On: The Little C Function From Hell

[malkia]

That's undefined behavior, the compiler can do whatever it pleases to. It can even print 666, and be done.

[AceJohnny2 → malkia]

No it's not. I had the same thought, but the author carefully points out from the standard: "The usual arithmetic conversions ensure that two operands to a “+” operator of type signed char are both promoted to signed int before the addition is performed."

He does assume sizeof(int) > sizeof(char), which is true on all platforms he has tried. It would be undefined on an AVR or other microcontroller where sizeof(int) == sizeof(char) though.

[nova → AceJohnny2]

undefined on an AVR or other microcontroller where sizeof(int) == sizeof(char) though.

Just noting that ints are 16 bits wide in AVR-GCC, unless you use the -mint8 option which violates C standards.

[JoachimSchipper → nova]

Absolutely true. Note, though, that sizeof(int) == sizeof(char) is allowed if you have 16-bit chars.

[cperciva → JoachimSchipper]

16-bit chars aren't allowed in modern versions of C -- CHAR_BIT is required to be 8.

[JoachimSchipper → cperciva]

Thanks for the update! (For the curious: C99 or POSIX both require CHAR_BIT == 8.)

[pascal_cuoq → JoachimSchipper]

C99 only requires CHAR_BIT to be at least 8 (5.2.4.2.1:1). POSIX requires it to be exactly 8.

[cperciva → pascal_cuoq]

Oops, quite right. I missed the "equal or greater in magnitude" line when I was reading that section.

[malkia → AceJohnny2]

Thanks! Should read the article more carefully.