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

       The backlog argument defines the maximum length to which the  queue  of
       pending  connections  for sockfd may grow.  If a connection request ar-
       rives when the queue is full, the client may receive an error  with  an
       indication  of ECONNREFUSED or, if the underlying protocol supports re-
       transmission, 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.

              (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

       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() opera-

       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 set-
       ting 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.

       See bind(2).

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

       This  page  is  part of release 5.05 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                             2020-02-09                         LISTEN(2)
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