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

6       listen - listen for connections on a socket
7

SYNOPSIS

9       #include <sys/types.h>          /* See NOTES */
10       #include <sys/socket.h>
11
12       int listen(int sockfd, int backlog);
13

DESCRIPTION

15       listen()  marks  the  socket referred to by sockfd as a passive socket,
16       that is, as a socket that will be used to  accept  incoming  connection
17       requests using accept(2).
18
19       The  sockfd  argument  is  a file descriptor that refers to a socket of
20       type SOCK_STREAM or SOCK_SEQPACKET.
21
22       The backlog argument defines the maximum length to which the  queue  of
23       pending  connections  for  sockfd  may  grow.   If a connection request
24       arrives when the queue is full, the client may receive an error with an
25       indication  of  ECONNREFUSED  or,  if  the underlying protocol supports
26       retransmission, the request may be ignored so that a later reattempt at
27       connection succeeds.
28

RETURN VALUE

30       On  success,  zero is returned.  On error, -1 is returned, and errno is
31       set appropriately.
32

ERRORS

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

55       POSIX.1-2001, POSIX.1-2008, 4.4BSD (listen() first appeared in 4.2BSD).
56

NOTES

58       To accept connections, the following steps are performed:
59
60           1.  A socket is created with socket(2).
61
62           2.  The socket is bound to a local address using bind(2),  so  that
63               other sockets may be connect(2)ed to it.
64
65           3.  A  willingness to accept incoming connections and a queue limit
66               for incoming connections are specified with listen().
67
68           4.  Connections are accepted with accept(2).
69
70       POSIX.1 does not require  the  inclusion  of  <sys/types.h>,  and  this
71       header  file  is not required on Linux.  However, some historical (BSD)
72       implementations required this header file,  and  portable  applications
73       are probably wise to include it.
74
75       The  behavior of the backlog argument on TCP sockets changed with Linux
76       2.2.  Now it specifies the  queue  length  for  completely  established
77       sockets  waiting  to  be  accepted, instead of the number of incomplete
78       connection requests.  The maximum length of the  queue  for  incomplete
79       sockets  can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog.  When
80       syncookies are enabled there is no logical maximum length and this set‐
81       ting is ignored.  See tcp(7) for more information.
82
83       If    the   backlog   argument   is   greater   than   the   value   in
84       /proc/sys/net/core/somaxconn, then it is  silently  truncated  to  that
85       value.   Since  Linux 5.4, the default in this file is 4096; in earlier
86       kernels, the default value is 128.   In  kernels  before  2.4.25,  this
87       limit was a hard coded value, SOMAXCONN, with the value 128.
88

EXAMPLES

90       See bind(2).
91

SEE ALSO

93       accept(2), bind(2), connect(2), socket(2), socket(7)
94

COLOPHON

96       This  page  is  part of release 5.07 of the Linux man-pages project.  A
97       description of the project, information about reporting bugs,  and  the
98       latest     version     of     this    page,    can    be    found    at
99       https://www.kernel.org/doc/man-pages/.
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103Linux                             2020-06-09                         LISTEN(2)
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