Comment by minitech

What they point to are capabilities, but the integer handles that user space gets are annoyingly like pointers. In some respects, better, since we don’t do arithmetic on them, but in others, worse: they’re not randomized, and I’ve never come across a sanitizer (in the ASan sense) for them, so they’re vulnerable to worse race condition and use-after-free issues where data can be quietly sent to the entirely wrong place. Unlike raw pointers’ issues, this can’t even be solved at a language level. And maybe worst of all, there’s no bug locality: you can accidentally close the descriptor backing a `FILE*` just by passing the wrong small integer to `close` in an unrelated part of the program, and then it’ll get swapped out at the earliest opportunity.