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I'm creating a library in C that contains common data structures, convenience functions, etc. that is intended for general use. Within, I've implemented a dynamic array, and I've chosen the golden ratio as the growth factor for the reason explained here. However, this necessarily involves multiplication of floating-point numbers, which can cause FE_INEXACT to be raised if they have large significands.

When I implemented it, I was under the impression that, as the library is for general use, floating point exceptions must be avoided if at all possible. I first tried something like

fenv_t fenv;
feholdexcept(&fenv);
// expand dynamic array
feclearexcept(FE_INEXACT);
feupdateenv(&fenv);

, but this had such an enormous time cost that it wasn't worth it.

Eventually, I came up with a solution that had negligible time cost. While not avoiding FE_INEXACT entirely, it made it highly unlikely. Namely,

size_t newCapacity = nearbyint((double)(float)PHI * capacity);

This would only raise FE_INEXACT if the current capacity was extremely large, at least for compilers that adhere to IEEE 754 standards.

I'm starting to wonder whether my efforts have gone into solving a relative nonissue. For library code, is it reasonable to expect the user to handle the raising of FE_INEXACT when necessary, or should it be avoided within the library? In the latter case, how important is the issue compared to other factors, such as efficiency?

Isaac Saffold
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1 Answers1

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To what lengths should I go...

None at all. Almost nobody uses fenv.h, compilers do not even fully support it (they make transformations that wrongly disregard or alter the floating point environment), and if someone calling your code is using it, it's completely reasonable to require them to save/restore exception state around calls to your library. Moreover, most of the time if you're doing an operation that raises FE_INEXACT, it's precisely because the result you're going to be returning is inexact, and it's thereby semantically appropriate to be raising it.

R.. GitHub STOP HELPING ICE
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