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)? Have a lovely night! Alex --