Can a conversion from double to int be written in portable C

Question

I need to write function like double_to_int(double val, int *err) which would covert double val to integer when it\'s possible; otherwise report an error (NAN/I

Answer 1

Can a conversion from double to int be written in portable C (?)

is there any way to implement double_to_int function in cross-platform way (basing only on C standard, even not considering target platforms to support IEEE-754).?

int double_to_int(double val, int *err)

Detail: (int)val truncates the fractional portion, so the range of convertible val using (int)val is mathematically:
INT_MIN - 0.9999... ≤ val ≤ INT_MAX + 0.9999... or
INT_MIN - 1 < val < INT_MAX + 1.

---

Yes a cross-platform way, by using exact floating-point math and constants, code can test for conversion success.

2.0*(INT_MAX/2+1) is certainly exactly converted to a FP constant.

val - INT_MIN > -1.0 is akin to val > INT_MIN - 1.0 but does not suffer imprecision (with the common 2's complement machines) possible with INT_MIN - 1.0. Recall that the integer type may have greater precision than double. Consider a 64-bit int and INT_MIN - 1.0 not exactly representable as a double.

Code does not use (double)INT_MAX which also may be imprecise.

---

To copy myself:

#include 
#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 val, int *err) {
  if (val < DBL_INT_MAXP1) {
    #if -INT_MAX == INT_MIN
    // rare non-2's complement machine 
    if (val > DBL_INT_MINM1) {
      *err = OK;
      return (int) val;
    }
    #else
    if (val - INT_MIN > -1.0) {
      *err = OK;
      return (int) val;
    }
    #endif 
    // Underflow
    *err = ERR_MINUS_INF;
    return INT_MIN;
  }
  if (x > 0) {
    // Overflow
    *err = ERR_PLUS_INF;
    return INT_MAX;
  }
  // NaN;
  *err = ERR_NAN;
  return 0;
}

Corner weakness: FLT == 10 and the integer type > 34 bits.