Shared Sockets in the SPI
Socket sharing between processes in Windows Sockets is implemented as follows. A source process calls WSPDuplicateSocket to obtain a special WSAPROTOCOL_INFO structure. 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.
It is the service provider's responsibility to perform whatever operations are needed in the source process context and to create a WSAPROTOCOL_INFO structure that will be recognized when it subsequently appears as a parameter to WSPSocket in the target processes' context. The dwProviderReserved member of the WSAPROTOCOL_INFO structure is available for the service provider's use, and may be used to store any useful context information, including a duplicated handle.
This mechanism is designed to be appropriate for both single-threaded and preemptive multithreaded versions of Windows. Note however, that sockets may be shared among threads in a given process without using the WSPDuplicateSocket function, since a socket descriptor is valid in all of a process' threads.
As is described in section Descriptor Allocation, when new socket descriptors are allocated IFS providers must call WPUModifyIFSHandle and non-IFS providers must call WPUCreateSocketHandle.
One possible scenario for establishing and using a shared socket in a handoff mode is illustrated in the following table.
| Source process | IPC | Destination process |
|---|---|---|
| 1) WSPSocket, WSPConnect | ||
| 2) Requests target process identifier. | ==> | |
| 3) Receives process identifier request and responds. | ||
| 4) Receives process identifier. | <== | |
| 5) Calls WSPDuplicateSocket to get a special WSAPROTOCOL_INFO structure. | ||
| 6) Sends WSAPROTOCOL_INFO structure to target. | ||
| ==> | 7) Receives WSAPROTOCOL_INFO structure. | |
| 8) Calls WSPSocket to create shared socket descriptor. | ||
| 9)Uses shared socket for data exchange. | ||
| 10) WSPClosesocket | <== |