[LV2] Need help sending a variable size atom from the UI of my plug-in

Clay Citadel claycitadel at gmail.com
Mon Sep 6 08:25:10 PDT 2021

I'm using c++ and lvtk framework, but I'll be perfectly happy with using
raw lv2 C functions to do this. Here is the code I have that works with
In the UI code:

      struct VectorOf5Ints {
        uint32_t size; // sizeof(LV2_Atom_Vector_Body) + (42 *
        uint32_t type; // map(expand("Vector"))
        uint32_t child_size; // sizeof(float)
        uint32_t child_type; // map(expand("Float"))
        uint32_t elems[5];
      VectorOf5Ints vec{sizeof(LV2_Atom_Vector_Body) + (5 *
                        {5, 9, 5, 9, 7}};
      write(5, sizeof(vec), map(LV2_ATOM__eventTransfer), &vec);

where write is a LV2UI_Write_Function.

In the turtle file:
    ... , [
    a lv2:InputPort, atom:AtomPort ;
    atom:bufferType atom:Vector ;
    lv2:designation lv2:control ;
    lv2:index 5 ;
    lv2:symbol "out_ui" ;
    lv2:name "UI to Business comm" ;
    rdfs:comment "UI -> communication" ;
    resize_port:minimumSize 15360 ;
    ] ...

In the plug-in connect_port function:
   configuration_from_ui = static_cast<int *>(data);

In the run function of my plugin:
    for (auto i{0}; i <= 5+9; ++i) {
         log.printf(map(LV2_LOG__Entry), "LV2LOGGER: %d\n",

This works, but it attaches the event header because I'm using
LV2_ATOM__eventTransfer, but I really just want to use
LV2_ATOM__atomTransfer is there a way to do this? when I change it in the
code I get the following message in the console:

UI write with unsupported protocol 48 (

1) Is it really unsupported or am I doing something wrong? Like for
example, do I need to change something in the turtle file?

2) What I'm trying to do is send a configuration state from the UI to the
business end. Ideally I would like it to avoid sending the message every
frame, what is it doing right now? is the host constantly copying a 15360
size buffer every frame? or only when I use the LV2UI_Write_Function it
will copy the buffer into the business end of the plugin? what is the best
way of doing this? Am I on the right track?

3) If I must use eventTransfer how can get just the event body when I'm
reading it on the business end, notice I'm just doing a "for loop" for
testing and I figured out the event header is 5 ints worth of size.
Similarly, how do I get the atom body other than doing what I currently do
which is just added a number to the pointer and taking that.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.lv2plug.in/pipermail/devel-lv2plug.in/attachments/20210906/2925b3fd/attachment.html>

More information about the Devel mailing list