LISTEN(2)                 (2020-06-09)                  LISTEN(2)

          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 SOCK_SEQPACKET.

          The backlog argument defines the maximum length to which the
          queue of pending connections for sockfd may grow.  If a con-
          nection 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 connec-
          tion 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).

               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()

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     LISTEN(2)                 (2020-06-09)                  LISTEN(2)

          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 informa-

          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 ker-
          nels 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 5.10 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

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