Comment by ridiculous_fish

4 years ago

Amusingly vfork semantics differ across OSes. This program prints 42 in Linux but 1 on Mac: https://godbolt.org/z/jn7Gaf5Me because on Linux they share address space.

4 comments

ridiculous_fish

Reply
cryptonector  4 years ago

Unfortunately there was this paper from the 80s titled "vfork() Considered Dangerous", which led to BSDs removing vfork(), and then later it was re-added because that paper was clearly quite wrong. But the news hasn't quite filtered through to Apple, I guess.

butterisgood  4 years ago

I am pretty sure Mac OS doesn't COW fork(), and that the address space is copied. At least it was the last time I looked. FreeBSD and Linux both seem to COW.

Perhaps there's a reason vfork is different too.

  • ridiculous_fish  4 years ago

    My (very possibly wrong) understanding is that xnu does CoW fork but doesn't overcommit, meaning that memory must be reserved (perhaps in swap) in case the pages need to be duplicated.

    There's other complications relating to inheriting Mach ports and the mach_task <-> BSD process "duality" in xnu, which Linux doesn't have. I'd love for someone to chime in who knows more about how this stuff works.