qsort has the following function protype
void qsort(
void* _Base,
size_t _NumOfElements,
size_t _SizeOfElements,
_CompareFunction
);
How is it possible that it is able to sort any array types (int, double, char, etc.)? How does qsort know the type of array that I am asking it to sort?
It is due to the comparison function that casts void pointers to required types when it compares elements of the array.
Here is a demonstration program.
#include <stdio.h>
#include <stdlib.h>
int cmp( const void *px, const void *py )
{
int x = *( const int * )px;
int y = *( const int * )py;
return ( y < x ) - ( x < y );
}
int main( void )
{
int a[] ={5, 3, 1, 9, 8, 2, 4, 7};
const size_t N = sizeof( a ) / sizeof( *a );
for (size_t i = 0; i < N; i++ )
{
printf( "%d ", a[i] );
}
putchar( '\n' );
qsort( a, N, sizeof( *a ), cmp );
for (size_t i = 0; i < N; i++ )
{
printf( "%d ", a[i] );
}
putchar( '\n' );
}
The program output is
5 3 1 9 8 2 4 7
1 2 3 4 5 7 8 9
As you can see within the comparison function cmp the passed pointers are casted to the required type.
int x = *( const int * )px;
int y = *( const int * )py;
To traverse the array the function qsort uses the pointer arithmetic the following way. Initially the pointer _Base points to the first element of the passed array. To move for example the pointer to the i-th element of the array it does something like the following
void *ith_element_ptr = ( char * )_Base + i * _SizeOfElements;