Violating of strict-aliasing in C, even without any casting?

Question

How can *i and u.i print different numbers in this code, even though i is defined as int *i = &u.i;? I can only assuming

Answer 1

Strict aliasing is underspecified in the C Standard, but the usual interpretation is that union aliasing (which supersedes strict aliasing) is only permitted when the union members are directly accessed by name.

For rationale behind this consider:

void f(int *a, short *b) { 

The intent of the rule is that the compiler can assume a and b don't alias, and generate efficient code in f. But if the compiler had to allow for the fact that a and b might be overlapping union members, it actually couldn't make those assumptions.

Whether or not the two pointers are function parameters or not is immaterial, the strict aliasing rule doesn't differentiate based on that.