Is scientific notation safe for integer constants in C?

Question

For a while, I\'ve been representing large powers of 10 in constants using scientific notation, just so I don\'t have to count the zeros. e.g.

#define DELAY_USE         


        

Answer 1

It seems that gcc assumes a constant defined using scientific notation as a floating point number unless it is cast.

A simple C code shows this:

#include 

#define DELAY_USEC_FP  1e6
#define DELAY_USEC_INT (unsigned int) 1e6

int main()
{
    printf("DELAY_USEC_FP: %f\n", DELAY_USEC_FP);
    printf("DELAY_USEC_INT: %u\n",  DELAY_USEC_INT);
    return 0;
}

On a x86-64 machine, gcc generates this assembly code ($ gcc -S define.c):

[...]
; 0x4696837146684686336 = 1e6 in double-precision FP IEEE-754 format
movabsq $4696837146684686336, %rax
[...]
call    printf
movl    $1000000, %esi
[...]
call    printf
movl    $0, %eax

As stated here, 10e15 and 10e22 are the maximum power of ten numbers that have an exact representation in simple and double-precision floating-point format, respectively.

Larger power of ten numbers can not be represented using 32-bit or 64-bit integer types.