posix pthreads in c

Question

Iam new to c programming and needs some help.

long *taskids[NUM_THREADS];

for(t=0; t<NUM_THREADS; t++)
{
  taskids[t] = (long *) malloc(sizeof(long));
 *taskids[t] = t;
  printf("Creating thread %ld\n", t);
  rc = pthread_create(&threads[t], NULL, PrintHello, (void *) taskids[t]);
  ...
}

This code fragment demonstrates how to pass a simple integer to each thread. The calling thread uses a unique data structure for each thread, insuring that each thread's argument remains intact throughout the program. Iam not able to understand how this is happening can somebody explain it??

Answer 1

What this code is doing is storing an array of pointers to allocated data. Each thread has space allocated for a long and is then passed a pointer to that value. This gives the thread a place to work that the main thread still has access to.

This could be done with a little less allocation:

long taskids[NUM_THREADS];

for(t=0; t<NUM_THREADS; t++)
{
  taskids[t] = t;
  printf("Creating thread %ld\n", t);
  rc = pthread_create(&threads[t], NULL, PrintHello, (void *) &taskids[t]);
  ...
}

Though this does decrease the flexibility for freeing the taskids. It also requires that this function doesn't return until all the threads have finished.

Answer 2

• malloc allocates enough memory to hold a long integer. It returns a pointer to long, a long*.
• This long* is stored into taskids[t] so that it can be used later.
• pthread_create is called. One of its arguments is a value to be passed to the new thread.
  • Since the pthread_create function does not know what kind of value the programmer wants to pass, it uses a void* which can be a pointer to anything. taskids[t] is the pointer to a long integer. It is cast (that is the (void *)) to a void pointer and given as an argument to pthread_create.
• pthread_create creates a new thread and calls the PrintHello function and passes the void pointer to taskids[t] as the value to PrintHello.
• Once PrintHello starts, it will cast the void pointer back to a long pointer and then it can access the long integer that contains its taskid.