[erlang-questions] Static callback in NIF

[Daniel Goertzen]

Static vs priv_data are functionally the same here so it doesn't matter
which way you go.  I can empathize with your sense of dread; there are a
lot of rules to keep track of for keeping Undefined Behavior at bay.  But
you seem to have a good handle on things.

On Thu, Jun 30, 2016 at 6:02 AM Igor Clark <igor.clark@REDACTED> wrote:

> Hi folks,
>
> I've got a NIF that uses some library code to talk to specific hardware.
> It's a hobby project with only one user (me) and no real performance
> concerns, so what I've got works well at the moment, but I think I'm
> doing some sneaky/dirty stuff and would like to know the best way to do
> what I need.
>
> Sending messages outwards from erlang->C->HW is easy, very quick, and
> works fine. I return a pointer to the HW reference back to erlang using
> the enif_*_resource functions, and manage keeping track of everything on
> the erlang side, which feels pretty natural.
>
> Coming the other way works fine too, but relies on a C function callback
> which gets called when the hardware has a message for me. Right now I
> just have a static function in the NIF C code which I pass to the
> library. I create a static ErlNifEnv on NIF load() which I keep around
> and use in the callback to send messages to a specified erlang Pid,
> passed in via enif_get_local_pid() in another NIF function and also
> stored statically. This works a treat, but I'm feeling some pretty
> strong dread that it's very much the wrong way to do things, and asking
> for scheduler headaches/explosions.
>
> I'm planning to try storing the various resources in priv_data at load()
> time, on the theory that that way the memory would at least be managed
> by the NIF system rather than just as enif_alloc()'ing static pointers,
> but I'm not sure if that would make any diffrence if code external to
> the scheduler calls back into it.
>
> I've looked into running this part as a C node or a port that sends
> messages with the HW data in a callback in its own process, and the
> communication seems straightforward enough, but it also seems like I
> immediately need to start designing mechanisms to deal with working out
> where to send received messages, almost a protocol in itself. Whereas
> with the NIF+callback method I have a lot of the work done for me -
> except, of course, for the synchronisation and memory management, which
> is the bit I'm worried about.
>
> FWIW the callback code doesn't modify any of the static data structures
> directly, it just calls library code which uses the stored references to
> work out which hardware device & channel to send the message to.
>
> What's the best practice here? Is a callback in a NIF OK if it's stored
> in priv_data, or is it never OK? What's the best way to do this if not?
>
> Would appreciate any tips!
>
> Cheers,
> Igor
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