Why can I modify the const pointer in C?

Question

Today I tried to use const indentifier, but I find the const variable can still be modified, which confuses me..

Following is the code, in compare(const void *a, const void *b) function, I tried to modify the value that a is pointing to:

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

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

int compare (const void *a, const void*b)
{
    *(int*)a=2;
/* Then the value that a points to will be changed! */
    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;
}

Then I also tried to change the value of a itself:

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

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

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

However, I found both of them works.. Can anyone explain to me why I need to use const in the parameter list of compare if they can still be changed?

Answer 1

It only works in this case because the pointer you're working against wasn't originally constant. Casting away constness and then modifying a value is undefined behaviour. UB means that the app can do anything from succeed to crash to make purple dragons fly out of your nostrils.

Answer 2

Its protecting you from silly mistakes, not when you try hard to make a mistake.
(int *)a when a is const something * is a bad practice, use (const int *)a instead.
a = b when a is const void * is ok because only the value pointed to is const. if you want both *a = x and a = x disallowed, declare a as const void * const.