main.c

#include <math.h>
#include <assert.h>

float my_nextafterf( float from, float to )
{
  union {
    float a;
    int b;
  } from_union = { .a = from };

  if(from > to)
    from_union.b--;
  else
    from_union.b++;

  return from_union.a;
}

int main(void)
{
  float from = 1;
  float to = my_nextafterf(from, 2);
  printf("%.64f (%a)\n", to, to);
  assert(to == 0x1.000002p+0);
}