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

     accept, accept4 --	accept a connection on a socket

     Standard C	Library	(libc, -lc)

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

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

     accept4(int s, struct sockaddr * restrict addr,
	 socklen_t * restrict addrlen, int flags);

     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() system call extracts the first connection	re-
     quest on the queue	of pending connections,	creates	a new socket, and al-
     locates a new file	descriptor for the socket which	inherits the state of
     the O_NONBLOCK and	O_ASYNC	properties and the destination of SIGIO	and
     SIGURG signals from the original socket s.

     The accept4() system call is similar, but the O_NONBLOCK property of the
     new socket	is instead determined by the SOCK_NONBLOCK flag	in the flags
     argument, the O_ASYNC property is cleared,	the signal destination is
     cleared and the close-on-exec flag	on the new file	descriptor can be set
     via the SOCK_CLOEXEC flag in the flags argument.

     If	no pending connections are present on the queue, and the original
     socket is not marked as non-blocking, accept() blocks the caller until a
     connection	is present.  If	the original socket is marked non-blocking and
     no	pending	connections are	present	on the queue, accept() returns an er-
     ror 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 argument that is filled-in with the address
     of	the connecting entity, as known	to the communications layer.  The ex-
     act format	of the addr argument is	determined by the domain in which the
     communication is occurring.  A null pointer may be	specified for addr if
     the address information is	not desired; in	this case, addrlen is not used
     and should	also be	null.  Otherwise, the addrlen argument is a value-re-
     sult argument; it should initially	contain	the amount of space pointed to
     by	addr; on return	it will	contain	the actual length (in bytes) of	the
     address 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.

     For some applications, performance	may be enhanced	by using an
     accept_filter(9) to pre-process incoming connections.

     When using	accept(), portable programs should not rely on the O_NONBLOCK
     and O_ASYNC properties and	the signal destination being inherited,	but
     should set	them explicitly	using fcntl(2);	accept4() sets these proper-
     ties consistently,	but may	not be fully portable across UNIX platforms.

     These calls return	-1 on error.  If they succeed, they return a non-nega-
     tive integer that is a descriptor for the accepted	socket.

     The accept() and accept4()	system calls 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 descrip-

     [EFAULT]		The addr argument is not in a writable part of the
			user address space.

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

     [ECONNABORTED]	A connection arrived, but it was closed	while waiting
			on the listen queue.

     The accept4() system call will also fail if:

     [EINVAL]		The flags argument is invalid.

     bind(2), connect(2), getpeername(2), getsockname(2), listen(2),
     select(2),	socket(2), accept_filter(9)

     The accept() system call appeared in 4.2BSD.

     The accept4() system call appeared	in FreeBSD 10.0.

BSD				October	9, 2014				   BSD


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