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listen(2) [centos man page]

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

listen - listen for connections on a socket SYNOPSIS
#include <sys/types.h> /* See NOTES */ #include <sys/socket.h> int listen(int sockfd, int backlog); DESCRIPTION
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 type SOCK_STREAM or SOCK_SEQPACKET. The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow. If a connection request arrives when the queue is 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 a later reattempt at connection succeeds. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
EADDRINUSE Another socket is already listening on the same port. EBADF The argument sockfd is not a valid descriptor. ENOTSOCK The argument sockfd is not a socket. EOPNOTSUPP The socket is not of a type that supports the listen() operation. CONFORMING TO
4.4BSD, POSIX.1-2001. The listen() function call first appeared in 4.2BSD. NOTES
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.1-2001 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 setting 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; the default value in this file is 128. In kernels before 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128. EXAMPLE
See bind(2). SEE ALSO
accept(2), bind(2), connect(2), socket(2), socket(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at Linux 2008-11-20 LISTEN(2)

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listen(3XNET)					   X/Open Networking Services Library Functions 				     listen(3XNET)

listen - listen for socket connections and limit the queue of incoming connections SYNOPSIS
cc [ flag ... ] file ... -lxnet [ library ... ] #include <sys/socket.h> int listen(int socket, int backlog); DESCRIPTION
The listen() function marks a connection-mode socket, specified by the socket argument, as accepting connections, and limits the number of outstanding connections in the socket's listen queue to the value specified by the backlog argument. If listen() is called with a backlog argument value that is less than 0, the function sets the length of the socket's listen queue to 0. The implementation may include incomplete connections in the queue subject to the queue limit. The implementation may also increase the specified queue limit internally if it includes such incomplete connections in the queue subject to this limit. Implementations may limit the length of the socket's listen queue. If backlog exceeds the implementation-dependent maximum queue length, the length of the socket's listen queue will be set to the maximum supported value. The socket in use may require the process to have appropriate privileges to use the listen() function. RETURN VALUES
Upon successful completions, listen() returns 0. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The listen() function will fail if: EBADF The socket argument is not a valid file descriptor. EDESTADDRREQ The socket is not bound to a local address, and the protocol does not support listening on an unbound socket. EINVAL The socket is already connected. ENOTSOCK The socket argument does not refer to a socket. EOPNOTSUPP The socket protocol does not support listen(). The listen() function may fail if: EACCES The calling process does not have the appropriate privileges. EINVAL The socket has been shut down. ENOBUFS Insufficient resources are available in the system to complete the call. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ SEE ALSO
accept(3XNET), connect(3XNET), socket(3XNET), attributes(5), standards(5) SunOS 5.11 10 Jun 2002 listen(3XNET)
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