Jonathan Lennox <lennox@xxxxxxxxxxxxxxx> writes:
> void my_get_func(object* the_object)
> {
> my_user_context* uc;
> errcode err;
>
> /* Breaks strict aliasing rules? */
> err = GetUserContext(the_object, (void**)&uc);
> }
>
> Now, up through GCC 3.2.x, this code produced no warnings. However, as of
> GCC 3.3.x, the line
> err = GetUserContext(the_object, (void**)&uc);
> produces the warning
> objectref.c:31: warning: dereferencing type-punned pointer will break strict-aliasing rules
>
> I have several questions:
> 1. Is the function my_get_func() indeed non-conformant C?
No. However, GetUserContext is if it actually dereferences its
argument. In other words, the warning is correct.
> 2. Might GCC actually mis-compile it? (It seems like it would need
> inter-module optimization to do so, assuming GetUserContext isn't an
> inline function and that all object pointers have the same
> representation.)
Correct. However, future gccs might have inter-module optimizations.
> 3. Is the function my_get_func2() conformant? (It generates no -W -Wall
> warnings with GCC 3.2.2.)
Looks OK to me.
> 4. Is there any way to write this code conformantly without needing the
> intermediate void* variable?
Have it return the void*, and use NULL to indicate an error.
> 5. If not, will GCC (with -O2) always realize that the void* variable is
> unnecessary, and optimize it out?
Very likely yes. You could check the assembly to verify.
--
Falk
