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/types.h>          /* See NOTES */
       #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 of

       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 appropriately.

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 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.

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 5.08 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                            2020-06-09                        LISTEN(2)

Pages that refer to this page: accept(2)accept4(2)bind(2)connect(2)socket(2)socketcall(2)syscalls(2)proc(5)procfs(5)services(5)systemd.socket(5)epoll(7)ip(7)sctp(7)signal-safety(7)sock_diag(7)socket(7)tcp(7)vsock(7)