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A fully standards conforming fenv.h does not have to define any FE_* macros, and if it does define them, then it defines macros only for the FP exceptions it actually supports. Correct usage requires a triple check: 1) Check BOOST_NO_FENV_H to see if the header is supported. 2) Include the header and then check FE_ALL_EXCEPT to see if any FP exceptions are supported. 3) Before using the individual FE_* macros, you need to check for their existence too as not all may be supported. The reason is that some (notably FPU-less) architectures, including mips*-nf, don't define/implement some of the floating point constants, even though fenv.h is available. Other projects have similar issues, e.g. pixman, and apply similar work-arounds: https://lists.freedesktop.org/archives/pixman/2014-February/003172.html Architectures are notably also allowed to define FE_ALL_EXCEPT to 0! Keeping this in mind, and knowing that the compiler will eliminate code that can't be executed, we can change BOOST_FPE_ALL to be 0 for the case of compiling using Clang and/or fenv.h being unavailable as well, which allows simplification of the #ifdef's in execution_monitor.ipp a bit.