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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
       arrives when the queue is full, the client may receive an error with an
       indication  of  ECONNREFUSED  or,  if  the underlying protocol supports
       retransmission, 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.

       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 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-11-20			     LISTEN(2)
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