Question

I have just started learning pthreads API and I am following the tutorial here

However, in an example program of pthread_create, the sample program creates a long variable and passes its value, typecasted as void*. In the thread entry function, it dereferences it just like a long.

Is this legit?
I understand if I pass the address of variable t, every thread would be acting on the same variable and not on a copy of it. Can we do this because it’s a void* and the compiler has no idea about what type we are sending?

#include <pthread.h>
#include <stdio.h>

#define NUM_THREADS     5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   printf("Hello World! It's me, thread #%ld!\n", tid);
   pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   long t;
   for(t=0; t<NUM_THREADS; t++){
      printf("In main: creating thread %ld\n", t);
      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
      if (rc){
         printf("ERROR; return code from pthread_create() is %d\n", rc);
         exit(-1);
      }
   }
   pthread_exit(NULL);
}

Answer 1

This works as long as sizeof(long) <= sizeof(void*), and that every value of long can be represented as a void*.

Better would be to pass the address of the variable. You can cast from a T* to void*, and back again safely and without assumption.