Question

Code below taken from here.

* qsort example */
#include <stdio.h>
#include <stdlib.h>

int values[] = { 40, 10, 100, 90, 20, 25 };

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

int main ()
{
  int n;
  qsort (values, 6, sizeof(int), compare);
  for (n=0; n<6; n++)
     printf ("%d ",values[n]);
  return 0;
}

We have a compare function with parameters in its signature but when we call it in qsort no arguments are passed. How are the values of a and b passed to the function? Thanks

Answer 1

In the context of this expression:

qsort (values, 6, sizeof(int), compare);

the subexpression compare that identifies a function decays into a pointer to that function (and not a function call). The code is effectively equivalent to:

qsort (values, 6, sizeof(int), &compare);

This is exactly the same thing that happens to arrays when used as arguments to a function (which you might or not have seen before but is more frequently asked):

void f( int * x );
int main() {
   int array[10];
   f( array );      // f( &array[0] )
}