listen(2)

1LISTEN(2)                  Linux Programmer's Manual                 LISTEN(2)
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NAME

6       listen - listen for connections on a socket
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SYNOPSIS

9       #include <sys/socket.h>
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11       int listen(int sockfd, int backlog);
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DESCRIPTION

14       listen()  marks  the  socket referred to by sockfd as a passive socket,
15       that is, as a socket that will be used to  accept  incoming  connection
16       requests using accept(2).
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18       The  sockfd  argument  is  a file descriptor that refers to a socket of
19       type SOCK_STREAM or SOCK_SEQPACKET.
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21       The backlog argument defines the maximum length to which the  queue  of
22       pending  connections  for sockfd may grow.  If a connection request ar‐
23       rives when the queue is full, the client may receive an error  with  an
24       indication  of ECONNREFUSED or, if the underlying protocol supports re‐
25       transmission, the request may be ignored so that a later  reattempt  at
26       connection succeeds.
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RETURN VALUE

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

33       EADDRINUSE
34              Another socket is already listening on the same port.
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36       EADDRINUSE
37              (Internet domain sockets) The socket referred to by  sockfd  had
38              not  previously been bound to an address and, upon attempting to
39              bind it to an ephemeral port, it was determined  that  all  port
40              numbers  in  the ephemeral port range are currently in use.  See
41              the  discussion  of  /proc/sys/net/ipv4/ip_local_port_range   in
42              ip(7).
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44       EBADF  The argument sockfd is not a valid file descriptor.
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46       ENOTSOCK
47              The file descriptor sockfd does not refer to a socket.
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49       EOPNOTSUPP
50              The  socket  is  not of a type that supports the listen() opera‐
51              tion.
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CONFORMING TO

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

57       To accept connections, the following steps are performed:
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59           1.  A socket is created with socket(2).
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61           2.  The socket is bound to a local address using bind(2),  so  that
62               other sockets may be connect(2)ed to it.
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64           3.  A  willingness to accept incoming connections and a queue limit
65               for incoming connections are specified with listen().
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67           4.  Connections are accepted with accept(2).
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69       The behavior of the backlog argument on TCP sockets changed with  Linux
70       2.2.   Now  it  specifies  the  queue length for completely established
71       sockets waiting to be accepted, instead of  the  number  of  incomplete
72       connection  requests.   The  maximum length of the queue for incomplete
73       sockets can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog.   When
74       syncookies are enabled there is no logical maximum length and this set‐
75       ting is ignored.  See tcp(7) for more information.
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77       If   the   backlog   argument   is   greater   than   the   value    in
78       /proc/sys/net/core/somaxconn, then it is silently capped to that value.
79       Since Linux 5.4, the default in this file is 4096; in earlier  kernels,
80       the  default  value is 128.  In kernels before 2.4.25, this limit was a
81       hard coded value, SOMAXCONN, with the value 128.
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EXAMPLES

84       See bind(2).
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COLOPHON

90       This page is part of release 5.13 of the Linux  man-pages  project.   A
91       description  of  the project, information about reporting bugs, and the
92       latest    version    of    this    page,    can     be     found     at
93       https://www.kernel.org/doc/man-pages/.
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97Linux                             2021-03-22                         LISTEN(2)