Monado OpenXR Runtime
Understanding and Writing Targets: Connecting the Pieces

Monado is designed to be a collection of related but independent modules. In a sense, the Monado project is almost more of a "runtime construction kit" than a single monolithic runtime. This makes it easy for adaptation and modification, as well as extension, but it also means that any call in an OpenXR application goes through quite a few modules before e.g. talking with the driver or the compositor.

The final build product that brings all the desired components together, potentially with additional code, is called the "target". There are several targets included in the Monado source tree (in src/xrt/targets/) including:

  • cli - builds monado-cli executable
  • openxr - builds OpenXR runtime shared object
  • gui - builds monado-gui executable
  • service - builds monado-service executable (if XRT_FEATURE_SERVICE is enabled)

There is also a directory common which builds two static libraries. Because the "target" is responsible for pulling in all the desired drivers, etc. it can lead to some repetition if multiple targets want the same driver collection. For this reason, the "all drivers" code shared between many targets is located here, though you could consider it a part of the individual targets. See this section for details on how the targets find the drivers to probe: Writing a new driver

Requirements of a Target

A target must first provide the entry point desired: int main() if it's an executable, or the well-known symbol name if it's a shared library. In some cases, the entry point might be provided by one of the modules being combined to form the target. For instance, an OpenXR runtime must expose xrNegotiateLoaderRuntimeInterface: this function is provided by the OpenXR state tracker st_oxr, so the OpenXR runtime target links the state tracker in and ensures that symbol is present and visible in the final build product.

Then, the target must provide an interface to the collection of devices desired. This is done by implementing the xrt_instance interface in your target and providing a definition of xrt_instance_create that instantiates your implementation.

All methods of xrt_instance are required, though the get_prober method may output a null pointer if the instance is not using a prober, and targets that do not need compositing may stub out the create_native_compositor method to always return an error. A fully-featured implementation is in src/targets/common/target_instance.c, which calls xrt_prober_create_with_lists passing the common target_lists variable to include all supported devices.

For more detailed information on this interface, see the documentation for xrt_instance.

Sample Call Trees

For clarity, call trees are included below for the OpenXR runtime in two general cases: XRT_FEATURE_SERVICE disabled, and XRT_FEATURE_SERVICE enabled.

Note that even with XRT_FEATURE_SERVICE enabled, the other targets (cli, gui) more closely resembler the XRT_FEATURE_SERVICE disabled diagram: they contain the device drivers internally rather than contacting the service. They use a modified version of the in-process target instance without compositor support.


This is the simplest architecture. It is also the architecture used by the various extra targets like monado-cli even when building with XRT_FEATURE_SERVICE enabled. (The CLI and GUI link against a slightly modified version, target_instance_no_comp, which stubs out the compositor creation call, but are otherwise the same.)


Note that in this case, there are two processes involved, which have different xrt_instance implementations.

  • The runtime has a "stub" or "client proxy" implementation that delegates to the service over the IPC.
  • The service has a normal or complete instance implementation that actually provides hardware device interaction, etc.