4.1cBSD/usr/man/man2/socket.2

.TH SOCKET 2 2/13/83
.SH NAME
socket \- create an endpoint for communication
.SH SYNOPSIS
.nf
.ft B
#include <sys/socket.h>
.PP
.ft B
s = socket(af, type, protocol)
int s, af, type, protocol;
.fi
.SH DESCRIPTION
.I Socket
creates an endpoint for communication and returns a descriptor.
The socket has the indicated
.I type
which specifies the semantics of communication.  Currently
defined types are:
.PP
.nf
.in +0.5i
SOCK_STREAM
SOCK_DGRAM
SOCK_RAW
.in -0.5i
.fi
.PP
A SOCK_STREAM type provides sequenced, reliable,
two-way connection based byte streams with an out-of-band data
transmission mechanism.
A SOCK_DGRAM socket supports
datagrams (connectionless, unreliable messages of
a fixed (typically small) maximum length).
SOCK_RAW sockets provide access to internal network interfaces.
The types SOCK_RAW,
which is available only to the super-user, and
SOCK_PKTSTREAM, which is planned, but not yet implemented, are not
described here.
.PP
The
.I af
parameter specifies an address format with which addrresses specified
in later operations using the socket should be interpreted.  These
formates are defined in the include file
.IR <sys/socket.h> .
The currently understood formats are
.PP
.nf
.in +0.5i
.ta 1.25i 1.75i
AF_UNIX	(UNIX path names),
AF_INET	(ARPA Internet addresses),
AF_PUP	(Xerox PUP-I Internet addresses), and
AF_IMPLINK	(IMP \*(lqhost at IMP\*(rq addresses).
.in -0.5i
.fi
.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 using a given address format.  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 \*(lqcommunication domain\*(rq in which communication
is to take place; see
.IR services (3N)
and
.IR protocols (3N).
.PP
Sockets of type SOCK_STREAM
are full-duplex byte streams, similar
to pipes.  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
.IR connect (2)
call.  Once connected, data may be transferred using
.IR read (2)
and
.IR write (2)
calls or some variant of the 
.IR send (2)
and
.IR recv (2)
calls.  When a session has been completed a
.IR close (2)
may be performed.
Out-of-band data may also be transmitted as described in
.IR send (2)
and received as described in
.IR recv (2).
.PP
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 \*(lqwarm\*(rq 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 exit.
.PP
SOCK_DGRAM and SOCK_RAW
sockets allow sending of datagrams to correspondents
named in
.IR send (2)
calls.  It is also possible to receive datagrams at
such a socket with
.IR recv (2).
.PP
A SIOCSPGRP ioctl can be used to specify a process group to receive
a SIGURG signal when the out-of-band data arrives.  If the
integer argument to SIOCSPGRP is negative, then it is taken to mean
a single process rather than a process group, given by the absolute
value of the argument.  The SIOCGPGRP ioctl returns the current
value of a socket's process group.
.SH "RETURN VALUE
A \-1 is returned if an error occurs, otherwise the return
value is a descriptor referencing the socket.
.SH "ERRORS
The \fIsocket\fP call fails if:
.TP 20
[EAFNOSUPPORT]
The specified address family is not supported in this version
of the system.
.TP 20
[ESOCKTNOSUPPORT]
The specified socket type is not supported in this address family.
.TP 20
[EPROTONOSUPPORT]
The specified protocol is not supported.
.TP 20
[EMFILE]
The per-process descriptor table is full.
.TP 20
[ENOBUFS]
No buffer space is available.  The socket cannot be created.
.SH SEE ALSO
accept(2), bind(2), connect(2), ioctl(2), listen(2), recv(2),
select(2), send(2), shutdown(2), socketopt(2)
.br
``A 4.2bsd Interprocess Communication Primer''.
.SH BUGS
The use of keepalives is a questionable feature for this layer.
.PP
SIOCSPGRP should be replaced by
.I setdopt
and
SIOCGPGRP by
.IR getdopt .