Is dialogue possible through pipes between mother and child process on Windows? - windows

Question

I use CreateProcess and CreatePipe to spawn a child process and set up pipes between mother and child to communicate through. Then I use WriteFile to write to the write handle of the child's input pipe and ReadFile to read from the read handle of the child's output pipe. After having finished writing to the child I do CloseHandle on the write handle of the input pipe.
This all works well. However, I don't want it to work like this. I want to feed one line to the child, have the child compute something and output the results as a line of output, and then read that line of output from the mother. Then feed another line of input to the child and so on.
Unfortunately, when I skip the CloseHandle function call the two processes hang and nothing happens. So how can I reuse the pipes and avoid closing them? If I close them I have to create the child process again, right? That's a heavy operation, I suppose, and I really want to avoid that. Is there a good solution using pipes? I want the child process to run indefinitely and the communication to be a dialogue, alternating between writes and reads.

Answer 1

I solved it by using Windows kernel ReadFile and WriteFile instead of standard C functions in the child code. Here is the child code:
HANDLE inp = (HANDLE)_get_osfhandle(0);
HANDLE out = (HANDLE)_get_osfhandle(1);
char buffer[0x400];
unsigned long N;
while (ReadFile(inp, buffer, sizeof(buffer), &N, NULL) && N > 0)
{
WriteFile(out, buffer, N, &N, NULL);
}
And here is the mother code:
process app("child.exe");
app.write(string("hello\n"));
app.read().print();
app.write(string("world\n"));
app.read().print();
It prints:
hello
world