Question

Ignoring why I would want to do this, the 754 IEEE fp standard doesn’t define the behavior for the following:

float h = NAN;
printf("%x %d\n", (int)h, (int)h);

Gives: 80000000 -2147483648

Basically, regardless of what value of NAN I give, it outputs 80000000 (hex) or -2147483648 (dec). Is there a reason for this and/or is this correct behavior? If so, how come?

The way I’m giving it different values of NaN are here:
How can I manually set the bit value of a float that equates to NaN?

So basically, are there cases where the payload of the NaN affects the output of the cast?

Thanks!

Answer 1

The result of a cast of a floating point number to an integer is undefined/unspecified for values not in the range of the integer variable (±1 for truncation).

Clause 6.3.1.4:

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.

If the implementation defines __STDC_IEC_559__, then for conversions from a floating-point type to an integer type other than _BOOL:

if the floating value is infinite or NaN or if the integral part of the floating value exceeds the range of the integer type, then the “invalid” floating-
point exception is raised and the resulting value is unspecified.

(Annex F [normative], point 4.)

If the implementation doesn’t define __STDC_IEC_559__, then all bets are off.