iphone: floats cast to unsigned ints get set to 0 if they are negative?

Question

try it out:

volatile float bob = -344.0f;
unsigned int fred = (unsigned int)bob;

printf(\"%d\\n\",fred);

output will be 0.

obvious

Answer 1

This conversion is undefined and therefore not portable.

According to C99 §6.3.1.4 footnote 50:

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).

And given that this conversion is known not to be portable, it's quite a reasonable interpretation to return 0 rather than a random particular conversion. There are at least two reasons for this: (1) to flag non-portable code rather than propagate it, and (2) just dropping the sign is wildly different from what happens when the same value of an integral type is converted, so it's unclear that any particular alternative is a better idea.