How should I obtain the fractional part of a floating-point value?

Question

I have a variable x of type float, and I need its fractional part. I know I can get it with

• x - floorf(x), or
• fmodf(x, 1.0f)

My questions: Is one of these always preferable to the other? Are they effectively the same? Is there a third alternative I might consider?

Notes:

• If the answer depends on the processor I'm using, let's make it x86_64, and if you can elaborate about other processors that would be nice.
• Please make sure and refer to the behavior on negative values of x. I don't mind this behavior or that, but I need to know what the behavior is.

Answer 1

Is there a third alternative I might consider?

There's the dedicated function for it. modff exists to decompose a number into its integral and fractional parts.

float modff( float arg, float* iptr );

Decomposes given floating point value arg into integral and fractional parts, each having the same type and sign as arg. The integral part (in floating-point format) is stored in the object pointed to by iptr.