Can a C pointer point to a value in an inner block and dereference the pointer after the block ends? [duplicate]

Question

I'm wondering if a C pointer can point to a value in an inner block and then dereference the pointer after the inner block ends?

#include <stdio.h>

int main() {
    int a = 42;
    int *r;
    {
        int b = 43;
        r = &b;
    }
    int c = 44;
    printf("%d\n", c);  // Output: 44

    // Is this OK?
    printf("%d\n", *r); // Output: 43
    return 0;
}

Am I getting the output "43" by luck or by design? Does C language specification say something about this situation? (Note that I'm not asking what a specific C compiler behaves)

Answer 1

This is not allowed. You were "lucky" you got an output of 43.

The lifetime of b ends after the scope it which it was declared ended, and accessing a variable whose lifetime has ended triggers undefined behavior.

This is described in section 6.2.4p2 of the C language:

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to (or just past) reaches the end of its lifetime.