Hi Florian, On Fri, Jul 05, 2019 at 07:49:21AM +0200, Florian Weimer wrote: > > We already have an option for that (-Werror=implicit-function-declaration), > > and it is an error by default with -pedantic-errors already. If you are > > asking to make it an error by default, I second that; there needs to be a > > wat to turn it off though. Maybe it should be an error for c99 and later > > only, anyway? > > Yes, it should be an error by default, without any flags. Which is > gnu11 mode by now, I think. So it's not sufficient to do this for > c99/c11 mode.
When I say "c99 and later", of course that includes gnu99 and gnu11. My point is that we probably shouldn't by default error on this in c90 mode, since it is a valid construct there, and not extraordinarily harmful. > >> According to my observations, lack of an error diagnostic has turned > >> into a major usability issue. For bugs related to pointer truncation, > >> we could perhaps change the C front end to produce a hard error if an > >> int value returned from an implicitly declared function is converted to > >> a pointer. > > > > No, if we allow implicit declarations at all, your proposal would error > > on valid (but improbable :-) ) code. We should only ever give hard > > errors if we *know* something is wrong. > > Yes, it should be valid code on 32-bit targets, which is why it's so > common. It is valid for *all* targets. C11 6.3.2.3/5: An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation. and GCC documents that implementation-defined behaviour as A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged. > >> Implicit int we should remove as well. Checking configure scripts for > >> both issues at the same time would not be much more work. > > > > We could enable -Wimplicit-int by default for c99 and later, maybe even > > its -Werror- version. This conflicts with at least -fms-extensions it, > > seems, dunno what to do there. > > We can keep this a separate discussion if it helps. Yes please. Can you make separate PRs? There is essentially no overlap. Segher
