listen

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



NAME
       listen - listen for connections on a socket

SYNOPSIS
       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int listen(int sockfd, int backlog);

DESCRIPTION
       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 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
       On success, zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

ERRORS
       EADDRINUSE
              Another socket is already listening on the same port.

       EADDRINUSE
              (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.

       ENOTSOCK
              The file descriptor sockfd does not refer to a socket.

       EOPNOTSUPP
              The socket is not of a type that supports the listen()
              operation.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() first appeared in 4.2BSD).

NOTES
       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; the default value in this file is 128.  In kernels before
       2.4.25, this limit was a hard coded value, SOMAXCONN, with the value
       128.

EXAMPLE
       See bind(2).

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

COLOPHON
       This page is part of release 5.03 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
       https://www.kernel.org/doc/man-pages/.



Linux                             2017-09-15                         LISTEN(2)