[LV2] worker thread question

David Robillard d at drobilla.net
Sun Dec 30 08:24:57 PST 2012

On Sun, 2012-12-30 at 11:20 +0100, hermann meyer wrote:
> Am 30.12.2012 09:16, schrieb Robin Gareus:
> > On 12/30/2012 08:43 AM, hermann meyer wrote:
> >> Thanks Robin, for your input, what is open now for me is that I
> >> need to read the port values in the worker thread, and that I would
> >> watch them "outside" from run(). From what I understand about
> >> glib-threads there is no problem as long I only read the values
> >> were my pointers point to (data).
> > I'm not aware of the spec mandating specific behavior of port values for
> > the host-side implementation of the worker thread. Dave?
> >
> > You can access the instance LV2_Handle in the worker thread and
> > you can also use the user-data field to pass information along to it.
> >
> > Since you likely have a local copy of the port data anyway (to compare
> > if value_changed()) it's safer to use that local copy from your
> > instance. Opposed to the data pointed to by ports, you are in control
> > of that variable.
> But those copy is set by use a other thread then run() to get the port 
> value.
> In fact it is set by the worker thread at the end of schedule.
> So the outstanding question is, why should it be un-save to  (only) read 
> the value my pointer points to, from a other thread then run() ?

Here is a valid host implementation approach in pseudo-code:

void run_plugin(Plugin plugin, float control_value, uint32_t nframes)
    plugin.connect_port(0, &control_value);

When this function returns, control_value (which is on the stack), does
not exist anymore and will be immediately overwritten in the next
function call.  A plugin thread that accesses that port will then read
garbage on the stack, not the control value.

Similar things happen if the host decides to connect the port to
something else before/after run, which is more common.

connect_port() is a mistake.  Pretend port buffers are a parameter to
run() because that is what they conceptually are, and literally should

Breaking thread rules in the spec can cause problems by definition,
because it violates assumptions in host implementations.  Of all the
things that it is bad to do, this is probably the worst.  The problems
you cause by doing that can be notoriously difficult to detect and fix.
It *might* work fine 9 times then crash on the tenth - during your live
performance.  Not exactly what you want from quality audio software.
On behalf of every host author (and user) ever, please don't do that.

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