WSPListen function

The WSPListen function establishes a socket to listen for incoming connections.


int WSPListen(
  _In_  SOCKET s,
  _In_  int    backlog,
  _Out_ LPINT  lpErrno


s [in]

Descriptor identifying a bound, unconnected socket.

backlog [in]

Maximum length to which the queue of pending connections can grow. If this value is SOMAXCONN, then the service provider should set the backlog to a maximum "reasonable" value. There is no standard provision to find out the actual backlog value.

lpErrno [out]

Pointer to the error code.

Return value

If no error occurs, WSPListen returns zero. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code is available in lpErrno.

Error codeMeaning

The network subsystem has failed.


Socket's local address is already in use and the socket was not marked to allow address reuse with SO_REUSEADDR. This error usually occurs at the time of bind, but could be delayed until this function if the bind was to a partially wildcard address (involving ADDR_ANY) and if a specific address needs to be committed at the time of this function.


Function is invoked when a callback is in progress.


Socket has not been bound with WSPBind.


Socket is already connected.


No more socket descriptors are available.


No buffer space is available.


The descriptor is not a socket.


Referenced socket is not of a type that supports the WSPListen operation.




To accept connections, a socket is first created with WSPSocket bound to a local address with WSPBind, a backlog for incoming connections is specified with WSPListen, and then the connections are accepted with WSPAccept. WSPListen applies only to sockets that are connection oriented (for example, SOCK_STREAM). The socket s is put into passive mode where incoming connection requests are acknowledged and queued pending acceptance by the Windows Sockets SPI client.

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.

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

A Windows Sockets SPI client can call WSPListen 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.

The backlog parameter is limited (silently) to a reasonable value as determined by the service provider. Illegal values are replaced by the nearest legal value. There is no standard provision to find out the actual backlog value.


Minimum supported client

Windows 2000 Professional [desktop apps only]

Minimum supported server

Windows 2000 Server [desktop apps only]



See also