LISTEN(3P) POSIX Programmer's Manual LISTEN(3P)

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

listen — listen for socket connections and limit the queue of incoming connections

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

The listen() function shall mark a connection-mode socket, specified by the socket argument, as accepting connections.

The backlog argument provides a hint to the implementation which the implementation shall use to limit the number of outstanding connections in the socket's listen queue. Implementations may impose a limit on backlog and silently reduce the specified value. Normally, a larger backlog argument value shall result in a larger or equal length of the listen queue. Implementations shall support values of backlog up to SOMAXCONN, defined in <sys/socket.h>.

The implementation may include incomplete connections in its listen queue. The limits on the number of incomplete connections and completed connections queued may be different.

The implementation may have an upper limit on the length of the listen queue—either global or per accepting socket. If backlog exceeds this limit, the length of the listen queue is set to the limit.

If listen() is called with a backlog argument value that is less than 0, the function behaves as if it had been called with a backlog argument value of 0.

A backlog argument of 0 may allow the socket to accept connections, in which case the length of the listen queue may be set to an implementation-defined minimum value.

The socket in use may require the process to have appropriate privileges to use the listen() function.

Upon successful completions, listen() shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.

The listen() function shall fail if:

The socket argument is not a valid file descriptor.

The socket is not bound to a local address, and the protocol does not support listening on an unbound socket.
The socket is already connected.
The socket argument does not refer to a socket.
The socket protocol does not support listen().

The listen() function may fail if:

The calling process does not have appropriate privileges.
The socket has been shut down.
Insufficient resources are available in the system to complete the call.

The following sections are informative.





accept(), connect(), socket()

The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .

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2017 IEEE/The Open Group