Search code examples
cpointersconstants

Changing the value of a constant

I thought that the & operator wouldn't work for constants because I thought their invokations are replaced with their values at compile time like with macros and that they're rvalues. I wrote this test and the output suprised me.

#include<stdio.h>
int main(void){
    const int a=1;
    *(int*)&a=2;
    printf("%i%i",a,*&a);
    return 0;
}

It outputs 12. I expected it to be 11. Thanks for reaching out.

Solution

  • It outputs 12. I expected it to be 11.

    Incorrect expectations.

    Casting a pointer to const data to a pointer to non-const data is OK - if the resulting pointer is not used to write data.

    const int a=1;
(int*)&a; // OK

    Attempting to write via a pointer that originated as a pointer to const data is undefined behavior (UB). Don't code like that. Anything may happen.

    *(int*)&a = ... // Not OK

    If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. C17dr § 6.7.3 7