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LISTEN(2)		   Linux Programmer's Manual		     LISTEN(2)

NAME
       listen -	listen for connections on a socket

SYNOPSIS
       #include	<sys/types.h>	       /* See NOTES */
       #include	<sys/socket.h>

       int listen(int sockfd, int backlog);

DESCRIPTION
       listen()	 marks	the  socket referred to	by sockfd as a passive socket,
       that is,	as a socket that will be used to  accept  incoming  connection
       requests	using accept(2).

       The  sockfd  argument  is  a file descriptor that refers	to a socket of
       type SOCK_STREAM	or SOCK_SEQPACKET.

       The backlog argument defines the	maximum	length to which	the  queue  of
       pending	connections  for sockfd	may grow.  If a	connection request ar-
       rives when the queue is full, the client	may receive an error  with  an
       indication  of ECONNREFUSED or, if the underlying protocol supports re-
       transmission, the request may be	ignored	so that	a later	 reattempt  at
       connection succeeds.

RETURN VALUE
       On  success,  zero is returned.	On error, -1 is	returned, and errno is
       set appropriately.

ERRORS
       EADDRINUSE
	      Another socket is	already	listening on the same port.

       EADDRINUSE
	      (Internet	domain sockets)	The socket referred to by  sockfd  had
	      not  previously been bound to an address and, upon attempting to
	      bind it to an ephemeral port, it was determined  that  all  port
	      numbers  in  the ephemeral port range are	currently in use.  See
	      the  discussion  of  /proc/sys/net/ipv4/ip_local_port_range   in
	      ip(7).

       EBADF  The argument sockfd is not a valid descriptor.

       ENOTSOCK
	      The argument sockfd is not a socket.

       EOPNOTSUPP
	      The  socket  is  not of a	type that supports the listen()	opera-
	      tion.

CONFORMING TO
       4.4BSD, POSIX.1-2001.  The listen() function  call  first  appeared  in
       4.2BSD.

NOTES
       To accept connections, the following steps are performed:

	   1.  A socket	is created with	socket(2).

	   2.  The  socket  is bound to	a local	address	using bind(2), so that
	       other sockets may be connect(2)ed to it.

	   3.  A willingness to	accept incoming	connections and	a queue	 limit
	       for incoming connections	are specified with listen().

	   4.  Connections are accepted	with accept(2).

       POSIX.1-2001  does not require the inclusion of _sys/types.h_, and this
       header file is not required on Linux.  However, some  historical	 (BSD)
       implementations	required  this	header file, and portable applications
       are probably wise to include it.

       The behavior of the backlog argument on TCP sockets changed with	 Linux
       2.2.   Now  it  specifies  the  queue length for	completely established
       sockets waiting to be accepted, instead of  the	number	of  incomplete
       connection  requests.   The  maximum length of the queue	for incomplete
       sockets can be set using	/proc/sys/net/ipv4/tcp_max_syn_backlog.	  When
       syncookies are enabled there is no logical maximum length and this set-
       ting is ignored.	 See tcp(7) for	more information.

       If   the	  backlog   argument   is   greater   than   the   value    in
       /proc/sys/net/core/somaxconn,  then  it	is  silently truncated to that
       value; the default value	in  this  file	is  128.   In  kernels	before
       2.4.25,	this  limit  was a hard	coded value, SOMAXCONN,	with the value
       128.

EXAMPLE
       See bind(2).

SEE ALSO
       accept(2), bind(2), connect(2), socket(2), socket(7)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-05-10			     LISTEN(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

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