(Host Side Question) Plugins from "Full Bucket Music" crash after "set_parent"

[Alpha21016]

Hi, I'm developing a DAW software and I use QT6 as my GUI framework. I have implemented clap sdk and tried some plugins listed in this page "https://clapdb.tech/". Most of the plugins worked well, but all the plugins from "Full Bucket Music" crashed after I executed the function "bool(CLAP_ABI *set_parent)(const clap_plugin_t *plugin, const clap_window_t *window);". My codes to load the plugin follow the official project "clap-host", I don't know where I did it wrong. The following is my loading process:

1. load clap library
2. get clap_entry
3. get_factory
4. get_plugin_descriptor
5. create_plugin (use first desc->id)
6. init plugin
7. activate plugin
8. check can use gui and check if is in main thread
9. select os api = "win32" (win11/x64)
10. create gui using os_api
11. generate a clap_window_t
12. create an empty qt widget
13. pass the wid from this qt widget to clap_window_t (w.win32 = reinterpret_cast<clap_hwnd>(wid);)
14. execute set_parent
15. the set_parent function returns true and the code execution continues, after a few milliseconds my program crashes
16. exit code = -1073741819

I'd be very grateful If someone could tell me what is wrong with my process, thank you

[baconpaul]

Are you calling gui create before set parent?

https://github.com/free-audio/clap/blob/main/include/clap/ext/gui.h

that header contains the call order

[Alpha21016]

Are you calling gui create before set parent?

https://github.com/free-audio/clap/blob/main/include/clap/ext/gui.h

that header contains the call order

Oh I forgot to write that in my question, thanks for your notice. Actually I did create gui between step 9 and 10, but it still crash. I compile and run the official demo project "clap-host", it works well with the "Full Bucket Music" plugins. I have compared each step with each other but I can hardly find the difference

[baconpaul]

Do other plugins like surge work in your host?

[Alpha21016]

Yes, I have tried SurgeXT, PlexiTape, Odin2, Regency, Vital and all Fabfilter plugins, all of them successfully attach the qt widget and no crash happens.

[xris1658]

The exit code is 0xC0000005, which means an access violation, you should run your program with Dr. Memory to spot illegal memory accesses.

Since your program crashes on showing plugin GUI, and the load process itself seems correct, it's a good idea to check if your clap_host implementations, especially clap_host_gui callbacks are correct.

[baconpaul]

this is an excellent idea.

In fact, having your host print out / log all the host side extensions which are queried would help. It may be that FBM is querying an extension you don't support and is not being robust to null; or it may be that it queries one you partially support.

[Alpha21016]

Thanks for your help, I finally find out the reason. I did not fully implement the host callback function "CLAP_ABI *get_extension", when these plugins request extension such as "host_gui", I simply return nullptr and causes them crashed.

[baconpaul]

Right the clap contract is: if you support an extension you must support all methods of it. But a plugin should be resilient if you support none. Glad you found it!