On Sun, 4 Jun 2017, Willem Jan Withagen wrote: > On 04/06/2017 05:28, kefu chai wrote: > > On Sat, Jun 3, 2017 at 2:50 AM, Willem Jan Withagen <wjw@xxxxxxxxxxx> wrote: > >> Hi, > >> > >> Now that we have -Wvla set: > >> === > >> Adds C++ warning flag for C Variable-Length Arrays. > >> > >> C VLAs are not supported in C++. However, the GNU compiler allows > >> them as an extension, which it does not warn about when not in > >> pedantic mode. Unfortunately, it's easy to accidentally write > >> a VLA, even unintentionally-- adding this warning will help us > >> catch that. > >> === > >> > >> Are there going to be attempts to eradicate this usage? > >> Note that Clang also supports VLAs: > >> === > >> GCC and C99 allow an array's size to be determined at run time. This > >> extension is not permitted in standard C++. However, Clang supports such > >> variable length arrays for compatibility with GNU C and C99 programs. > >> === > >> > > > > it's not in C++11 standard. but it's a handy feature in practice. i > > still have trouble understanding we want to have -Wvla. i was trying > > to silence this warning option last night but failed to move further > > after several attempts. see https://github.com/ceph/ceph/pull/15441. > > > > probably, Jesse have more insights on this. > > It starts with old discussions from before 2009, and it would be hard > for compiler makers to actually build it. > > Both GCC and Clang support it. > > And next the major complaint I find about it, is that these structures > are build on the stack, which might cause stackoverlows (and that mostly > on embedded computers) That would be something for programmers to be > aware of. I'd consider this a feature more than a risk. It means we can allocate a modest-sized array without hitting the allocator. My vote is to just revert -Wvla... > I would probably think there are beter things to do, than to chase after > this. And IMHO the order should have been: Fix the VLAs and only then > turn the warning on on the master repo. As to not have every body start > looking at these warning that have just started to pop up. But perhaps > that is just me, because each and every warning is again a case where I > need to look for possible incompatabilities. Yep, my bad for assuming the cleanup was already done (or trivial)! sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
