What is it?

This is an example program to demonstrate how to communicate with a child process through pipe. The child perfoms input/output as usual using stdin/stdout like scanf(.) and printf(.). Pipe technique redirects the child's stdin/stdout to the in/out streams created by the parent program.

To compile under MinGW

Type:

gcc Parent.c -o Parent.exe
gcc Child.c -o Child.exe

To run

Type:

./Parent.exe HelloMsg.txt

Note

I modified the original code [1]. In its original form, the Parent code did not terminate when the child process terminated. The parent code blocked at ReadFile(.) till the child wrote to STDOUT. When the child process terminated, the parent was not informed and the latter blocked forever on the ReadFile(.).

I modified the code to wait for the child process to signal its termination using WaitForSingleObject(childProcInfo->hProcess, 0). The value 0 means the function does not wait, but returns immediately whether the object has signaled or not. This is to prevent blocking, so that the parent can continue its reading and printing. This idea came to me after reading [5].

Another, but cleaner way [6] to do the same is perhaps using

GetExitCodeProcess(childProcInfo.hProcess, &exitCode) && exitCode == STILL_ACTIVE

But I have not tried yet.

I previously attempted using asynchronous ReadFile(.) using OVERLAPPED to timeout if the child stopped writing for sometime (because it terminated) as suggested here [2]. However, it did not seem to work. It blocked on ReadFile(.) forever. I guess it is because OVERLAPPED only works for serial comm. like UART, but not stdin/stdout. See commit: 14352e24b5d39606 if you are interested to look at the code.

StringCchPrintf(.) is not available under MinGW. So I replaced it with snprintf(.) as suggested here [4].

References

1. https://msdn.microsoft.com/en-us/library/windows/desktop/ms682499(v=vs.85).aspx

2. http://stackoverflow.com/questions/7955199/device-driver-windows-readfile-function-timeout

3. https://blogs.msdn.microsoft.com/oldnewthing/20110707-00/?p=10223

4. http://betterlogic.com/roger/2010/08/undefined-reference-to-stringcchprintf/

5. http://www.catch22.net/tuts/undocumented-createprocess

6. http://stackoverflow.com/questions/4063051/possible-to-have-a-timeout-on-readfile

7. "Introduction to CreateProcess()", http://www.cplusplus.com/forum/beginner/48283/