path: root/man2/socket.2

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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
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.\" Modified 1998, 1999 by Andi Kleen <ak@muc.de>
.\" Modified 2002-07-17 by Michael Kerrisk <mtk-manpages@gmx.net>
.\" Modified 2004-06-17 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"
.TH SOCKET 2 2004-06-17 "Linux 2.6.7" "Linux Programmer's Manual"
.SH NAME
socket \- create an endpoint for communication
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <sys/socket.h>
.sp
.BI "int socket(int " domain ", int " type ", int " protocol );
.SH DESCRIPTION
.B Socket
creates an endpoint for communication and returns a descriptor.
.PP
The
.I domain
parameter specifies a communication domain; this selects the protocol
family which will be used for communication.  These families are defined in 
.BR <sys/socket.h> .
The currently understood formats include:
.PP
.TS
tab(:);
l l l.
Name:Purpose:Man page
T{
.B "PF_UNIX,\ PF_LOCAL"
T}:T{
Local communication
T}:T{
.BR unix (7)
T}
T{
.B PF_INET
T}:IPv4 Internet protocols:T{
.BR ip (7)
T}
T{
.B PF_INET6
T}:IPv6 Internet protocols:
T{
.B PF_IPX
T}:IPX \- Novell protocols:
T{
.B PF_NETLINK
T}:T{
Kernel user interface device
T}:T{
.BR netlink (7)
T}
T{
.B PF_X25
T}:ITU-T X.25 / ISO-8208 protocol:T{
.BR x25 (7)
T}
T{
.B PF_AX25
T}:T{
Amateur radio AX.25 protocol
T}:
T{
.B PF_ATMPVC
T}:Access to raw ATM PVCs:
T{
.B PF_APPLETALK
T}:Appletalk:T{
.BR ddp (7)
T}
T{
.B PF_PACKET
T}:T{
Low level packet interface
T}:T{
.BR packet (7)
T}
.TE
.PP
The socket has the indicated
.IR type ,
which specifies the communication semantics.  Currently defined types
are:
.TP
.B SOCK_STREAM
Provides sequenced, reliable, two-way, connection-based byte streams.
An out-of-band data transmission mechanism may be supported.  
.TP
.B SOCK_DGRAM
Supports datagrams (connectionless, unreliable messages of a fixed
maximum length).
.TP
.B SOCK_SEQPACKET
Provides a sequenced, reliable, two-way connection-based data
transmission path for datagrams of fixed maximum length; a consumer is
required to read an entire packet with each read system call.  
.TP
.B SOCK_RAW
Provides raw network protocol access.
.TP
.B SOCK_RDM
Provides a reliable datagram layer that does not guarantee ordering.
.TP
.B SOCK_PACKET
Obsolete and should not be used in new programs;
see 
.BR packet (7).
.PP
Some socket types may not be implemented by all protocol families; for example,
.B SOCK_SEQPACKET
is not implemented for 
.BR AF_INET .
.PP
The
.I 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, in which a case
.I protocol
can be specified as 0.
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 specific to the \*(lqcommunication domain\*(rq
in which communication is to take place; see
.BR protocols (5).
See
.BR getprotoent (3)
on how to map protocol name strings to protocol numbers.
.PP
Sockets of type
.B SOCK_STREAM
are full-duplex byte streams, similar to pipes.  They do not preserve
record boundaries. A stream socket must be in
a
.I connected
state before any data may be sent or received on it.  A connection to
another socket is created with a
.BR connect (2)
call.  Once connected, data may be transferred using
.BR read (2)
and
.BR write (2)
calls or some variant of the 
.BR send (2)
and
.BR recv (2)
calls.  When a session has been completed a
.BR close (2)
may be performed.  Out-of-band data may also be transmitted as described in
.BR send (2)
and received as described in
.BR recv (2).
.PP
The communications protocols which implement a
.B 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
to be dead.
When
.B SO_KEEPALIVE 
is enabled on the socket the protocol checks in a protocol-specific
manner if the other end is still alive.
A
.B SIGPIPE
signal is raised if a process sends or receives
on a broken stream; this causes naive processes,
which do not handle the signal, to exit.
.B SOCK_SEQPACKET
sockets employ the same system calls as
.B SOCK_STREAM
sockets.  The only difference is that
.BR read (2)
calls will return only the amount of data requested, and any remaining in
the arriving packet will be discarded. Also all message boundaries in
incoming datagrams are preserved. 
.PP
.B SOCK_DGRAM
and
.B SOCK_RAW
sockets allow sending of datagrams to correspondents named in
.BR send (2)
calls.  Datagrams are generally received with
.BR recvfrom (2),
which returns the next datagram with its return address.
.PP
.B SOCK_PACKET
is an obsolete socket type to receive raw packets directly from the
device driver. Use
.BR packet (7)
instead.
.PP
An 
.BR fcntl (2)
call with the the 
.B F_SETOWN 
argument
can be used to specify a process group to receive a
.B SIGURG
signal when the out-of-band data arrives or 
.B SIGPIPE 
signal when a 
.B SOCK_STREAM 
connection breaks unexpectedly.  
It may also be used to set the process or process group that receives
the I/O and asynchronous notification of I/O events via
.BR SIGIO. 
Using
.B F_SETOWN
is equivalent to an
.BR ioctl (2)
call with the
.B FIOSETOWN
or
.B SIOCSPGRP
argument.
.PP
When the network signals an error condition to the protocol module (e.g.
using a ICMP message for IP) the pending error flag is set for the socket.
The next operation on this socket will return the error code of the pending
error. For some protocols it is possible to enable a per-socket error queue
to retrieve detailed information about the error; see
.B IP_RECVERR 
in
.BR ip (7) .
.PP
The operation of sockets is controlled by socket level
.IR options .
These options are defined in 
.BR <sys/socket.h> .
The functions
.BR setsockopt (2)
and
.BR getsockopt (2)
are used to set and get options, respectively.
.SH "RETURN VALUE"
On success, a file descriptor for the new socket is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EACCES
Permission to create a socket of the specified type and/or protocol
is denied.
.TP
.B EAFNOSUPPORT
The implementation does not support the specified address family.
.TP
.B EINVAL
Unknown protocol, or protocol family not available.
.TP
.B EMFILE
Process file table overflow.
.TP
.B ENFILE
The system limit on the total number of open files has been reached.
.TP
.BR ENOBUFS " or " ENOMEM
Insufficient memory is available.  The socket cannot be
created until sufficient resources are freed.
.TP
.B EPROTONOSUPPORT
The protocol type or the specified protocol is not
supported within this domain.
.PP
Other errors may be generated by the underlying protocol modules.
.SH "CONFORMING TO"
4.4BSD, SUSv2, POSIX 1003.1-2001.
The
.B socket
function call appeared in 4.2BSD. It is generally portable to/from
non-BSD systems supporting clones of the BSD socket layer (including
System V variants).
.SH NOTE
The manifest constants used under BSD 4.* for protocol families
are PF_UNIX, PF_INET, etc., while AF_UNIX etc. are used for address
families. However, already the BSD man page promises: "The protocol
family generally is the same as the address family", and subsequent
standards use AF_* everywhere.
.SH BUGS
.B SOCK_UUCP
is not implemented yet.
.SH "SEE ALSO"
.BR accept (2),
.BR bind (2),
.BR connect (2),
.BR fcntl (2),
.BR getpeername (2),
.BR getsockname (2),
.BR getsockopt (2),
.BR ioctl (2),
.BR listen (2),
.BR read (2),
.BR recv (2),
.BR select (2),
.BR send (2),
.BR shutdown (2),
.BR socketpair (2),
.BR write (2),
.BR getprotoent (3),
.BR ip (7),
.BR socket (7),
.BR tcp (7),
.BR udp (7),
.BR unix (7)
.PP
\(lqAn Introductory 4.3 BSD Interprocess Communication Tutorial\(rq
is reprinted in
.I UNIX Programmer's Supplementary Documents Volume 1.
.PP
\(lqBSD Interprocess Communication Tutorial\(rq
is reprinted in
.I UNIX Programmer's Supplementary Documents Volume 1.