C++, How to determine if a Windows Process is running?

This is concerning Windows XP processes.

I have a process running, let's call it Process1. Process1 creates a new process, Process2, and saves its id.

Now, at some point Process1 wants Process2 to do something, so it first needs to make sure that Process2 is still alive and that the user has not not killed it.

How can I check that this process is still running? Since I created it, I have the Process ID, I would think there is some library function along the lines of IsProcessIDValid( id ) but I can't find it on MSDN

You can use GetExitCodeProcess. It will return STILL_ACTIVE (259) if the process is still running (or if it happened to exit with that exit code :( ).

The process handle will be signaled if it exits.

So the following will work (error handling removed for brevity):

BOOL IsProcessRunning(DWORD pid)
{
    HANDLE process = OpenProcess(SYNCHRONIZE, FALSE, pid);
    DWORD ret = WaitForSingleObject(process, 0);
    CloseHandle(process);
    return ret == WAIT_TIMEOUT;
}

Note that process ID's can be recycled - it's better to cache the handle that is returned from the CreateProcess call.

You can also use the threadpool API's (SetThreadpoolWait on Vista+, RegisterWaitForSingleObject on older platforms) to receive a callback when the process exits.

EDIT: I missed the "want to do something to the process" part of the original question. You can use this technique if it is ok to have potentially stale data for some small window or if you want to fail an operation without even attempting it. You will still have to handle the case where the action fails because the process has exited.