3), netgraph(4), protocols(5)
An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7.
BSD Interprocess Communication Tutorial, PS1, 8.
socket() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”). The
POSIX standard specifies only the
constants for address families, and requires the use of
AF_* constants for the domain
SOCK_CLOEXEC flag is expected to conform to the next
revision of the POSIX standard. The
PF_* constants, and
other address families are FreeBSD extensions.
socket() system call appeared in
|August 19, 2018||Debian|