What's happening here? Pointer math messing up? [duplicate]

Question

    int i = 0, j = 0;

int *a , *b;

a = &i + 1;
b = &j;

printf(" %p \n ",a);
printf(" %p \n ",b);

if(a==b)
printf(" yo \n");

Output:

0x7ffda8e133a4

0x7ffda8e133a4

a is same as b, still printf does not execute.

ideone link to this code

This works well, when I make a and b as normal variables. (int a, b;)

Answer 1

ISO/IEC 9899:201x

§6.5.6 Equality operators:

7. Two pointers compare equal if and only if both are null pointers, both are pointers to the same object (including a pointer to an object and a subobject at its beginning) or function, both are pointers to one past the last element of the same array object, or one is a pointer to one past the end of one array object and the other is a pointer to the start of a different array object that happens to immediately follow the first array object in the address space.

but then again:

8. For the purposes of these operators, a pointer to an object that is not an element of an array behaves the same as a pointer to the first element of an array of length one with the type of the object as its element type.

So ... a and b should behave the same as a pointer to an array and i and j happen to be next to each other in memory ... then &i + 1 could point to the beginning of the first "array element" of j (b). Well, that's a big if. I wouldn't rely on that. Maybe in packed structs.

@OmG

The last version of the code works for me! [with initialized i and j]

gcc 9.1 -O3, 2 x printf() then ret.

        sub     rsp, 24
        mov     edi, OFFSET FLAT:.LC0
        xor     eax, eax
        lea     rsi, [rsp+12]
        mov     DWORD PTR [rsp+8], 0
        mov     DWORD PTR [rsp+12], 0
        call    printf                       ; <----------
        lea     rsi, [rsp+12]
        mov     edi, OFFSET FLAT:.LC0
        xor     eax, eax
        call    printf                       ; <----------
        xor     eax, eax
        add     rsp, 24
        ret                                  ; <----------

godbolt Compiler Explorer, gcc 9.1