Can we change the value of an object defined with const through pointers?

Question

#include 
int main()
{
    const int a = 12;
    int *p;
    p = &a;
    *p = 70;
}

Will it work?

Answer 1

It's undefined behaviour. Proof:

/* program.c */

int main()
{
        const int a = 12;
        int* p;
        p = &a;
        *p = 70;
        printf("%d\n", a);
        return 0;
}


gcc program.c

and run it. Output will be 70 (gcc 4.3)

Then compile it like this:

gcc -O2 program.c

and run it. The output will be 12. When it does optimisation, the compiler presumably loads 12 into a register and doesn't bother to load it again when it needs to access a for the printf because it "knows" that a can't change.