_ISOCxx_SOURCE

[Joseph Myers]

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

-- 
Joseph S. Myers
josmyers@redhat.com