Handling overflow when casting doubles to integers in C

Question

Today, I noticed that when I cast a double that is greater than the maximum possible integer to an integer, I get -2147483648. Similarly, when I cast a double that is less

Answer 1

What is the best way to detect this under/overflow?

Compare the truncated double to exact limits near INT_MIN,INT_MAX.

The trick is to exactly convert limits based on INT_MIN,INT_MAX into double values. A double may not exactly represent INT_MAX as the number of bits in an int may exceed that floating point's precision. In that case, the conversion of INT_MAX to double suffers from rounding. The number after INT_MAX is a power-of-2 and is certainly representable as a double. 2.0*(INT_MAX/2 + 1) generates the whole number one greater than INT_MAX.

The same applies to INT_MIN on non-2s-complement machines.

INT_MAX is always a power-of-2 - 1.
INT_MIN is always:
-INT_MAX (not 2's complement) or
-INT_MAX-1 (2's complement)

int double_to_int(double x) {
  x = trunc(x);
  if (x >= 2.0*(INT_MAX/2 + 1)) Handle_Overflow();
  #if -INT_MAX == INT_MIN
  if (x <= 2.0*(INT_MIN/2 - 1)) Handle_Underflow();
  #else
  if (x < INT_MIN) Handle_Underflow();
  #endif
  return (int) x;
}

---

To detect NaN and not use trunc()

#define DBL_INT_MAXP1 (2.0*(INT_MAX/2+1)) 
#define DBL_INT_MINM1 (2.0*(INT_MIN/2-1)) 

int double_to_int(double x) {
  if (x < DBL_INT_MAXP1) {
    #if -INT_MAX == INT_MIN
    if (x > DBL_INT_MINM1) {
      return (int) x;
    }
    #else
    if (ceil(x) >= INT_MIN) {
      return (int) x;
    }
    #endif 
    Handle_Underflow();
  } else if (x > 0) {
    Handle_Overflow();
  } else {
    Handle_NaN();
  }
}