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

The disrepancy is issued by -fstrict-aliasing optimization option. Its behavior and possible traps are described in GCC documentation:

Pay special attention to code like this:

      union a_union {
        int i;
        double d;
      };

      int f() {
        union a_union t;
        t.d = 3.0;
        return t.i;
      }

The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected. See Structures unions enumerations and bit-fields implementation. However, this code might not:

      int f() {
        union a_union t;
        int* ip;
        t.d = 3.0;
        ip = &t.i;
        return *ip;
      }

Note that conforming implementation is perfectly allowed to take advantage of this optimization, as second code example exhibits undefined behaviour. See Olaf's and others' answers for reference.