debugging a process spawned with CreateProcess in Visual Studio

Question

I've created a Windows/C++/WTL application that spawns a child process. The two processes communicate via anonymous pipes.

I'd like to be able to debug the child process.

Since both parent and child projects are in the same solution in Visual Studio 2008, is there any way to tell VS2008 that I'd like the debugger to debug both processes?

When I start the debugger with the parent process, the debugger won't break on any breakpoints in the child process code.

And since the child process is spawned by the parent process, I can't think of an easy way of attaching the child process (maybe via another instance of VS2008) when it's spawned.

Any insights greatly appreciated!

Answer 1

Plenty of options:

• You can debug multiple .exes with one instance of the debugger by using Debug + Attach. I however always use two instances of Visual Studio, use Debug + Attach in the second one.

• You could put a __debugbreak() in the child's code, the JIT debugger window will prompt you to select a debugger.

• You can use the "Image File Execution Options" registry key to automatically launch a debugger as soon as the child .exe is started. Add a key named HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Image File Execution Options\\YourApp.exe, set its "Debugger" value to "vsjitdebugger.exe"

• There is a VS add-in that makes it easier yet, haven't tried it myself yet.

Answer 2

You can temporarily put in a call to DebugBreak() somewhere in the startup code of your child process. This will cause Windows to prompt you if you want to debug that process.

Answer 3

You could put a global named mutex around your CreateProcess call, and then try to grab the mutex in the child process. If you then put a breakpoint on the CreateProcess call, you should have time to attach to the child before it does anything substantial.

Note that you would miss anything that happens before main in the child process.

edit: As an example, something like this, untested:

// parent.cpp
HANDLE hMutex = ::CreateMutex(NULL, TRUE, "Global\\some_guid");
::CreateProcess(...);
::ReleaseMutex(hMutex); // breakpoint here
::CloseHandle(hMutex);

// child.cpp
int main(...)
{
  HANDLE hMutex = ::OpenMutex(MUTEX_ALL_ACCESS, FALSE, "Global\\some_guid");
  ::WaitForSingleObject( hMutex, INFINITE );
  ::CloseHandle(hMutex);

  ...
}

You would probably want to wrap it with #if _DEBUG or environment variable checks.