Re: _ISOCxx_SOURCE

[Alejandro Colomar <alx@kernel.org>]

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Hi Joseph,

On 2026-06-16T16:02:44+0000, Joseph Myers wrote:
> On Mon, 15 Jun 2026, Alejandro Colomar wrote:
> 
> > Hi Joseph,
> > 
> > On 2026-06-15T17:20:17+0000, Joseph Myers wrote:
> > > On Sat, 13 Jun 2026, Alejandro Colomar via Gcc wrote:
> > > 
> > > > I was reviewing the feature_test_macros(7) page, and found the
> > > > documentation of the _ISOC{99,11,23,...}_SOURCE macros to be incorrect.
> > > > And thinking about fixing that led me to think that the design of those
> > > > macros is less than ideal.
> > > 
> > > Users should normally use -std= options with the compiler to get matching 
> > > language and library features.  These feature test macros are mainly for 
> > > niche cases with old compilers.  So I don't think any new user-facing 
> > > design for them should be added (the C23 and later ones already use the 
> > > newer __GLIBC_USE style of conditionals in the header implementation).
> > 
> > Makes sense.  Should I document that users should avoid setting them
> > directly (and that they should use the corresponding compiler flag)?
> 
> Indeed.

Thanks!  Will do.

>  Also, users are probably using a compiler defaulting to 
> -std=gnu23 or -std=gnu11 so the older _ISOC99_SOURCE and _ISOC11_SOURCE 
> are doubly irrelevant for that reason.

I was also wondering...

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).

If the way to pass the language version from the compiler to the libc
were through a single macro, then the library would be able to at least
diagnose, and/or turn on the latest dialect it knows about.

Is there any limitation of which libc versions are supported by modern
GCC?  Is this a bug?


Have a lovely night!
Alex

> 
> -- 
> Joseph S. Myers
> josmyers@redhat.com
> 

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

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