How can you get the return value of a Windows thread? - c++

Question

I was just wondering if it is possible (and if so, how) to get the return value of a thread in C++ (Windows). I have several threads, and I use WaitForMultipleObjects(...) on them. This waits until a thread is done, and returns the index of said thread, and all is well. However, I want to be able to obtain the return value of the thread that finished using its handle.
For example:
DWORD WINAPI Thread1(void *parameter){
...
if(...) return 0;
else return -1;
}
DWORD WINAPI Thread2(void *parameter){
...
if(...) return 1;
else return 0;
}
int main(){
HANDLE t1 = CreateThread(0, 0, Thread1, NULL, 0, 0);
HANDLE t2 = CreateThread(0, 0, Thread2, NULL, 0, 0);
HANDLE *threads = new HANDLE[2];
threads[0] = t1;
threads[1] = t2;
int result = WaitForMultipleObjects(2, threads, false, INFINITE);
if(result == 0){
//get the threads value here:
int retVal = SomeFunction(t1); //What is SomeFunction?
}
...
}
I have tried to use GetExitCodeThread(thread) but I'm assuming this returns a system exit code, as it always gives me a very strange integer. Does anyone know a way, or a workaround?

Answer 1

GetExitCodeThread is the correct function. Here is the MSDN description of what it does:
This function returns immediately. If the specified thread has not
terminated and the function succeeds, the status returned is
STILL_ACTIVE. If the thread has terminated and the function succeeds,
the status returned is one of the following values:
The exit value specified in the ExitThread or TerminateThread function.
The return value from the thread function.
The exit value of the thread's process.

Answer 2

The problem with most programmer is that they don't pay attention to compiler warnings. At level 4 (/W4), the compiler produces following warning when returning -1 from a function that is returning DWORD:
warning C4245: 'return' : conversion from 'int' to 'DWORD', signed/unsigned mismatch

Answer 3

you need to check for STILL_ACTIVE for the values one of these threads may still be active

Answer 4

You can use C++ 11's threads concepts using std::future.
See an example below.
int ReturnFrmThread() {
return 100;
}
int main() {
std::future<int> GetRetVal= std::async(ReturnFrmThread); // Execution of ReturnFrmThread starts
int answer = GetAnAnswer.get(); // gets value as 100;
// Waits until ReturnFrmThread has finished
}