GetWindowThreadProcessId Exhibits Weird Behavior

[Jack41EL]

I wrote a program that calls GetWindowThreadProcessId. This function will either return a DWORD with the process ID if the second argument is NULL, or the second argument can be a pointer to a DWORD that the process ID is written to. When I call this function, different values are written to the PID variable depending on whether or the second argument is a pointer, or the other technique is used. May someone please explain this weird behavior?



[ code]
#include <iostream>
#include <Windows.h>

int main()
{
HWND hWnd;
DWORD PID;
HANDLE hProc;

hWnd = FindWindow(NULL, L"Untitled - Notepad");

if (!hWnd)
{
std::cout << "Failed to find window" << std::endl;
return 0;
}

PID = GetWindowThreadProcessId(hWnd, NULL);
std::cout << PID << std::endl;
GetWindowThreadProcessId(hWnd, &PID);
std::cout << PID << std::endl;

if (!PID)
{
std::cout << "Failed to get process ID" << std::endl;
return 0;
}

hProc = OpenProcess(PROCESS_ALL_ACCESS, TRUE, PID);

if (!hProc)
{
std::cout << "Failed to oppen the process." << std::endl;
return 0;
}

std::cout << "Success!" << std::endl;

return 0;
}
[ /code]

[ahcfan]

https://msdn.microsoft.com/en-us/library/windows/desktop/ms633522%28v=vs.85%29.aspx

According to this, GetWindowThreadProcessId() returns the thread id, not process id. The second argument can receive the process id.

[Texan40]

Yes, ahcfan is right. Call should look more like this:

DWORD tID, pID;
tID = GetWindowThreadProcessId(hWnd,&pID);

cout << "Thread ID: " << tID << endl;
cout << "Process ID: " << pID << endl;

[Jack41EL]

OK, thanks. I really should skim less.