table of contents
SOCKET(2) | System Calls Manual | SOCKET(2) |
NAME¶
socket
—
LIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <sys/types.h>
#include <sys/socket.h>
int
socket
(int
domain, int type,
int protocol);
DESCRIPTION¶
Thesocket
() system call creates an endpoint for
communication and returns a descriptor.
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
<sys/socket.h>
. The
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 “PF_
”,
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
A 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
length). A 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 is protocol
specific, and presently unimplemented. 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.
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 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) 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 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). By default, a SIGPIPE
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
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) 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
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¶
Thesocket
() system 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.
- [
EPERM
] - User has insufficient privileges to carry out the requested operation.
- [
EPROTONOSUPPORT
] - The protocol type 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), getpeername(2), getsockname(2), getsockopt(2), ioctl(2), listen(2), read(2), recv(2), select(2), send(2), shutdown(2), socketpair(2), write(2), getprotoent(3), netgraph(4), protocols(5)An Introductory 4.3 BSD Interprocess Communication Tutorial, PS1, 7.
BSD Interprocess Communication Tutorial, PS1, 8.
STANDARDS¶
Thesocket
() function conforms to IEEE
Std 1003.1-2008 (“POSIX.1”). The POSIX standard specifies
only the AF_INET
, AF_INET6
,
and AF_UNIX
constants for address families, and
requires the use of AF_*
constants for the
domain argument of socket
(). The
SOCK_CLOEXEC
flag is expected to conform to the next
revision of the POSIX standard. The SOCK_RDM
type, the PF_*
constants, and
other address families are FreeBSD extensions.
HISTORY¶
Thesocket
() system call appeared in
4.2BSD.
March 19, 2013 | Linux 4.9.0-9-amd64 |