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CAsyncSocket::Listen 

Call this member function to listen for incoming connection requests.


BOOL Listen(
   int nConnectionBacklog = 5 
);

Parameters

nConnectionBacklog

The maximum length to which the queue of pending connections can grow. Valid range is from 1 to 5.

Nonzero if the function is successful; otherwise 0, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED   A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN   The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEADDRINUSE   An attempt has been made to listen on an address in use.

  • WSAEINPROGRESS   A blocking Windows Sockets operation is in progress.

  • WSAEINVAL   The socket has not been bound with Bind or is already connected.

  • WSAEISCONN   The socket is already connected.

  • WSAEMFILE   No more file 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.

To accept connections, the socket is first created with Create, a backlog for incoming connections is specified with Listen, and then the connections are accepted with Accept. Listen applies only to sockets that support connections, that is, those of type SOCK_STREAM. This socket is put into "passive" mode where incoming connections are acknowledged and queued pending acceptance by the process.

This function is typically used by servers (or any application that wants to accept connections) 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 ports (descriptors). It will accept connections until the queue is emptied. If ports 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.

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