listen(2) — Linux manual page


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

NAME         top

       listen - listen for connections on a socket

SYNOPSIS         top

       #include <sys/socket.h>

       int listen(int sockfd, int backlog);

DESCRIPTION         top

       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

       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.

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

              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()

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() first appeared in

NOTES         top

       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

           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.

EXAMPLES         top

       See bind(2).

SEE ALSO         top

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

COLOPHON         top

       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)

Pages that refer to this page: accept(2)bind(2)connect(2)socket(2)socketcall(2)syscalls(2)proc(5)services(5)systemd.socket(5)epoll(7)ip(7)sctp(7)signal-safety(7)sock_diag(7)socket(7)tcp(7)vsock(7)