What does this mean in C?

Question

I am currently learning about the qsort function, and I encountered the compare function:

int cmpfunc (const void * a, const void * b)
{
    return ( *(int*)a - *(int*)b );
}

I don't understand why we are using *(int*)a. What is this thing, and what is its use? Why const void *a instead of simple int a?

Answer 1

Since qsort is supposed to be able to sort an array of any kind of things (including structs), it passes (as arguments to its compare method) a pointer to each of the things to compare. These can be anything, so the type used is "void *" which can be a pointer to anything. Your compare method then should cast that pointer to a pointer of the correct type ( in this example, int * because you are comparing two ints. then you dereference that cast pointer in order to compare the values pointed to, hence the *(int *)

Answer 2

The (int*) do a typecast to the memory address of the variable 'a' that was before a void pointer. The * before (int*) access the integer value of the memory address. void *a is used because this is a generic function, you could use it to compare two char, for example.

Answer 3

Your code is functionally equivalent to this:

int cmpfunc(const int* a, const int* b)
{
    return(*a - *b);
}

In other words, (int*)a casts your void* a into an integer pointer type.