How can a (badly chosen) typedef name trigger _undefined behavior_, and not just, say, a compilation error...?
I find it difficult to imagine what that would even mean.
I find it difficult to imagine what that would even mean.
typedef struct foo foo_t;
It is not required to do so, hence undefined behavior. You might get a wrong underlying type under that name.
I mean, no typedef names are defined in the global scope without including any headers right? Like I find it really weird that a type ending in _t would be UB if there is no such typedef name declared at all.
Or is this UB stuff merely a way for the ISO C committee to enforce this without having to define <something more complicated>?
The purpose of this particular naming rule is to allow adding new typedefs such as int128_t. The "undefined behaviour" part is for declaration of any reserved identifier (not specifically for this naming rule). I don't know why the standard uses "undefined behaviour" instead of the other classes (https://en.cppreference.com/w/cpp/language/ub); I suspect because it gives compilers the most flexibility.