PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of
       this interface may differ (consult the corresponding Linux  manual  page  for  details  of
       Linux behavior), or the interface may not be implemented on Linux.

NAME

       listen — listen for socket connections and limit the queue of incoming connections

SYNOPSIS

       #include <sys/socket.h>

       int listen(int socket, int backlog);

DESCRIPTION

       The  listen()  function  shall  mark  a  connection-mode  socket,  specified by the socket
       argument, as accepting connections.

       The backlog argument provides a hint to the implementation which the implementation  shall
       use  to  limit  the  number  of  outstanding  connections  in  the  socket's listen queue.
       Implementations may impose a limit on backlog and silently  reduce  the  specified  value.
       Normally,  a larger backlog argument value shall result in a larger or equal length of the
       listen queue. Implementations shall support values of backlog up to SOMAXCONN, defined  in
       <sys/socket.h>.

       The  implementation  may include incomplete connections in its listen queue. The limits on
       the number of incomplete connections and completed connections queued may be different.

       The implementation may have an upper limit on the length of the listen queue—either global
       or  per accepting socket. If backlog exceeds this limit, the length of the listen queue is
       set to the limit.

       If listen() is called with a backlog argument value that is  less  than  0,  the  function
       behaves as if it had been called with a backlog argument value of 0.

       A  backlog  argument  of  0  may allow the socket to accept connections, in which case the
       length of the listen queue may be set to an implementation-defined minimum value.

       The socket in use may require the process  to  have  appropriate  privileges  to  use  the
       listen() function.

RETURN VALUE

       Upon  successful completions, listen() shall return 0; otherwise, -1 shall be returned and
       errno set to indicate the error.

ERRORS

       The listen() function shall fail if:

       EBADF  The socket argument is not a valid file descriptor.

       EDESTADDRREQ
              The socket is not bound to a local address,  and  the  protocol  does  not  support
              listening on an unbound socket.

       EINVAL The socket is already connected.

       ENOTSOCK
              The socket argument does not refer to a socket.

       EOPNOTSUPP
              The socket protocol does not support listen().

       The listen() function may fail if:

       EACCES The calling process does not have appropriate privileges.

       EINVAL The socket has been shut down.

       ENOBUFS
              Insufficient resources are available in the system to complete the call.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       accept(), connect(), socket()

       The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1-2017, Standard for Information Technology -- Portable  Operating  System  Interface
       (POSIX),  The  Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
       the Institute of Electrical and Electronics Engineers, Inc and The  Open  Group.   In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .