[erlang-questions] Static callback in NIF
Fri Jul 1 19:38:03 CEST 2016
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
I haven't been doing any digging in erts headers, as I'd be pretty
nervous about doing anything undocumented :-)
Am I missing something?
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.
>> 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 <
>>> <mailto:>> 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
>>> concerns, so what I've got works well at the moment, but I think
>>> 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
>>> 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
>>> 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.
>>> 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!
>>> erlang-questions mailing list
>> erlang-questions mailing list
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