LISTEN

Section: Linux Programmer's Manual (2)
Updated: 23 July 1993
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NAME

listen, listen_secure - listen for connections on a socket

SYNOPSIS

#include <sys/socket.h>

int listen(int s, int backlog);

int listen_secure(int s, int backlog, security_id_t sid, int useclient);

DESCRIPTION

To accept connections, a socket is first created with socket(2), a willingness to accept incoming connections and a queue limit for incoming connections are specified with listen, and then the connections are accepted with accept(2). The listen call applies only to sockets of type SOCK_STREAM or SOCK_SEQPACKET.

The backlog parameter defines the maximum length the queue of pending connections may grow to. If a connection request arrives with the queue 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 retries may succeed.

Listen_secure may be used to specify the SID to use for sockets created by connections to this socket. Otherwise, by default, sockets created by a connection will be labeled with the SID of this socket. If sid is non-zero, then each socket created by a connection will be labeled with the specified SID. If useclient is non-zero, then each socket created by a connection will be labeled with SID of its peer socket. If both sid and useclient are specified, then EINVAL will be returned.

NOTES

The behaviour of the backlog parameter 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 the tcp_max_syn_backlog sysctl; see tcp(4). When syncookies are enabled there is no logical maximum length and this sysctl setting is ignored.

RETURN VALUE

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

ERRORS

EBADF: The argument s is not a valid descriptor.
ENOTSOCK: The argument s is not a socket.
EOPNOTSUPP: The socket is not of a type that supports the listen operation.

CONFORMING TO

SVr4, 4.4BSD (the listen function call first appeared in 4.2BSD).

BUGS

If the socket is of type AF_INET, and the backlog argument is greater than the constant SO_MAXCONN (128 in 2.0.23), it is silently truncated to SO_MAXCONN. For portable applications, don't rely on this value since BSD (and some BSD-derived systems) limit the backlog to 5.

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Time: 17:08:27 GMT, December 18, 2000