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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.

              (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 file descriptor.

              The file descriptor sockfd does not refer to a socket.

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


       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.  Since Linux 5.4, the default in this file  is  4096;
       in  earlier kernels, the default value 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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