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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  type  SOCK_STREAM  or

       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.

       EBADF  The argument sockfd is not a valid descriptor.

              The argument sockfd is not a socket.

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


       4.4BSD, POSIX.1-2001.  The listen() function call 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.1-2001  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
       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 128.


       See bind(2).


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


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