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//-----------------------------------
// Округление val к rval.
//-----------------------------------
float round (float val, float rval)
{
    val /= rval;
    if (val <0 )
    {
        val -= 0.5;
        val = ceil(val);
    }
    else
    {
        val += 0.5;
        val = floor(val);
    }
    return (val*rval);
}

//Примеры вызова:
float pi100 = 3.14159265 * 100;
printf("%f", round(pi100, 0.01));
printf("%f", round(pi100, 0.1));
printf("%f", round(pi100, 1));
printf("%f", round(pi100, 10));
printf("%f", round(pi100, 100));