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       listen - listen for connections on a socket


       Standard C library (libc, -lc)


       #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 to indicate the


              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).

       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  capped to that value.  Since Linux 5.4, the default in this file is 4096; in
       earlier kernels, the default value is 128.  Before Linux 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)