Coordinating Access to a Local Named Pipe with Semaphores

When a process writes to a named pipe, the process at the other end (or handle) of the pipe might require notification that data is available to be read. Similarly, when a process reads from a named pipe, the process at the other end might require notification that write space has become available. As long as the communicating processes are on the same computer system, shared event semaphores and muxwait semaphores can be used to provide this notification. Using shared semaphores for this purpose is more efficient than dedicating a thread to periodically poll each pipe, particularly when a server process is communicating with a large number of client processes. Whenever data is available in the pipe, the system posts a semaphore to the server or client (whichever is reading from the pipe). This means that the reading process can use DosWaitEventSem or DosWaitMuxWaitSem to wait for data to arrive, rather than devote a thread to periodically polling the pipe.

A process associates a semaphore with a named pipe by using DosSetNPipeSem. First, create an event semaphore with DosCreateEventSem, specifying the initial state of the semaphore as reset. Then call DosSetNPipeSem to attach the event semaphore to a particular named-pipe handle. Up to two event semaphores can be attached to each named pipe, one for the server process and one for the client process. If there is already a semaphore associated with one end of the pipe, that semaphore is replaced. A process can check the state of the semaphores by using DosQueryNPipeSemState.

The server or client process must then call DosWaitEventSem. The particular thread that calls this function will block until data is either read from or written to the pipe. At that time, the system posts the event semaphore, enabling the blocked thread to resume its execution.

If a process requires notification whenever any one of multiple named pipes has been written to or read from, it can either attach the same event semaphore to multiple pipes, or it can create a muxwait semaphore:

• If the same event semaphore is attached to multiple pipes, then the KeyHandle parameter of DosSetNPipeSem is used to assign a unique value to each pipe. After the event semaphore has been posted, the process calls DosQueryNPipeSemState. This function returns information about each of the named pipes that are attached to the semaphore, including key-handle values. The calling process can use this information to determine which one of the named pipes has either data or write space available.

• To use a muxwait semaphore, a process first creates an event semaphore for each of the pipes that it wants to monitor. Each semaphore must then be attached to a pipe by calling DosSetNPipeSem. Again, a unique key-handle value must be assigned to each pipe.

  Next, the process calls DosCreateMuxWaitSem to create the muxwait semaphore, specifying DCMW_WAIT_ANY as one of the flAttr flags. The muxwait semaphore will consist of a linked list of the previously created event semaphores.

  The process calls DosWaitMuxWaitSem so that it will be notified the next time data is read from or written to any of the pipes. However, it must call DosQueryNPipeSemState to determine which one of the pipes is ready to be read from or written to.