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Date: Wed, 30 Aug 2023 15:04:50 -0700
From: Fangrui Song <>
Subject: Re: [PATCH] Use __builtin_FILE/__builtin_LINE if available

On Sat, Mar 4, 2023 at 7:23 PM Zhihao Yuan <> wrote:
> On Thu, Mar 2, 2023 at 12:19 AM Fangrui Song <> wrote:
>> On Mon, Feb 27, 2023 at 2:27 PM Szabolcs Nagy <> wrote:
>> > > > 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: ^^
> The definition of 'assert' is mandatory only when NDEBUG
> is defined as a macro name. reads:
> "[...], if expression (which shall have a scalar type) is false (that is, compares equal to 0),
> the assert macro writes information about the particular call that failed (including the text of the
> argument, the name of the source file, the source line number, and the name of the enclosing function
> — the latter are respectively the values of the preprocessing macros __FILE__ and __LINE__ and of
> the identifier __func__) on the standard error stream in an implementation-defined format."
> The text in parentheses did not mandate the use of __FILE__
> and __LINE__, and C++ did not permit 'assert' not to work in
> modules. Therefore an implementation that fails to compile
> the code snippet in the original email is not conforming.
>> > > > Changing the semantics of assert in C seems like a bad thing to do.
>> > > >
> The semantics is unchanged, and people are doing it:
> Add custom ODR-safe assert. (!1166) · Merge requests · libeigen / eigen · GitLab
>> >
>> > 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.)
> This is a different topic. In related news, CWG Issue 2678 (
> will likely need to be revisited (i.e., what does 'odr' mean
> is subject to change). Regardless, __builtin_FILE is
> a vendor extension and serves customers' needs in
> implementing 'assert.'
> --
> Zhihao Yuan, ID lichray
> The best way to predict the future is to invent it.
> _______________________________________________


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