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  connec‐
       tion 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() 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 con‐
               nections 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 speci‐
       fies  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 syncook‐
       ies 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 3.74 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)