Evan Jones wrote: > Andrew Haley wrote: >> so, this: >> >> int *sp = &int_var; >> void *vp = sp; >> *(short*)vp = 22; >> >> is undefined behaviour, and may well not do what you want. > > Ignoring platform specific issues like byte order and size, is it safe > to use memcpy instead? Example: > > int *sp = &int_var; > void *vp = sp; > short value = 22; > memcpy(vp, &value, sizeof(value)); This is okay, albeit non-portable. > In my experiments with strict aliasing and GCC, replacing accesses like > this with memcpy resolves the problem, even when GCC inlines the memcpy > into a simple load/store. It seems to me that this should be okay, since > the write through a void pointer is permitted to alias anything? It's okay if and only if you're not doing anything with pointers to incompatible types. > Or is the only safe way to use the mayalias attribute? > > I think I'm starting to understand why many people insist on using > -fno-strict-aliasing. Why not write in Standard C? I don't understand this determination to try to subvert the type system via pointers. What application actually needs it? If you *really* need to access the bits in a float you can do that via a union. Andrew.
