[LV2] Questions about passing strings and other data from UI to LV2 plugin

[Hanspeter Portner]

On 27.11.2016 09:58, Torbjörn Rathsman wrote:
> I need to pass a string from the UI to the plugin. From the eg-sample, it
> appears that an LV2 atom should be written to a atom port.
> 
> If I understand it correctly
> 
>  1. Allocate a LV2_Atom_Forge. May that object be on the stack or does it have
>     to survive after the UI event callback has returned?

This could be on the stack, but it makes sense to make it part of UI/plugin
struct/class directly as the forge needs to be initialized before use
(lv2_atom_forge_init). e.g. initialize once (in your initialize routine), use
many times...

>  2. Call lv2_atom_forge_set_buffer. How do I know the required size of the
>     buffer? Is it the size of the payload (the string), or does it include any
>     headers. The example sets it to 1024 bytes for no reason. May the buffer be
>     allocated on the stack or does it have to survive the UI after the UI event
>     callback has returned?

It does include headers (and padding), so for your string, a safe minimum is:
  uint32_t capacity = lv2_atom_pad_size(sizeof(LV2_Atom) + strlen(str) + 1)

Don't forget to initialize the forge structure (lv2_atom_forge_init), this sets
vital variables in the structure.

I use lv2_atom_forge_set_buffer manly on the DSP side (where you cannot use
malloc et al.). On the UI side it's more convenient to use
lv2_atom_forge_set_sink with which you can implement growing buffers and don't
have to ask yourself about maximal buffer sizes in the first place, e.g. [1].

>  3. Add some objects. The API uses key-value pairs. Should the size of keys be
>     included in the buffer size?

Not sure what API you mean, but as mentioned above, just use a growing buffer.

>  4. Write the data to an atom input port

Yes, but don't forget to use the proper port protocol in the writer function,
e.g. atom:eventTransfer [2].

[1]
https://gitlab.com/OpenMusicKontrollers/tracker.lv2/blob/master/tracker_ui.c#L595
[2]
https://gitlab.com/OpenMusicKontrollers/tracker.lv2/blob/master/tracker_ui.c#L603