将Float转换为Int32，负数失败，C

Question

I have the following function which is suppose to convert a floating point number to int32. The problem is that for negative numbers it just doesn't work (my if statement isn't executing). I've tried a similar program for a conversion from float to int16 and everything works just fine. Sry if this is too simple , but I just can't figure out what I'm missing and why it doesn't work for negative values.

#define MaxInt32 2147483647
#define MinInt32 -2147483648
…
bool CastFloatToInt32 ( float  fNumber, int32 *ConvertedValue) {
    bool CastStatus = False;

    if ( ( fNumber >= MinInt32 ) && ( fNumber <= MaxInt32 ) ) {
        *ConvertedValue = ( int32 ) ( fNumber );
        CastStatus = True;

    } else {
        if (fNumber < MinInt32) {
            *ConvertedValue = MinInt32;

        } else {
            *ConvertedValue = MaxInt32;
        }
    }

    return CastStatus;
}

Answer 1

You can see why here: https://stackoverflow.com/a/20910712/1073171

Thus, you can fix this code by changing your defines to either:

#define MaxInt32 (int32)0x7FFFFFFF
#define MinInt32 (int32)0x80000000

Or else:

#define MaxInt32 (int32)2147483647
#define MinInt32 (int32)(-2147483647 - 1)

The reasoning's are given in the answer I linked above. If you're using GCC, you could always move to -std=gnu99 or similar too!

Answer 2

The compiler parses "-2147483648" in 2 stages: text to number and then negation.

2147483648 is likely an unsigned long/unsigned value. Negating that does not change the type and curiously retains the same unsigned integer value of 2147483648 .

Thus fNumber >= MinInt32 is a signed / unsigned compare which converts fNumber to an unsigned integer before comparing.

Suggest using @Brian Sidebotham solution or if that is not acceptable, at least cast the MinInt32 to its expected type.

#define MinInt32 ((int32)-2147483648)