LISTEN(2)                   Linux Programmer's Manual                  LISTEN(2)

       listen - listen for connections on a socket

       #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

       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 error.

              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.  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)

       This page is part of release 5.13 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

Linux                              2021-03-22                          LISTEN(2)