compare function for anagram program

Question

hey guys i have this code here but i cannot understand what the cmpfunc function does actually i don't understand the return statement can someone explain it to me ? than

#include <stdio.h>
#include <stdlib.h>

int values[] = { 88, 56, 100, 2, 25 };

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

int main()
{
   int n;

   printf("Before sorting the list is: \n");
   for( n = 0 ; n < 5; n++ ) {
      printf("%d ", values[n]);
   }

   qsort(values, 5, sizeof(int), cmpfunc);

   printf("\nAfter sorting the list is: \n");
   for( n = 0 ; n < 5; n++ ) {
      printf("%d ", values[n]);
   }

  return(0);
}

Answer 1

Start with a qsort description - here it is.

void qsort (void* base, size_t num, size_t size,
            int (*compar)(const void*,const void*));

So you need to write a function with such inputs and output to use in qsort .

int cmpfunc (const void * a, const void * b)

is ok, but inputs are *void and you need to compare integers in your case. So you will need to convert types. That's why there are

 *(int*)a

Finally, return values of cmpfunc must match requirements from qsort description. In article I mentioned at the begging of this post you can find this implementation:

int compareMyType (const void * a, const void * b)
{
  if ( *(MyType*)a <  *(MyType*)b ) return -1;
  if ( *(MyType*)a == *(MyType*)b ) return 0;
  if ( *(MyType*)a >  *(MyType*)b ) return 1;
}   

Your version is just a proper simplification of it.

Answer 2

cmpfnc is comparing the values of of pointers provided. It uses void* so that any kind of data can be passed to it and compared after being converted. First it converts the void* values to int* and then deferences them via * to subtract the integers they are pointing at

Answer 3

When you want to sort an array of some kind you need a comparison function. the comparison function is used by qsort in order to compare two objects in the array. In this case the compare function compares two integers.