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Technical Standard: Networking Services (XNS), Issue 5.2 Draft 2.0
Copyright © 1999 The Open Group


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 marks a connection-mode socket, specified by the socket argument, as accepting connections.

The backlog argument provides a hint to the implementation which the implementation will use to limit the number of outstanding connections in the socket's listen queue. Normally, a larger backlog argument value will result in a larger or equal length of the listen queue.

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-dependent minimum value.

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


Upon successful completions, listen() returns 0. Otherwise, -1 is returned and errno is set to indicate the error.


The listen() function will 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 the appropriate privileges.

The socket has been shut down.

Insufficient resources are available in the system to complete the call.


accept(), connect(), socket(), <sys/socket.h>.


First released in Issue 4.

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