[erlang-questions] enif_select() questions

[Sverker Eriksson]

On 05/05/2017 09:00 PM, Daniel Goertzen wrote:
> - When the resource stop() callback is invoked with is_direct_call==false,
> is the provided environment considered a "process" environment? (ie, should
> I pass it as the first parameter to enif_send() or leave null?)  Same
> question for resource down() callback.
As a general rule you should pass the environment that you got
passed to your NIF or callback function. Pass NULL when you don't
have an environment which is only in a created thread.

Maybe the documentation should be changed to talk about
"calling context" environments vs "dynamic term" environments
or something like that.

The idea with the is_direct_call argument was to make lock handling
easier. If the resource has a mutex protecting its data, then enif_select
can be called without unlocking the mutex and use is_direct_call to 
determine
if the mutex is already locked or not.

> - enif_select() appears to not add a reference to the associated resource.
> What should happen if the resource gets garbage collected (dtor invoked)
> while it has outstanding an enif_select()?
>
>
enif_select *does* add a reference to the resource in the first call
and it releases the reference after the stop callback has returned.

https://github.com/erlang/otp/blob/OTP-20.0-rc1/erts/emulator/sys/common/erl_check_io.c#L1352 



enif_monitor_process, however, does not add a reference to the resource.


Suggestions of how to improve the documentation is always welcome.

/Sverker, Erlang/OTP
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