Defining (1 << 31) or using 0x80000000? Result is different

Question

#define SCALE (1 << 31)

#define fix_Q31_80(x) ( (int) ( (float)(x)*(float)0x80000000 ) )
#define fix_Q31_SC(x) ( (int) ( (float)(x)*(float)SCALE      ) )

int         


        

Answer 1

0x80000000 is a big number which needs 32 bit to represent that number. This means on a 32 bit system (or in a 32 bit compatible application) an int is too small. So use unsigned long instead:

#define SCALE (1u << 31)

#define fix_Q31_80(x) ( (unsigned long) ( (float)(x)*(float)0x80000000u ) )
#define fix_Q31_SC(x) ( (unsigned long) ( (float)(x)*(float)SCALE       ) )

int main()
{
    unsigned long fix_80 = fix_Q31_80(0.5f);
    unsigned long fix_sc = fix_Q31_SC(0.5f);
}