“ISO C language standard requires that C compilers support the following three pragmas, and some C++ compiler vendors support them, to varying degrees, in their C++ frontends:

  #pragma STDC FENV_ACCESS arg      (1)
  #pragma STDC FP_CONTRACT arg      (2)
  #pragma STDC CX_LIMITED_RANGE arg (3)

⇒ it seems C compilers must support this (but C++ compilers need not)

At least some GCC devs claimed that the standard does not impose any requirements on these macros if a compiler does not claim strict ISO/IEC 6055 (aka IEE 754) conformance (which is still optional, and gcc and clang do not claim it).

Other devs disagree.

But it does kind of support the whole "GCC C is not standard C" point.

    4   Conformance

    A strictly conforming program shall use only those features of the language and
    library specified in this document.

    [...]

    The two forms of conforming implementation are hosted and freestanding. A
    conforming hosted implementation shall accept any strictly conforming program.

That said, I was surprised that it's mandatory by the standard (and wasn't made optional later like VLAs); feels rather unlike C to require such precise control over something it doesn't even specify any behavior of - imagine if we had mandatory macros for integer overflow behavior and whatnot!

...That "that said" said, C11 does also contain:

> Each of the macros FE_DOWNWARD FE_TONEAREST FE_TOWARDZERO FE_UPWARD is defined if and only if the implementation supports getting and setting the represented rounding direction by means of the fegetround and fesetround functions.

so presumably one could just not define those and get away with not implementing (at least the rounding mode part of) fenv.h, properly allowing the `#define FLT_ROUNDS 1` hack? gcc+glibc does define those, though.