> I'm trying to understand what '-fstrict-aliasing' actually means.
>
> Looking at the man page, one can read:
>
> -fstrict-aliasing
> Allow the compiler to assume the strictest aliasing
> rules applicable to the language being compiled. For C
> [...]
>
> What does "the strictest aliasing rules ..." means ?
It means the compiler is allowed to only guarantee behaviour that
the relevant standards guarantee, treat undefined behaviour as
undefined, etc.
> Also I don't really understand the examples provided by the man page. Ok
> type punning using union is still allowed.
It is allowed in C99, and a GCC extension for C90. GCC guarantees
behaviour for it always, also with -fstrict-aliasing.
> But the second example which is:
>
> However, this code might not:
>
> int f() {
> union a_union t;
> int* ip;
> t.d = 3.0;
> ip = &t.i;
> return *ip;
> }
>
> _might_ not work. Why using 'might' ?
It isn't guaranteed to work. It can work by accident. A lot
of incorrect code sometimes works.
> Does it work or not ?
It isn't correct code.
> AFAIK, this type of aliasing is defined by the standard.
It is not. You're accessing an object of type "double" using
an lvalue of type "int". This is not ok; see C99 6.5/7.
> The third example is:
>
> Similarly, access by taking the address, casting the
> resulting pointer and dereferencing the result has
> undefined behavior, even if the cast uses a union type,
> e.g.:
>
> int f() {
> double d = 3.0;
> return ((union a_union *) &d)->i;
> }
>
> Again in my understanding of the standard, this is an undefined
> behaviour. So why does man page mention this case ?
Because it is a common misunderstanding.
Segher
