[erlang-questions] Static callback in NIF
Thu Jun 30 20:31:50 CEST 2016
Thanks Daniel, good to hear.
Thanks also Roger & Sergej for your replies. I'll try out
enif_alloc_env()'ing a new ErlNifEnv each time the callback uses
enif_send(), rather than just leaving it static.
On 30/06/2016 07:45, Daniel Goertzen wrote:
> 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
> <mailto:igor.clark@REDACTED>> wrote:
> Hi folks,
> I've got a NIF that uses some library code to talk to specific
> 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
> 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
> 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
> for scheduler headaches/explosions.
> I'm planning to try storing the various resources in priv_data at
> 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
> 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,
> is the bit I'm worried about.
> FWIW the callback code doesn't modify any of the static data
> 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
> in priv_data, or is it never OK? What's the best way to do this if
> Would appreciate any tips!
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