WinAPI's sleep doesn't work inside child thread - c++

Question

I'm a beginner and I'm trying to reproduce a rae condition in order to familirize myself with the issue. In order to do that, I created the following program:
#include <Windows.h>
#include <iostream>
using namespace std;
#define numThreads 1000
DWORD __stdcall addOne(LPVOID pValue)
{
int* ipValue = (int*)pValue;
*ipValue += 1;
Sleep(5000ull);
*ipValue += 1;
return 0;
}
int main()
{
int value = 0;
HANDLE threads[numThreads];
for (int i = 0; i < numThreads; ++i)
{
threads[i] = CreateThread(NULL, 0, addOne, &value, 0, NULL);
}
WaitForMultipleObjects(numThreads, threads, true, INFINITE);
cout << "resulting value: " << value << endl;
return 0;
}
I added sleep inside a thread's function in order to reproduce the race condition as, how I understood, if I just add one as a workload, the race condition doesn't manifest itself: a thread is created, then it runs the workload and it happens to finish before the other thread which is created on the other iteration starts its workload. My problem is that Sleep() inside the workload seems to be ignored. I set the parameter to be 5sec and I expect the program to run at least 5 secs, but insted it finishes immediately. When I place Sleep(5000) inside main function, the program runs as expected (> 5 secs). Why is Sleep inside thread unction ignored?
But anyway, even if the Sleep() is ignored, the program outputs this everytime it is launched:
resulting value: 1000
while the correct answer should be 2000. Can you guess why is that happening?

Answer 1

WaitForMultipleObjects only allows waiting for up to MAXIMUM_WAIT_OBJECTS (which is currently 64) threads at a time. If you take that into account:
#include <Windows.h>
#include <iostream>
using namespace std;
#define numThreads MAXIMUM_WAIT_OBJECTS
DWORD __stdcall addOne(LPVOID pValue) {
int* ipValue=(int*)pValue;
*ipValue+=1;
Sleep(5000);
*ipValue+=1;
return 0;
}
int main() {
int value=0;
HANDLE threads[numThreads];
for (int i=0; i < numThreads; ++i) {
threads[i]=CreateThread(NULL, 0, addOne, &value, 0, NULL);
}
WaitForMultipleObjects(numThreads, threads, true, INFINITE);
cout<<"resulting value: "<<value<<endl;
return 0;
}
...things work much more as you'd expect. Whether you'll actually see results from the race condition is, of course, a rather different story--but on multiple runs, I do see slight variations in the resulting value (e.g., a low of around 125).

Answer 2

Jerry Coffin has the right answer, but just to save you typing:
#include <Windows.h>
#include <iostream>
#include <assert.h>
using namespace std;
#define numThreads 1000
DWORD __stdcall addOne(LPVOID pValue)
{
int* ipValue = (int*)pValue;
*ipValue += 1;
Sleep(5000);
*ipValue += 1;
return 0;
}
int main()
{
int value = 0;
HANDLE threads[numThreads];
for (int i = 0; i < numThreads; ++i)
{
threads[i] = CreateThread(NULL, 0, addOne, &value, 0, NULL);
}
DWORD Status = WaitForMultipleObjects(numThreads, threads, true, INFINITE);
assert(Status != WAIT_FAILED);
cout << "resulting value: " << value << endl;
return 0;
}
When things go wrong, make sure you've asserted the return value of any Windows API function that can fail. If you really badly need to wait on lots of threads, it is possible to overcome the 64-thread limit by chaining. I.e., for every additional 64 threads you need to wait on, you sacrifice a thread whose sole purpose is to wait on 64 other threads, and so on. We (Windows Developer's Journal) published an article demonstrating the technique years ago, but I can't recall the author name off the top of my head.