Question

This should be an easy one: I am creating a program that spawns a process using the win32 CreateProcess() function. Once this process is loaded, I find its window using FindWindow and send it messages using SendMessage(). The question is, how do I know when that window is ready to accept messages?

Consider the following:

HWND wnd;

BOOL Start()
{
  // Spawn the process
  if (! CreateProcess(...))
    return FALSE;

  // Find the process's window (class and name already known)
  wnd = FindWindow(MY_WINDOW_CLASS, MY_WINDOW_NAME);

  // Always returns FALSE because window has not yet been created.
  return (wnd != NULL);
}

The code above will (almost?) always fail; the window cannot be created and found that quickly. If I put a thread wait, say Sleep(1000), between the CreateProcess and FindWindow calls, it works fine. But this feels like a very bad hack.

How can I improve this?

Answer 1

(Edit): User IInspectable pointed out problems with WaitForInputIdle(), and suggested CBT Hooks instead.

(…) callback function used with the SetWindowsHookEx
function. The system calls this function before activating,
creating, (…) a window; (… many other things).

Also, CBT is short for computer-based training, for whatever reason.

(Old, beware, see comments.) You are looking for WaitForInputIdle(). Quote:

When a parent process creates a child
process, the CreateProcess function
returns without waiting for the child
process to finish its initialization.
Before trying to communicate with the
child process, the parent process can
use the WaitForInputIdle function to
determine when the child’s
initialization has been completed.