Simple experiment with floating-point rounding modes in C

test.c

#include <fenv.h>
#include <stdio.h>
#include <stdlib.h>

double do_the_thing(const char *mode) {
  float b, v;
  int i;
  b = 0.01;
  // Perform a lot of operations on b to accumulate rounding.
  for (i = 0; i < 100000; i++)
    b *= 1.00001;
  v = 0.1 - b;
  printf("%s %f\n", mode, v);
  return v;
}

int main(int argc, char *argv[]) {
  do_the_thing("round default: ");
  fesetround(FE_UPWARD);
  do_the_thing("round up:      ");
  fesetround(FE_DOWNWARD);
  do_the_thing("round down:    ");
  return 0;
}

// gcc -O0 test.c -Wall -lm && ./a.out