Check for infinity while using -Wfloat-equal?

Question

My pet project uses -nostdlib so I can't use any C libraries however I can include them for constants.

Below is my code. If I compile using gcc -Wfloat-equal -nostdlib -O2 -c a.c on gcc or clang it'll give me the error below. I was curious if there's a way to check without triggering this warning. I suspect I can't unless I call code I don't compile

warning: comparing floating-point with ‘==’ or ‘!=’ is unsafe [-Wfloat-equal]

#include <math.h>
int IsInfinity(double d) {
    return d != (double)INFINITY && (float)d == INFINITY;
}

Not saying this is wise, but can you cast everything to `uint64_t`? — Fiddling Bits, Dec 15 '22 at 20:30
So you want this warning *unless* you're comparing with `INFINITY`? — Barmar, Dec 15 '22 at 20:30
@Barmar really I'm just going through all my warnings using -Weverything, that was one of them. I disabled a few warnings but thought maybe I can leave that on — , Dec 15 '22 at 20:33
[How to disable GCC warnings for a few lines of code](https://stackoverflow.com/questions/3378560/how-to-disable-gcc-warnings-for-a-few-lines-of-code) could be useful. — Retired Ninja, Dec 15 '22 at 20:34
@Henry B, Please report what result you want for `+INFINITY, -INFINITY, +DBL_MAX, -DBL_MAX`. — chux - Reinstate Monica, Dec 15 '22 at 20:57
@chux-ReinstateMonica turns out my use case never uses negatives. I completely didn't realize it was unhandled — , Dec 15 '22 at 20:57

Eric Postpischil · Accepted Answer · 2022-12-15T20:56:24.347

1

You can use return d < INFINITY && (float) d >= INFINITY;.

Another option is return isfinite(d) && isinf((float) d); if you want to classify negative values symmetrically. (These are macros defined in math.h.)

edited Dec 15 '22 at 20:56

answered Dec 15 '22 at 20:45

Eric Postpischil

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score 0 · Answer 2 · answered Dec 15 '22 at 20:43

Code looks like it is trying to test if d is in the finite range and, as a float, is more than FLT_MAX.

// OP's
int IsInfinity(double d) {
  return d != (double)INFINITY && (float)d == INFINITY;
}

if there's a way to check without triggering this warning.

Alternate:

Check via the max values:

int IsInfinity_alt(double d) {
  if (d < 0) d = -d;  // OP can not use fabs().
  return ((float)d > FLT_MAX && d <= DBL_MAX);
}

Had code used the below, some double, just bigger than FLT_MAX function differently than above. It is possible that this behavior is preferred by OP.

return (d > FLT_MAX && d <= DBL_MAX);

I just want to say this is also an extremely good answer – Dec 15 '22 at 20:57 — , Dec 15 '22 at 20:57

Check for infinity while using -Wfloat-equal?

2 Answers2