query about a multithreading program

Question

this might a simple query.

when we are creating a thread we are passing the (void *)t as an argument to a function PrintHello.we are copying the value in the pointer threadid(typacasting it to long) in tid which is a long variable again.i am confused with the parameter passing. is this a pass by reference or pass by value.over all is this correct? (value *)t. what value does it pass exactly?

long t;
rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;

please help me to avoid my confusion:(

Answer 1

It should be pass by reference.

Make sure parameter u pass should either be global or dynamically allocated in order to access in thread function .

You r passing addresss so write this ....

rc = pthread_create(&threads[t], NULL, PrintHello, (void *)&t);

Answer 2

What you are doing is pass by value. It is fine as long as sizeof(T) <= sizeof(void*) for the type T you are trying to pass.

If that is not the case, you should to create a T on the heap as it might be out of scope when the created thread tries to access it:

T* t = new T(someValue);
rc = pthread_create(&threads[t], NULL, PrintHello, (void*)t);