gistfile1.txt

#include <stdio.h>
#include <math.h>

float a(float x)
{
    return x + 0.25f - 0.25f;
}

float b(float x)
{
    return x - 0.25f + 0.25f;
}

void show(float x)
{
    printf("x = %10.10e, a = %10.10e, b = %10.10e\n", x, a(x), b(x));
}

int main()
{
    show(ldexpf(1.0f, 22));
    show(ldexpf(-1.0f, 22));
    show(ldexpf(1.0f, -26));
    show(ldexpf(-1.0f, -26));
    
    /* 他の解については http://blog.practical-scheme.net/shiro/20160513-flonum-question 参照 */
}

/*
$ ./a.out
x = 4.1943040000e+06, a = 4.1943037500e+06, b = 4.1943040000e+06
x = -4.1943040000e+06, a = -4.1943040000e+06, b = -4.1943037500e+06
x = 1.4901161194e-08, a = 0.0000000000e+00, b = 1.4901161194e-08
x = -1.4901161194e-08, a = -1.4901161194e-08, b = 0.0000000000e+00
*/