When is casting void pointer needed in C?

c pointers casting void
Amoeba picture Amoeba · Dec 9, 2013 · Viewed 25.2k times · Source

I've been looking at Advanced Linux Programming by Mitchell, Oldham and Samuel. I've seen in the section on pthreads something about void pointers and casting that confuses me.

Passing an argument to pthread_create(), they don't cast the pointer to a void pointer even though that is what the function expects. 

pthread_create( &thread, NULL, &compute_prime, &which_prime );

Here, which_prime is of type int.

But taking a value returned from the thread using pthread_join, they DO cast the variable to void pointer.

pthread_join( thread, (void*) &prime );

Here, prime is of type int again.

Why is casting done in the second instance and not in the first?

Answer

Fred Foo picture Fred Foo · Dec 9, 2013

The second example is a good example of why casting to void* is usually a mistake. It should be

void *primep = ′  // no cast needed
pthread_join(thread, &primep);

because pthread_join takes a void** as its second argument. The void* only makes sure the bug passes the compiler because the void* is converted to void** automatically.

So, when do you need to cast to void* or back:

  • when working with pointers stored as integers ((u)intptr_t);
  • when passing pointers to functions that have an incomplete prototype and take void* (or take a different type of pointer and you have void*); that usually means functions taking a variable number of arguments such as printf.

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