Linus Torvalds wrote: > > On Tue, 30 Sep 2008, Mike Travis wrote: >> One pain is: >> >> typedef struct __cpumask_s *cpumask_t; >> const cpumask_t xxx; >> >> is not the same as: >> >> typedef const struct __cpumask_s *const_cpumask_t; >> const_cpumask_t xxx; >> >> and I'm not exactly sure why. > > Umm. The const has different > > One is > > typedef const struct __cpumask_s *const_cpumask_t; > > which becomes > > (const struct __cpumask_s) * > > while the other is > > const cpumask_t xxx > > which is > > const (struct __cpumask_s *) > > and if you look a bit more closely, you'll see that they are _obviously_ > not the same thing at all. Thanks, yes that explains what I should have figured out. (Nice way to explain it btw... ;-) > > Quite frankly, I personally do hate typedefs that end up being pointers, > and used as pointers, without showing that in the source code. > > When you do > > type_t a; > > fn(a); > > I expect the code to essentially do a pass-by-value. But when the type_t > is a pointer, that doesn't really work. I agree, and as I mentioned, Rusty was working towards an alternative method of declaring cpumask's which does not hide this. My goal was to create an (apparent) opaque handle for cpumask_t and modify the code so all changes to the contents of the cpumask are via functions. > > Your issue with 'const' is just another version of the same. You don't > want the _pointer_ to be const, you want what it points _to_ to be const. > But because you hid the pointerness inside the typedef, you simply cannot > do that. > > The problem with cpumask's, of course, is that for the "small mask" case, > we really don't want it to be a pointer. So now it's sometimes a pointer > and sometimes not. The typedef hides that, and I understand why it's a > good idea, but I'm surprised you didn't understand what the implications > were for 'const', and I'm now a bit more leery about this whole thing just > because the typedef ends up hiding so much - it doesn't just hide the > basic type, it hides a very basic *code* issue. Perhaps then defining the cpumask as a list of unsigned longs (remove the outer struct) would play more "naturally". Lists by definition are always referenced by pointers. I have another version of the patchset that shows this and I'll post just the cpumask.h and doc files. > > Linus Thanks! Mike -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
