passing one function to different threads - c++

Question

I have to create an application where I'll have to make multiple threads. SoI thought to try making one function and passing it to different threads. Initially I've created two threads and have declared one function to be passed to both of them. All I am trying to do is to pass different integers to those threads and display it in the thread function,here is my code:
DWORD WINAPI Name(LPVOID lpParam)
{
int *Ptr=(int*)lpParam;
for(int j=0;j<2;j++)
{
cout<<"Thread"<<endl;
cout<<*Ptr<<endl;
}
return 0;
}
int main()
{
int a=10,b=15,c=25;
HANDLE thread1,thread2;
DWORD threadID,threadID2;
thread2= CreateThread(NULL,0,Name,LPVOID(a),0,&threadID2);
thread1= CreateThread(NULL,0,Name,LPVOID(b),0,&threadID);
for(int i=0;i<5;i++)
{
cout<<"Main Thread"<<endl;
}
if(thread1==NULL)
{
cout<<"Couldn't Create Thread:("<<endl;
exit(0);
}
if(thread2==NULL)
{
cout<<"Couldn't Create Thread:("<<endl;
exit(0);
}
return 0;
}
but this code is not running properly,i.e compliles fine,starts fine but afterwards gives a debugging error.
Could someone let let me know of my mistake and how I could correct it coz being able to utilize one function for multiple threads will be really helpful for me.

Answer 1

Wait for your child threads to return. Do this:
int main()
{
int a=10,b=15,c=25;
HANDLE thread[2];
DWORD threadID,threadID2;
thread[1]= CreateThread(NULL,0,Name,LPVOID(a),0,&threadID2);
thread[0]= CreateThread(NULL,0,Name,LPVOID(b),0,&threadID);
for(int i=0;i<5;i++)
{
cout<<"Main Thread"<<endl;
}
if(thread[0]==NULL)
{
cout<<"Couldn't Create Thread:("<<endl;
exit(0);
}
if(thread[1]==NULL)
{
cout<<"Couldn't Create Thread:("<<endl;
CloseHandle(thread[0]);
exit(0);
}
WaitForMultipleObjects(2, thread, TRUE, INFINITE);
CloseHandle(thread[0]);
CloseHandle(thread[1]);
return 0;
}
The handle of a thread is signaled when the thread is terminated (refer CreateThread).

Answer 2

You are passing the address of a local variable in the function into your thread. By the time the thread gets around to running your function has probably exited main already. So the thread will try to access a variable that no longer exists on the stack so will be reading some random value which when you try to dereference it as a pointer will likely crash.
You main needs to wait. For a simple test just put in a Sleep(10000) or something before it exits. Obviously that's no use for a real program.

Answer 3

To summarize the comments: There are two ways you can pass data. Either directly inside the void pointer, because "void pointer" is an integral type and thus can represent integers (but it doesn't necessarily have the same width as int), or indirectly by passing an actual address of the thing you care about.
Method 1 (pass the value):
DWORD WINAPI Name(LPVOID lpParam)
{
intptr_t n = reinterpret_cast<intptr_t>(lpParam);
// ...
}
int main()
{
intptr_t a = 10;
thread1 = CreateThread(NULL, 0, Name, reinterpret_cast<void *>(a), 0, &threadID);
// ... ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
}
For this method, we use the integral type intptr_t, which has the same width as void *. We use reinterpret-casts to store and retrieve arbitrary integral values.
Method 2 (pass a pointer):
DWORD WINAPI Name(LPVOID lpParam)
{
T * p = static_cast<T *>(lpParam);
// ... use *p ...
}
int main()
{
T thing_I_care_about;
thread1 = CreateThread(NULL, 0, Name, &thing_I_care_about, 0, &threadID);
// ... ^^^^^^^^^^^^^^^^^^^
}
This is the more general method, but requires that thing_I_care_about remain alive, and it becomes a brittle shared state, so lifetime management and synchronisation become issues. Note that any object pointer is implicitly convertible to void *, so there's no need for the cast at the call site.
Finally, as others have commented, don't forget to join or detach your threads.