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ACCEPT(2)		    BSD	System Calls Manual		     ACCEPT(2)

     accept -- accept a	connection on a	socket

     #include <sys/types.h>
     #include <sys/socket.h>

     accept(int	s, struct sockaddr *addr, socklen_t *addrlen);

     The argument s is a socket	that has been created with socket(2), bound to
     an	address	with bind(2), and is listening for connections after a
     listen(2).	 The accept() argument extracts	the first connection request
     on	the queue of pending connections, creates a new	socket with the	same
     properties	of s and allocates a new file descriptor for the socket.  If
     no	pending	connections are	present	on the queue, and the socket is	not
     marked as non-blocking, accept() blocks the caller	until a	connection is
     present.  If the socket is	marked non-blocking and	no pending connections
     are present on the	queue, accept()	returns	an error as described below.
     The accepted socket may not be used to accept more	connections.  The
     original socket s remains open.

     The argument addr is a result parameter that is filled in with the	ad-
     dress of the connecting entity, as	known to the communications layer.
     The exact format of the addr parameter is determined by the domain	in
     which the communication is	occurring.  The	addrlen	is a value-result pa-
     rameter; it should	initially contain the amount of	space pointed to by
     addr; on return it	will contain the actual	length (in bytes) of the ad-
     dress returned.  This call	is used	with connection-based socket types,
     currently with SOCK_STREAM.

     It	is possible to select(2) a socket for the purposes of doing an
     accept() by selecting it for read.

     For certain protocols which require an explicit confirmation, such	as ISO
     or	DATAKIT, accept() can be thought of as merely dequeueing the next con-
     nection request and not implying confirmation.  Confirmation can be im-
     plied by a	normal read or write on	the new	file descriptor, and rejection
     can be implied by closing the new socket.

     One can obtain user connection request data without confirming the	con-
     nection by	issuing	a recvmsg(2) call with an msg_iovlen of	0 and a	non-
     zero msg_controllen, or by	issuing	a getsockopt(2)	request.  Similarly,
     one can provide user connection rejection information by issuing a
     sendmsg(2)	call with providing only the control information, or by	call-
     ing setsockopt(2).

     In	the non-threaded library accept() is implemented as the	accept

     In	the threaded library, the accept syscall is assembled to
     _thread_sys_accept() and accept() is implemented as a function which
     locks s for read and write, then calls _thread_sys_accept().  If the call
     to	_thread_sys_accept() would block, a context switch is performed.  Be-
     fore returning, accept() unlocks s.

     The call returns -1 on error.  If it succeeds, it returns a non-negative
     integer that is a descriptor for the accepted socket.

     The accept() will fail if:

     [EBADF]	  The descriptor is invalid.

     [EINTR]	  The accept() operation was interrupted.

     [EMFILE]	  The per-process descriptor table is full.

     [ENFILE]	  The system file table	is full.

     [ENOTSOCK]	  The descriptor references a file, not	a socket.

     [EINVAL]	  listen(2) has	not been called	on the socket descriptor.

     [EFAULT]	  The addr parameter is	not in a writable part of the user ad-
		  dress	space.

		  The socket is	marked non-blocking and	no connections are
		  present to be	accepted.

     bind(2), connect(2), getpeername(2), listen(2), select(2),	socket(2)

     The accept() function appeared in 4.2BSD.

4.2 Berkeley Distribution      December	11, 1993     4.2 Berkeley Distribution


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