c - What happens when casting floating point types to unsigned integer types when the value would overflow?

Question

I'm wondering what happens when casting from a floating point type to an unsigned integer type in C when the value can't be accurately represented by the integer type in question. Take for instance

func (void)
{
float a = 1E10;
unsigned b = a;
}

The value of b I get on my system (with unsigned on my system being able to represent values from 0 to 2³²-1) is 1410065408. This seems sensible to me because it's simply the lowest order bits of the result of the cast.

I believe the behavior of operations such as these is undefined by the standard. Am I wrong? What can I expect in practice if I do things like this?

Also, what happens with signed types? If b is of type int, I get -2147483648, which doesn't really make sense to me.

Answer 1

In both cases value is out of range, so it's undefined behaviour.

6.3.1.4 Real floating and integer

1. 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. ⁶¹⁾

_{61) The remaindering operation performed when a value of integer type is converted to unsigned type need not be performed when a value of real floating type is converted to unsigned type. Thus, the range of portable real floating values is (?1, Utype_MAX+1).}

To make this well defined code, you should check that value is within possible range before doing the conversion.