[PD-dev] why must one never send a message from a perform routine ?

[Christof Ressi]

To expand on Miller's reply:

Conceptually, messaging and DSP are two separate domains. Sending a 
message from a perform routine violates this separation. Instead you 
should use a clock with delay 0 to defer the message to the begin of the 
next scheduler tick.

Miller already mentioned the greatest danger, but there are other, more 
subtle issues. DSP objects typically operate on the premise that the 
object's state won't change from the outside during the perform routine. 
For example, imagine a delay object with a buffer that can be resized 
with a message; by sending a Pd message from the perform routine, it 
might accidentally feed back into the object and reallocate the buffer 
while still in progress.

Unfortunately, very little of this is documented. Ideally, this should 
be covered in the externals-how-to 
(https://github.com/pure-data/externals-howto); I just added an item on 
my (long) TODO list.

Finally, although Pd is currently single-threaded, this could change in 
the future. FWIW, here is a PoC for multi-threaded DSP: 
https://github.com/spacechild1/pure-data/tree/multi-threading. This is 
only possible because perform routines may only use a restricted set of 
API functions - which, in my fork, are annoted with the (empty) 
THREADSAFE macro (and made thread-safe, if necessary).

Christof

On 21.08.2023 20:55, Joseph Larralde wrote:
> Hmm, I see ... unfortunately my random bug is totally unrelated to 
> this weakness of my code.
> Thanks Miller for the explanation and pointers to examples !
> And thanks Claude for the extra example.
> I'll check all my objects to see if there are other ones I can 
> consolidate.
>
> Cheers !
>
> Joseph
>
> Le 21/08/2023 à 19:08, Claude Heiland-Allen a écrit :
>> See bang~ in pure-data/src/d_misc.c for an example that uses a clock 
>> to send a message from DSP.
>>
>> On 21/08/2023 18:02, Miller Puckette wrote:
>>> The built-in objects "delay", "metro" and "pipe" use clocks in 
>>> various ways.
>>>
>>> On 8/21/23 18:02, Joseph Larralde wrote:
>>>> I just read in an answer from Christof to Alexandre : "never ever 
>>>> send a Pd message directly from a perform routine ! Always use a 
>>>> clock !"
>>
>>
>>
>>
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