https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85995
--- Comment #5 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Fri, 1 Jun 2018, vincent-gcc at vinc17 dot net wrote: > Programmers normally use conditionals on '__STDC__' to ask whether > it is safe to use certain features of ISO C, such as function > prototypes or ISO token concatenation. > > Precisely this is what I wish to do, with 2 versions of a macro: one that > assumes C conformance (requiring -fexcess-precision=standard with GCC, in > particular), and another one that doesn't but may be slower. The typical features people want to test are ones for which __STDC__ gives the correct answer. Likewise if they use __STDC_VERSION__ with a view to testing for support for VLAs or compound literals, for example. You can test for a conformance mode by testing __STRICT_ANSI__ (though someone can still use -f options affecting conformance together with a -std option that enables a conformance mode). You can also test __GCC_IEC_559 > 0 if you wish to test that no options have been used that specifically disable IEEE 754 floating-point features and that floating-point exceptions and rounding modes are supported as far as GCC knows (the precise rules for __GCC_IEC_559 depend on whether an option such as -std=c11 is used as well).
