[erlang-questions] Best way to kill an async thread in a linked in driver?

Mazen Harake <>
Fri Aug 13 00:14:27 CEST 2010

  Hmm... I'm not sure what you mean but the part I do understand is that 
select() blocks... right? I can't have anything that blocks in the 
driver since I must be able to interact with it as I'm waiting for 
input. E.g. I need to update a certain part of the screen even if the 
user is not pressing any buttons.

I don't see how select() would make any difference? Perhaps I 
misunderstood you, could you explain a bit more on how that would work? 
Or perhaps point me to some direction?

Thanks for feedback,


On 12/08/2010 14:42, Kresten Krab Thorup wrote:
> I'd write the driver using select and non-blocking reads; that way you would not need async threads at all.
> Kresten
> On Aug 12, 2010, at 10:31 , Mazen Harake wrote:
>>   It was late yesterday and despite 2 coffees and a nice breakfast I
>> still can't solve this one. Maybe a nudge in the right direction anyone?
>> Scenario:
>> I have a linked in driver; cecho [1], when ever I want to read input I
>> disable input to Erlang (-noinput) and use the getch() function in C
>> instead (never mind why ;)). This function is run in a seperate async
>> thread using driver_async() [2][3], and blocks at getch(). When a
>> character is read it sends a message back to Erlang (driver_output())
>> and then the thread exits, when the message comes back to Erlang the
>> character is sent to the requester and the thread is restarted (which
>> again blocks at getch()).
>> Later if I do an erlang:halt(0) ("graceful halt") the thread is still
>> lingering despite having closed the port[4] (which, Afaik should close
>> the thread as well) but it seems stay alive somehow. erlang:halt(1) kill
>> it immediately so it is not a problem.
>> Questions:
>> 1) Is my "analysis" correct? Is this infact what is happening? Anyone
>> who can explain this behaviour (I.e. why it lingers on)?
>> 2) If Question 1 then; what is the best way to kill the thread when I
>> exit? driver_async_cancel() only works for threads that are not
>> executing (according to Documentation) so what to do? I did an ugly
>> workaround which sets a global flag and then I timeout the getch()
>> operation and read for ERR. If I get an ERR I just read the abort flag
>> and if it is set I just die otherwise I try to getch() again. This
>> avoids hanging around for more then a tenth of a second but it is pretty
>> ugly in my oppinion and I'm forced to go into halfdelay mode which means
>> I can not allow this option outside of the driver because I become
>> dependent on it. I'd rather do it the "right way" if there is such a way.
>> References: (Links given as commit Ids so that if someone searches then
>> they get relevant code and not the latest)
>> [1]
>> http://github.com/mazenharake/cecho/tree/199fd342ba186e1154c32dbfdcf2d9222e25f9f4
>> [2]
>> http://github.com/mazenharake/cecho/blob/199fd342ba186e1154c32dbfdcf2d9222e25f9f4/c_src/cecho.c#L119
>>      static void request(ErlDrvData drvstate, char *buf, int buflen) {
>> state *st = (state *)drvstate;
>> driver_async(st->drv_port, NULL, loop_getch, (void *)st, NULL);
>>      }
>> [3]
>> http://github.com/mazenharake/cecho/blob/199fd342ba186e1154c32dbfdcf2d9222e25f9f4/c_src/cecho.c#L557
>> void loop_getch(void *arg) {
>>      state *st = (state *)arg;
>>      ei_x_buff eixb;
>>      int keycode;
>>      ei_x_new_with_version(&eixb);
>>      keycode = getch();
>>      integer(&eixb, keycode);
>>      driver_output(st->drv_port, eixb.buff, eixb.index);
>> }
>> [4]
>> http://github.com/mazenharake/cecho/blob/199fd342ba186e1154c32dbfdcf2d9222e25f9f4/src/cecho_srv.erl#L76
>> terminate(_Reason, State) ->
>>      do_call(State#state.port, ?ENDWIN),
>>      do_call(State#state.port, ?CURS_SET, ?ceCURS_NORMAL),
>>      erlang:port_close(State#state.port),
>>      erl_ddll:unload("cecho").
>> ________________________________________________________________
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> Kresten Krab Thorup, CTO, Trifork

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