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() operation.

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 setting 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)