Disappointing that errors are still signaled by assigning to `errno` (you can apparently override this to some other global, but it has to be a global or global-like lvalue).

The kernel actually signals errors by returning a negative error code (on most arches), which seems like a better calling convention. Storing errors in something like `errno` opens a whole can of worms around thread safety and signal safety, while seemingly providing very little benefit beyond following tradition.