Const variable changed with pointer in C

Question

The variable i is declared const but still I am able to change the value with a pointer to the memory location to it. How is it possible?

int ma         


        

Answer 1

It's not the attempt to modify i that causes your program's behavior to be undefined, it's the initialization of ip.

const int i = 11;
int *ip = &i;

&i is of type const int*. ip is of type int*. Attempting to initialize an int* with a const int* value is a constraint violation. A conforming implementation is requires to issue a diagnostic; once it's done that, it may or may not reject the translation unit. If it accepts it, the C standard says nothing about the resulting program's behavior.

Compilers that accept such things typically generate code equivalent to a conversion from const int* to int*, making the declaration effectively equivalent to:

int *ip = (int*)&i;

but the language doesn't require this behavior.

Don't ignore warnings.

(Note that with the cast, the code doesn't violate a constraint; then the behavior of the following

*ip = 100;

is undefined because it attempts to modify a const-qualified object.)

gcc in particular has a lot of cases where diagnostics for constraint violations are handled (by default) as warnings rather than fatal errors. I personally dislike that about gcc; it lets through too much bad code.

(Your program's behavior is also undefined because you call printf without a visible declaration; add #include . And int main() should be int main(void).)