Clarify API layer function limitations

[Supreeeme]

In the loader spec, section 3.2 ("Application API Layer Usage"):

An API layer cannot introduce new OpenXR core API commands, but may introduce new extension-specific OpenXR commands that can be queried through the extension interface.

There's no direct definition of a "core API command", but there is the definition of a "core function" in the glossary:

A function that is defined in a version of OpenXR and not an extension. For example, xrCreateInstance is a core function defined in the OpenXR API as well as openxr.h.

If this is what is meant by "core API command", I don't even see how it would be possible for an API layer to define such a thing anyway. What is the intent of the line?

[rpavlik]

You are right that command means function here. I think the intent of the line is the second part: API layers can add new functions, but they have to be enabled through extensions. You're welcome to clarify that line if you wish. Does that help?

[Supreeeme]

So to clarify, any new functions that an API layer implements must have a corresponding extension? What's the reason for this restriction?

[rpavlik]

That's how OpenXR functionality is controlled and exposed, through extensions. For an API layer that primarily implements new entry points on top of a runtime, enabling/disabling the extension provides a way for the app to control if it is in use, etc. xrGetInstanceProcAddr shouldn't return success for anything except functions in core or enabled extensions. It's not really a limitation, just a structure.

The extension doesn't have to be published or reviewed, though it should use a reserved extension number, and the repo provides a way to generate standalone headers of such extensions. See the extension process and style guide docs for more.

[rpavlik-bot]

An issue (number 2502) has been filed to correspond to this issue in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2502 ), to facilitate working group processes.

This GitHub issue will continue to be the main site of discussion.