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LISTEN(3P)                 POSIX Programmer's Manual                LISTEN(3P)
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 argu-
       ment 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 implementa-
       tion-defined minimum value.
       The socket in use may require the process to  have  appropriate  privi-
       leges 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.1-2008, <sys_socket.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                           LISTEN(3P)