How do I modify CreateThread function? - c++

Question

Like my title says, I'm trying to modify the CreateThread function. I want it to report the sequence number of the thread using the loop.
I'm fairly new to programming in C++ and am unsure which parameter needs to be altered. I suspect it may be the 'lpParameter' but after looking on microsofts website (https://msdn.microsoft.com/en-us/library/windows/desktop/ms682453%28v=vs.85%29.aspx) I still do not understand how the parameters work.
So far I have:
int int_tmain(int argc, _TCHAR* argv[])
{
HANDLE hThread[numThreads];
int tNum[10];
for (int i = 0; i < numThreads; i++)
{
tNum[i] = i;
hThread[i] =
CreateThread(NULL, 0, helloFun, NULL, 0, NULL);
} WaitForMultipleObjects(numThreads, hThread, TRUE, INFINITE);
return 0;
}
Which produces a blank thread.
EDIT:
Sorry for the confusion guys. I have declared the thread routine:
const int numThreads = 4;
DWORD WINAPI helloFun(LPVOID pArg)
{
printf("Hello Thread \n");
return 0;
}

Answer 1

It's not clear what you are trying to do, but it is clear that you are calling WaitForMultipleObjects in the wrong place.
You are trying to wait for numThreads after only creating one thread. The rest of the hThread array is still uninitialized, and passing wild handles to WaitForMultipleObjects is a terrible idea.
Move the wait call outside the loop.

Answer 2

Your thread routine must be declared
DWORD name (LPVOID whatever)
Whatever is passed as lpParameter is passed to the thread. In this case, cast integer to pointer and back.

Answer 3

As you mentioned, if you want to pass a parameter to the thread, then you use the lpParameter. It expects a pointer, so you could pass, for instance, &i, although that's not exactly a good idea. You could create a pointer to int, and pass it, and then delete after WaitForMultipleObjectsreturns.
As Ben said, you are calling WaitForMultipleObjects in the wrong place.
Something like this should work:
int int_tmain(int argc, _TCHAR* argv[])
{
HANDLE hThread[numThreads];
int tNum[10];
int *parameters[numThreads];
for (int i = 0; i < numThreads; i++)
{
tNum[i] = i;
parameters[i] = new int(i);
hThread[i] = CreateThread(NULL, 0, helloFun, (void *)parameters[i], 0, NULL);
}
WaitForMultipleObjects(numThreads, hThread, TRUE, INFINITE);
for (int i = 0; i < numThreads; i++)
delete parameters[i];
return 0;
}
You will have to cast the lpParameter to int in your helloFun.

Answer 4

Try something more like this:
const int maxThreads = 4;
DWORD WINAPI helloFun(LPVOID pArg)
{
int num = * (int*) pArg;
printf("Hello from Thread %d\n", num);
return 0;
}
int _tmain(int argc, _TCHAR* argv[])
{
HANDLE hThread[maxThreads];
int tNum[maxThreads];
int numThreads = 0;
for (int i = 0; i < maxThreads; i++)
{
tNum[numThreads] = i+1;
hThread[numThreads] = CreateThread(NULL, 0, &helloFun, &tNum[numThreads], 0, NULL);
if (hThread[numThreads] == NULL)
{
printf("Unable to create a thread! Error: %u\n", GetLastError());
break;
}
++numThreads;
}
if (numThreads > 0)
{
WaitForMultipleObjects(numThreads, hThread, TRUE, INFINITE);
for (int i = 0; i < numThreads; i++)
CloseHandle(hThread[i]);
}
return 0;
}