_ISOCxx_SOURCE

[Alejandro Colomar]

Hi Joseph,

On 2026-06-16T21:52:57+0000, Joseph Myers wrote:
> On Tue, 16 Jun 2026, Alejandro Colomar via Gcc wrote:
> 
> > If one uses a modern GCC with an old glibc (which doesn't know C23),
> > then, using -std=c23 will be problematic: the compiler will enable C23
> > language mode, but the library will entirely and silently ignore
> > _ISOC23_SOURCE, because it doesn't know about it, thus falling back to
> > C89 mode (I guess), and thus will for example enable things like
> > gets(3).
> 
> Since the relevant checks in features.h are based on __STDC_VERSION__ > 
> something or __STDC_VERSION__ >= something,

Ahh, yup, it has || for either _ISOCxx_SOURCE or __STDC_VERSION__.  That
makes sense.

> it will fall back to the most 
> recent C standard version known by that glibc version, which is probably 
> what you want.

Hmmm, it is.  Then it's all fine.  Thanks!


Cheers,
Alex

-- 
<https://www.alejandro-colomar.es>
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