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LISTEN(2)                  Linux Programmer's Manual                 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  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 appropriately.

              Another socket is already listening on the same port.

       EBADF  The argument sockfd is not a valid descriptor.

              The argument sockfd is not a socket.

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

       4.4BSD, POSIX.1-2001.  The listen() function call 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.1-2001  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  proba-
       bly 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 trun-
       cated 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.

       See bind(2).

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

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

Linux                             2008-11-20                         LISTEN(2)