socket(2N) socket(2N)
NAME
socket - create an endpoint for communication
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
s = socket(af, type, protocol)
int s, af, type, protocol;
cc ... -lnet
DESCRIPTION
socket creates an endpoint for communication and returns a
descriptor.
The af parameter specifies an address format with which
addresses specified in later operations using the socket
should be interpreted. These formats are defined in the
include file <sys/socket.h>. The currently understood
formats are:
AF_UNIX (UNIX path names),
AF_INET (ARPA Internet addresses),
AF_PUP (Xerox PUP-I Internet addresses),
AF_IMPLINK (IMP ``host at IMP'' addresses).
The socket has the indicated type which specifies the
semantics of communication. Currently defined types are:
SOCKSTREAM
SOCKDGRAM
SOCKRAW
SOCKSEQPACKET
SOCKRDM
A SOCKSTREAM type provides sequenced, reliable, two-way
connection based byte streams with an out-of-band data
transmission mechanism. A SOCKDGRAM socket supports
datagrams (connectionless, unreliable messages of a fixed
(typically small) maximum length). SOCKRAW sockets provide
access to internal network interfaces. The types SOCKRAW,
which is available only to the super-user, and
SOCKSEQPACKET and SOCKRDM, which are planned, but not yet
implemented, are not described here.
The 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
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socket(2N) socket(2N)
the communication domain in which communication is to take
place; see services(4N) and protocols(4N).
Sockets of type SOCKSTREAM 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(2N)
call. Once connected, data may be transferred using read(2)
and write(3) calls or some variant of the send(2N) and
recv(2N) calls. When a session has been completed a
close(2) may be performed. Out-of-band data may also be
transmitted as described in send(2N) and received as
described in recv(2N).
The communications protocols used to implement a SOCKSTREAM
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 ``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 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.
SOCKDGRAM and SOCKRAW sockets allow sending of datagrams
to correspondents named in send(2N) calls. It is also
possible to receive datagrams at such a socket with
recv(2N).
An fcntl(2) call can be used to specify a process group to
receive a SIGURG signal when the out-of-band data
The operation of sockets is controlled by socket level
options. These options are defined in the file
<sys/socket.h> and explained below. setsockopt and
getsockopt(2N) are used to set and get options,
respectively.
SODEBUG turn on recording of debugging
information
SOREUSEADDR allow local address reuse
SOKEEPALIVE keep connections alive
SODONTROUTE do no apply routing on outgoing messages
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socket(2N) socket(2N)
SOLINGER
linger on close if data present
SODONTLINGER do not linger on close
SODEBUG enables debugging in the underlying protocol
modules. SOREUSEADDR indicates the rules used in
validating addresses supplied in a bind(2N) call should
allow reuse of local addresses. SOKEEPALIVE enables the
periodic transmission of messages on a connected socket.
Should the connected party fail to respond to these
messages, the connection is considered broken and processes
using the socket are notified via a SIGPIPE signal.
SODONTROUTE indicates that outgoing messages should bypass
the standard routing facilities. Instead, messages are
directed to the appropriate network interface according to
the network portion of the destination address. SOLINGER
and SODONTLINGER control the actions taken when unsent
messages are queued on socket and a close(2) is performed.
If the socket promises reliable delivery of data and
SOLINGER is set, the system will block the process on the
close attempt until it is able to transmit the data or until
it decides it is unable to deliver the information (a
timeout period, termed the linger interval, is specified in
the setsockopt call when SOLINGER is requested). If
SODONTLINGER is specified and a close is issued, the system
will process the close in a manner which allows the process
to continue as quickly as possible.
RETURN VALUE
A -1 is returned if an error occurs, otherwise the return
value is a descriptor referencing the socket.
ERRORS
The socket call fails if:
[EAFNOSUPPORT] The specified address family is not
supported in this version of the
system.
[ESOCKTNOSUPPORT] The specified socket type is not
supported in this address family.
[EPROTONOSUPPORT] The specified protocol is not
supported.
[EMFILE] The per-process descriptor table is
full.
[ENOBUFS] No buffer space is available. The
socket cannot be created.
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socket(2N) socket(2N)
LINKING
This library is accessed by specifying -lnet as the last
argument to the compile line, e.g.:
cc -o prog prog.c -lnet
SEE ALSO
accept(2N), bind(2N), connect(2N), getsockname(2N),
getsockopt(2N), ioctl(2), listen(2N), recv(2N), select(2N),
send(2N), shutdown(2N).
BUGS
The use of keepalives is a questionable feature for this
layer.
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