Casting float to int (bitwise) in C

Question

Given the 32 bits that represent an IEEE 754 floating-point number, how can the number be converted to an integer, using integer or bit operations on the representation (rather than using a machine instruction or compiler operation to convert)?

I have the following function but it fails in some cases:

Input: int x (contains 32 bit single precision number in IEEE 754 format)

  if(x == 0) return x;

  unsigned int signBit = 0;
  unsigned int absX = (unsigned int)x;
  if (x < 0)
  {
      signBit = 0x80000000u;
      absX = (unsigned int)-x;
  }

  unsigned int exponent = 158;
  while ((absX & 0x80000000) == 0)
  {
      exponent--;
      absX <<= 1;
  }

  unsigned int mantissa = absX >> 8;

  unsigned int result = signBit | (exponent << 23) | (mantissa & 0x7fffff);
  printf("\nfor x: %x, result: %x",x,result);
  return result;

Answer 1

C has the "union" to handle this type of view of data:

typedef union {
  int i;
  float f;
 } u;
 u u1;
 u1.f = 45.6789;
 /* now u1.i refers to the int version of the float */
 printf("%d",u1.i);

Answer 2

&x gives the address of x so has float* type.

(int*)&x cast that pointer to a pointer to int ie to a int* thing.

*(int*)&x dereference that pointer into an int value. It won't do what you believe on machines where int and float have different sizes.

And there could be endianness issues.

This solution was used in the fast inverse square root algorithm.