const int a = 0; /* This variable is "unmodifiable", the compiler
                    should throw an error when this variable is changed */
int b = 0; /* This variable is modifiable */

b += 10; /* Changes the value of 'b' */
a += 10; /* Throws a compiler error */

The const qualification only means that we don’t have the right to change the data. It doesn’t mean that the value cannot change behind our back.

_Bool doIt(double const* a) {
   double rememberA = *a;
   // do something long and complicated that calls other functions

   return rememberA == *a;
}

During the execution of the other calls *a might have changed, and so this function may return either false or true.

Warning

Variables with const qualification could still be changed using pointers:

const int a = 0;

int *a_ptr = (int*)&a; /* This conversion must be explicitly done with a cast */
*a_ptr += 10;          /* This has undefined behavior */

printf("a = %d\\n", a); /* May print: "a = 10" */

But doing so is an error that leads to undefined behavior. The difficulty here is that this may behave as expected in simple examples as this, but then go wrong when the code grows.