The WSADuplicateSocket function is introduced to enable socket sharing across processes. A source process calls WSADuplicateSocket to obtain a special WSAPROTOCOL_INFO structure for a target process ID. It uses some interprocess communications (IPC) mechanism to pass the contents of this structure to a target process. The target process then uses the WSAPROTOCOL_INFO structure in a call to WSPSocket. The socket descriptor returned by this function will be an additional socket descriptor to an underlying socket which thus becomes shared. Sockets can be shared among threads in a given process without using the WSADuplicateSocket function because a socket descriptor is valid in all threads of a process.
The two (or more) descriptors that reference a shared socket can be used independently as far as I/O is concerned. However, the Windows Sockets interface does not implement any type of access control, so the processes must coordinate any operations on a shared socket. A typical example of sharing sockets is to use one process for creating sockets and establishing connections. This process then hands off sockets to other processes that are responsible for information exchange.
The WSADuplicateSocket function creates socket descriptors and not the underlying socket. As a result, all the states associated with a socket are held in common across all the descriptors. For example, a setsockopt operation performed using one descriptor is subsequently visible using a getsockopt from any or all descriptors. A process can call closesocket on a duplicated socket and the descriptor will become deallocated. The underlying socket, however, will remain open until closesocket is called with the last remaining descriptor.
Notification on shared sockets is subject to the usual constraints of the WSAAsyncSelect and WSAEventSelect functions. Issuing either of these calls using any of the shared descriptors cancels any previous event registration for the socket, regardless of which descriptor was used to make that registration. Thus, for example, it would not be possible to have process A receive FD_READ events and process B receive FD_WRITE events. For situations when such tight coordination is required, it is suggested that developers use threads instead of separate processes.