[erlang-questions] Static callback in NIF

Igor Clark igor.clark@REDACTED
Sat Jul 2 00:41:38 CEST 2016

Ah, got you. I'm not explicitly creating any threads at all; I just pass the callback to the library code in load() and return, so it sounds like that's OK. Great news for me!

Thanks for your help Sverker.


> On Jul 1, 2016, at 11:08 AM, Sverker Eriksson <sverker.eriksson@REDACTED> wrote:
> My mistake. I read "I create a static ErlNifEnv ..." and thought you did
> static ErlNifEnv my_env;
> which is not allowed and can only compile if you include internal headers.
> As long as you only have one callback thread, then it's totally fine to allocate
> one "static" ErlNifEnv, use that in your callback to create the message,
> call enif_send() and then call enif_clear_env() to prepare for next callback.
> /Sverker
>> On 07/01/2016 07:38 PM, Igor Clark wrote:
>> Hi Sverker, thanks.
>> I'm only using enif_alloc_env() to create the static ErlNifEnv, in the NIF load(). I haven't had compiler warnings about it - certainly haven't noticed any, and just did a clean+build to check.
>> (I'm using the rebar3 port compiler on erlang 18.3, erts-7.3 on OSX 10.11.5. It doesn't seem to show warnings without errors, and I can't find a verbose mode - so I used 'rebar comp --verbose' to inspect 'cc' lines and warnings, and there's nothing there about the static ErlNifEnv, either.)
>> I haven't been doing any digging in erts headers, as I'd be pretty nervous about doing anything undocumented :-)
>> Am I missing something?
>> Cheers,
>> Igor
>>> On 01/07/2016 08:18, Sverker Eriksson wrote:
>>> The only legit way to create an ErlNifEnv is to call enif_alloc_env().
>>> I bet the compiler warned about your static ErlNifEnv
>>> just before you went hunting for erts internal header files :-)
>>> In OTP-19.0 the 'msg_env' argument to enif_send() can be NULL
>>> in which case the message will be copied and you can reuse
>>> the environment for the next message with enif_clear_env().
>>> This is effective if the message is small.
>>> /Sverker, Erlang/OTP
>>>> On 06/30/2016 08:31 PM, Igor Clark wrote:
>>>> 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.
>>>> Cheers!
>>>> Igor
>>>>> 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
>>>>>    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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