Date: Thu, 2 Mar 2023 00:19:04 -0800 From: Fangrui Song <i@...kray.me> To: musl@...ts.openwall.com, lichray@...il.com Subject: Re: [PATCH] Use __builtin_FILE/__builtin_LINE if available On Mon, Feb 27, 2023 at 2:27 PM Szabolcs Nagy <nsz@...t70.net> wrote: > > * Fangrui Song <i@...kray.me> [2023-02-21 11:09:14 -0800]: > > > On Sat, Feb 18, 2023 at 4:17 AM Jon Chesterfield > > <jonathanchesterfield@...il.com> wrote: > > > > > > On Sat, 18 Feb 2023, 02:54 Fangrui Song, <i@...kray.me> wrote: > > > > > > On Fri, Feb 17, 2023 at 6:03 PM Rich Felker <dalias@...c.org> wrote: > > > > > > > > On Fri, Feb 17, 2023 at 05:33:33PM -0800, Fangrui Song wrote: > > > > > C++ inline functions are requred to have exact same sequence of tokens > > > > > in every translation unit, but __FILE__ and __LINE__ may expand to > > > > > different tokens. The ODR violatioin is usually benign, but it can lead > > > > > to errors when C++20 modules are used. > > > > > > > > > > > > It is sad that C++ modules broke 'assert' but not surprising. Modules were largely created out of aversion to macros. This isn't something libc can fix though, I suggest a defect report against C++ instead. To lichray: ^^ > > > Changing the semantics of assert in C seems like a bad thing to do. > > > > > > Thanks > > > > I disagree. This is a footgun where the right fix (or workaround, if > > you prefer) is on the libc side. It is fairly reasonable for a header > > to use assert and not expect two includes using different paths to not > > cause C++ module problems. > > > > The current module behavior regarding macros is a reasonable > > compromise. It can be evolved (e.g. > > https://gracicot.github.io/modules/2018/05/14/modules-macro.html). > > i dont see how that solves the fundamental problem: > > the *behavior* of assert changes depending on which include path is > used and thus inline functions that are supposed to be equivalent > aren't. (__builtin_FILE makes the pp-token sequence the same across > the instances, but the actual code will have different paths, which > while not an odr violation per the literal words of the spec, it > clearly violates the reason the rule is there in the first place.) > libc can avoid printing the file path in the assert fail message for > c++. this makes assert less useful but it solves the conformance issue. > if c++ does not specify which path assert should print (or allow it to > be unpredictable) then it is difficult to do better than this. > > it would have been more useful to have a __builtin_canonical_FILE() > or similar that gives a path that is somehow independent of include > path, but we don't have that now. __FILE_NAME__ / __builtin_FILE_NAME just expands to the basename (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108978), which may help. I created this musl patch as I saw glibc made a similar change on 2023-02-10. Rejecting this patch is fine. It probably needs some time for standard C++ modules to become mainstream to expose this deployment problem.
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