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       To  accept  connections,  a  socket  is first created with socket(2), a
15       willingness to accept incoming connections and a queue limit for incom‐
16       ing  connections  are specified with listen(), and then the connections
17       are accepted with accept(2).  The listen() call applies only to sockets
18       of type SOCK_STREAM or SOCK_SEQPACKET.
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20       The  backlog  parameter defines the maximum length the queue of pending
21       connections may grow to.  If a  connection  request  arrives  with  the
22       queue  full the client may receive an error with an indication of ECON‐
23       NREFUSED or, if the underlying protocol  supports  retransmission,  the
24       request may be ignored so that retries succeed.
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NOTES

27       The  behaviour  of  the  backlog  parameter on TCP sockets changed with
28       Linux 2.2.  Now it specifies the queue  length  for  completely  estab‐
29       lished  sockets waiting to be accepted, instead of the number of incom‐
30       plete connection requests. The maximum length of the queue  for  incom‐
31       plete  sockets  can  be set using the tcp_max_syn_backlog sysctl.  When
32       syncookies are enabled there is no  logical  maximum  length  and  this
33       sysctl setting is ignored.  See tcp(7) for more information.
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RETURN VALUE

37       On  success,  zero is returned.  On error, -1 is returned, and errno is
38       set appropriately.
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ERRORS

41       EADDRINUSE
42              Another socket is already listening on the same port.
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44       EBADF  The argument sockfd is not a valid descriptor.
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46       ENOTSOCK
47              The argument sockfd is not 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       4.4BSD,  POSIX.1-2001.   The  listen()  function call first appeared in
55       4.2BSD.
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BUGS

58       If the socket is of type AF_INET, and the backlog argument  is  greater
59       than  the  constant  SOMAXCONN (128 in Linux 2.0 & 2.2), it is silently
60       truncated to SOMAXCONN.
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