listen

The Windows Sockets listen function establishes a socket to listen for an incoming connection.

int listen (

SOCKET s,
int backlog
);

Parameters

s

[in] A descriptor identifying a bound, unconnected socket.

backlog

[in] The maximum length to which the queue of pending connections can GROW. If this value is SOMAXCONN, then the underlying service provider responsible for socket s will set the backlog to a maximum "reasonable" value.

Remarks

To accept connections, a socket is first created with socket, a backlog for incoming connections is specified with listen, and then the connections are accepted with accept. listen applies only to sockets that are connection oriented, for example, those of type SOCK_STREAM. The socket s is put into "passive'' mode where incoming connection requests are acknowledged and queued pending acceptance by the process.

This function is typically used by servers that could have more than one connection request at a time: if a connection request arrives with the queue full, the client will receive an error with an indication of WSAECONNREFUSED.

listen attempts to continue to function rationally when there are no available descriptors. It will accept connections until the queue is emptied. If descriptors become available, a later call to listen or accept will refill the queue to the current or most recent "backlog'', if possible, and resume listening for incoming connections.

An application can call listen more than once on the same socket. This has the effect of updating the current backlog for the listening socket. Should there be more pending connections than the new backlog value, the excess pending connections will be reset and dropped.

Compatibility

The backlog parameter is limited (silently) to a reasonable value as determined by the underlying service provider. Illegal values are replaced by the nearest legal value.

Return Values

If no error occurs, listen returns zero. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

Error Codes

WSANOTINITIALISED	A successful WSAStartup must occur before using this function.
WSAENETDOWN	The network subsystem has failed.
WSAEADDRINUSE	An attempt has been made to listen on an address in use.
WSAEINPROGRESS	A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
WSAEINVAL	The socket has not been bound with bind.
WSAEISCONN	The socket is already connected.
WSAEMFILE	No more socket descriptors are available.
WSAENOBUFS	No buffer space is available.
WSAENOTSOCK	The descriptor is not a socket.
WSAEOPNOTSUPP	The referenced socket is not of a type that supports the listen operation.

See Also

accept, connect, socket

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