The following code prints '-0x1.d6f3457f35ba8p+26' for the value of high. Using the gcc or TI arm compiler, the value is '-0x1.d6f3457f35ba7p+26'. It appears to be a floating point rounding error caused by the addition (+) or subtraction (-) arithmetic operators. #include <cmath> #include <stdio.h> #define BOUND 0.00000001 double _x9 = -123456789.987654321; double _y9 = 9.0; double ans9 = -123456789.987654321; int main() {     double val = std::nextafter(_x9, _y9);     double low = ans9 - BOUND;     double high = ans9 + BOUND;     printf("%a %a (%zu)\n", _x9, _y9, sizeof(double));     printf("%a < %a < %a ?\n", low, val, high);     if (val >= low && val <= high) printf("Yes!\n");     else printf("No!\n"); }