listen - listen for connections on a socket


   #include <sys/types.h>          /* See NOTES */
   #include <sys/socket.h>

   int listen(int sockfd, int backlog);


   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

   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.


   On  success,  zero is returned.  On error, -1 is returned, and errno is
   set appropriately.


          Another socket is already listening on the same port.

          (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

   EBADF  The argument sockfd is not a valid file descriptor.

          The file descriptor sockfd does not refer to a socket.

          The  socket  is  not  of  a  type  that  supports  the  listen()


   POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() first appeared in 4.2BSD).


   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 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


   See bind(2).


   accept(2), bind(2), connect(2), socket(2), socket(7)


   This  page  is  part of release 4.09 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


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