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       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 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)


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