What is the use of feholdexcept etc.?

The documentation (in the standards) for all of fenv.h is rather confusing, but I'm especially confused about feholdexcept and the concept of "non-stop mode" for a floating point exception. As far as I can tell, on any IEEE floating point implementation, exceptions are non-signaling/"non-stop" by default, and the fenv.h interfaces seem to provide 开发者_开发百科no way to enable a signaling mode unless it was the default. Is the whole concept of feholdexcept useless except on non-IEEE systems or systems with nonstandard extensions for setting the signaling exception mask?

Suppose that you're implementing a library, and you don't know anything about what your callers might do the the floating-point environment before calling your code. They might unmask an exception, and install a custom trap handler that causes division-by-zero to produce the value 42. Suppose that your library depends on having default IEEE-754 behavior for division-by-zero. The feholdexcept function gives you a means to enforce this behavior. The caller's environment, complete with their unmasked exception, can then be restored using the fesetenv function.

This is admittedly a fairly obscure corner case of usage, but frankly everything in fenv.h is fairly obscure to most programmers.