Windows Named Pipe Problem

Question

I'm writing a driver that communicates with a userland application through a named pipe. The userland application creates the named pipe by calling CreateNamedPipe() and then passes the pipe name to the driver by invoking an IOCTL. The driver then opens the pipe by calling ZwCreateFile().

The userland application then hits a loop that reads requests from the pipe, processes the requests and writes back the result to the pipe i.e.:

while(1) {
ReadFromPipe
ProcessRequest
WriteToPipe
}

The driver basically writes requests to the pipe and then directly reads back the answer:

WriteRequestToPipe
ReadAnswerFromPipe

My problem is that if the ReadAnswerFromPipe happens in the driver before the WriteToPipe happens in the application, ReadAnswerFromPipe never returns. So basically doing

WriteRequestToPipe
Sleep(10 seconds)
ReadAnswerFromPipe

fixes the problem.

Why am I seeing this?

Clarification: I'm using two different unidirectional pipes and the ReadAnswerFromPipe call is never returning although the application eventually successfully calls WriteToPipe...

Answer 1

Since named pipes support one-way communication at any given time, you will have to either use 2 pipes (as already suggested) for true asynchronous acces, or synchronize access to a single pipe (like a walkie-talkie). A semaphore object should work well for that. If you just set the Maximum Count parameter of CreateSemaphore() to 1 it will act as a binary semaphore. You can then ZwWaitForSingleObject() on that semaphore inside your while(true) message loop on both your client and server and you will have synchronized access.

Edit: Just remembered you are writing a driver as the server component of this, so you will need the kernel-specific functions and structs, the KSEMAPHORE struct on the driver side.