> memset (sprtemp,-1, sizeof(sprtemp));
Yikes. I think this article undersells the point somewhat. This line of code undermines the type system by spraying -1's into an array of structs, so the only surprise to me is that it took this long to break.
Yikes. I think this article undersells the point somewhat. This line of code undermines the type system by spraying -1's into an array of structs, so the only surprise to me is that it took this long to break.
typedef struct
{
// If false use 0 for any position.
// Note: as eight entries are available,
// we might as well insert the same name eight times.
boolean rotate;
// Lump to use for view angles 0-7.
short lump[8];
// Flip bit (1 = flip) to use for view angles 0-7.
byte flip[8];
} spriteframe_t;
So yeah, on C99 and C++ this always had UB and could've broken at any time – though I presume compiler devs were not particularly eager to make it ill-behaved just because. But in pre-C99 it's entirely fine, and `rotate == true || rotate == false` could easily be false without UB.
---
(*) other than `char` for which setting the MSB is… not UB but also not the best idea in general.
> When boolean is an enum, the compiler does exactly what I expected – the == false condition checks if the value is equal to 0, and the == true condition checks if the value is equal to 1.
> However, when boolean is actually _Bool, the == false check is transformed into != 1, and the == true check is transformed into != 0 – which makes perfect sense in the realm of boolean logic. But it also means that for a value of 255, hilarity ensures: since 255 is neither 0 nor 1, both conditions pass!
So a value of 255 also makes both checks fail for the enum, but because of the ordering of the code, it was expected to evaluate as != false.
Had the check: ``` if (sprtemp[frame].rotate == false) ```
been written as: ``` if (sprtemp[frame].rotate != true) ```
then it would work for the `bool` type, but not the `enum` type, at least in C23 mode. Assumedly the C++ mode (effectively) treated the boolean as an enum, or possibly as `false == 0`, `true != 0`.