Why cast to a pointer then dereference?

Question

I was going through this example which has a function outputting a hex bit pattern to represent an arbitrary float.

void ExamineFloat(float fValue)
{
    pr         


        

Answer 1

Floating-point values have memory representations: for example the bytes can represent a floating-point value using IEEE 754.

The first expression *(unsigned long *)&fValue will interpret these bytes as if it was the representation of an unsigned long value. In fact in C standard it results in an undefined behavior (according to the so-called "strict aliasing rule"). In practice, there are issues such as endianness that have to be taken into account.

The second expression (unsigned long)fValue is C standard compliant. It has a precise meaning:

C11 (n1570), § 6.3.1.4 Real floating and integer

When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.