When is casting void pointer needed in C?

Question

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 tha

Answer 1

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.