[erlang-questions] enif_send, how to call off a smp thread?

Vans S <>
Tue Mar 21 15:59:15 CET 2017


Im going to try that, for now I made a test case that fails for me 
https://github.com/vans163/nif_test.

The 3rd receive we get ok instead of 56.

I am on Erlang 19.1.2 though, maybe I should try the R19.3 or master HEAD?



On Tuesday, March 21, 2017 10:28 AM, Sverker Eriksson <> wrote:



enif_send can be called from a non scheduler thread in an SMP enabled VM 
(beam.smp)
but not in a non-SMP VM (beam).

The outcome being dependent on the message term sounds very strange.


Run with debug compiled VM to get more runtime sanity checks.


$> (cd $ERL_TOP/erts/emulator && make TYPE=debug smp)

$> $ERL_TOP/bin/cerl -debug


/Sverker



On 03/21/2017 02:57 PM, Vans S wrote:
> Iv tried so many combinations of passing the default env in the user_data of the callback/setting a global or setting that to NULL and using the msg_env, same result. Even if I do it like this or that:
>
> static ErlNifPid my_temp_pid;
> static void on_callback(void* user_data)
> {
> ErlNifEnv* msg_env = enif_alloc_env();
>
> enif_send(NULL, &my_temp_pid, msg_env, mk_atom(msg_env, "okk"));
> enif_free_env(msg_env);
>
> msg_env = enif_alloc_env();enif_send(NULL, &my_temp_pid, msg_env, mk_atom(msg_env, "aaokk"));
> enif_free_env(msg_env);
> }
>
>
> static void on_callback2(void* user_data)
> {
> ErlNifEnv* msg_env = enif_alloc_env();
> enif_send(NULL, &my_temp_pid, msg_env, mk_atom(msg_env, "okk"));
> enif_clear_env(msg_env);
> enif_send(NULL, &my_temp_pid, msg_env, mk_atom(msg_env, "aaokk"));
> enif_free_env(msg_env);
> }
>
>
> Only the first msg comes, its like it caches/remembers it and ignores the rest. I wonder if its due to not being able to
> call enif_send from a random non SMP thread and if I need to make a queue that I poll later?
>
> By only the first msg comes, if I later do a enif_send with "okk2" atom as the first, it wont send unless the atom is exactly "okk".
>
> The same thing if you substitute the atom with an integer or binary.
>
> On Tuesday, March 21, 2017 6:52 AM, Sverker Eriksson <> wrote:
>
>
>
> enif_send docs says:
>
> "The message environment msg_env with all its terms (including msg)
> is invalidated by a successful call to enif_send. The environment is to
> either be freed with enif_free_env of cleared for reuse with
> enif_clear_env."
>
> which means you have to call enif_clear_env before reusing msg_env
> in the second call to enif_send.
>
> Or if you're running OTP 19 you can pass msg_env as NULL,
> in which case the message term is copied and its environment left intact
> by enif_send.
>
>
> /Sverker, Erlang/OTP
>
>
>
> On 03/21/2017 05:10 AM, Vans S wrote:
>> I am having a weird issue with enif_send.
>>
>> What is happening is I am interoping with a 3rd party C library which has
>> its own event loop and procs callbacks from its own thread.
>>
>> When the callback procs (in the C NIF) I want to call enif_send to get
>> the message back to an erlang pid.
>>
>> When I call enif_send the first term I pass it works, then every subsequent message
>> gets dropped unless its exactly the same as the first time passed.
>>
>> For example
>>
>>
>> static ErlNifPid my_temp_pid;
>> static void on_callback(void* user_data)
>> {
>> ErlNifEnv* msg_env = enif_alloc_env();
>> enif_send(NULL, &my_temp_pid, msg_env, mk_atom(msg_env, "okk"));
>> enif_send(NULL, &my_temp_pid, msg_env, mk_atom(msg_env, "aaokk"));
>> enif_free_env(msg_env);}
>>
>> *on_callback proced
>> 6> receive X-> X after 1 -> none end.
>> okk
>>
>> 7> receive X-> X after 1 -> none end.none
>> *on_callback proced
>> 8> receive X-> X after 1 -> none end.
>> okk
>> 9> receive X-> X after 1 -> none end.
>> none
>> *on_callback proced
>> *on_callback proced
>> 10> receive X-> X after 1 -> none end.
>> okk
>> 11> receive X-> X after 1 -> none end.
>> okk
>> 12> receive X-> X after 1 -> none end.
>> none
>> What is the correct way to do this? Thanks.
>> _______________________________________________
>> erlang-questions mailing list
>> 
>> http://erlang.org/mailman/listinfo/erlang-questions

>>
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> 
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