Monado OpenXR Runtime
Code Style and Conventions

Here are some general code style guidelines we follow.

Note that we aim to "code with respect", to avoid terminology that may limit our community or hurt those in it, as well as to conform with emerging industry standards. Good guidelines to look to include the Android Coding with Respect policy and the Write Inclusive Documentation page from the Google developer documentation style guide. The latter also links to a word list for clear documentation, which, while not binding on this project, is useful in making sure your code, comments, and docs are understandable by the worldwide Monado community.

APIs

Internal APIs, when it makes sense, should be C APIs. Headers that define general communication interfaces between modules (not only use of utilities) belong in the xrt/include/xrt directory, and should not depend on any other module outside that directory. (As a historical note: this directory gets its name from a compressed version of the phrase "XR RunTime", a generic term for Monado and an early development codename. Also, it's shorter than monado_ and so nicer to use in code.)

What follows are some basic API usage rules. Note that all the module usage relations must be expressed in the build system, so module usage should form a directed-acyclic-graph.

  • Any module can implement or use APIs declared in xrt/include/xrt
  • Any module (except the xrt interface headers themselves) can (and should!) use APIs declared in xrt/auxiliary/util.
  • Any module except for auxiliary/util and the xrt interface headers themselves can use APIs declared in other xrt/auxiliary modules.

Naming

  • C APIs:
    • lower_snake_case for types and functions.
    • UPPER_SNAKE_CASE for macros. e.g. U_TYPED_CALLOC (which is how all allocations in C code should be performed)
    • Prefix names with a "namespace" - the library/module where they reside. e.g. u_var_add_root, math_pose_validate
      • Related: only things prefixed by xrt_ belong in the xrt/include/xrt directory, and nothing named starting with xrt_ should be declared anywhere else. (Interfaces declared in xrt/include/xrt are implemented in other modules.)
    • Generally, we do not declare typedefs for struct and enum types, but instead refer to them in long form, saying struct or enum then the name. The exception to not using typedefs is function pointers used as function arguments as these become very hard to both read and type out.
    • If a typedef is needed, it should be named ending with _t. Function pointer typedefs should end with _func_t.
    • Parameters: lower_snake_case or acronyms.
      • Output parameters should begin with out_.
      • Of special note: Structures/types that represent "objects" often have long type names or "conceptual" names. When a pointer to them is passed to a function or kept as a local variable, it is typically named by taking the first letter of each (typically _-delimited) word in the structure type name. Sometimes, it is an abbreviated form of that name instead. Relevant examples:
        • xrt_comp_native_create_swapchain() is a member function of the interface xrt_compositor_native, and takes a pointer to that interface named xcn. It creates an xrt_swapchain, which it populates in the parameter named out_xscn: out_ because it's a purely output parameter, xscn from xrt_swapchain_native specifically the letters Xrt_SwapChain_Native. xrt_swapchain and related types are a small exception to the rules - there are only 2 words if you go by the _ delimiters, but for clarity we treat swapchain as if it were two words when abbreviating. A few other places in the xrt headers use x + an abbreviated name form, like xinst for xrt_instance, xdev for xrt_device, xsysc sometimes used for xrt_system_compositor.
    • create and destroy are used when the functions actually perform allocation and return the new object, or deallocation of the passed-in object.
    • If some initialization or cleanup is required but the type is not opaque and is allocated by the caller, the names to use are init and, if needed, one of cleanup/fini/teardown. (We are not yet consistent on these names.) One common example is when there is some shared code and a structure partially implementing an interface: a further-derived object may need to call an init function on the shared structure, but it was allocated by the derived object and held by value.
  • C++:
    • Where a C API is exposed, it should follow the C API naming schemes.
    • If only a C++ API is exposed, a fairly conventional C++ naming scheme is used:
      • Namespaces: nested to match directory structure, starting with xrt::.
        • There are no C++ interfaces in the xrt/include/xrt, by design, so this is not ambiguous.
        • Place types that need to be exposed in a header for technical reasons, but that are still considered implementation details, within a further-nested detail namespace, as seen elsewhere in the C++ ecosystem.
      • Types/classes: CamelCase
      • Methods/functions: lowerCamelCase
      • Constants/constexpr values: kCamelCase
      • If a header is only usable from C++ code, it should be named with the extension .hpp to signify this.
  • Math:
    • For different types of transforms T between two entities A and B, try to use variable names like T_A_B to express the transform such that B = T_A_B * A. This is equivalent to "`B` expressed w.r.t. `A`" and "the transform that converts a point in `B` coordinates into `A` coordinates". T can be used for 4x4 isometry matrices, but you can use others like P for poses, R for 3x3 rotations, Q for quaternion rotations, t for translations, etc.

Patterns and Idioms

This is an incomplete list of conventional idioms used in the Monado codebase.

C "Inheritance" through first struct member

Despite being in C, the design is fairly object-oriented. Types implement interfaces and derive from other types typically by placing a field of that parent type/interface as their first element, conventionally named base. This means that a pointer to the derived type, and a pointer to the base type, have the same value.

For example, consider client_gl_swapchain

Structures/types that represent "objects" are often passed as the first parameter to many functions, which serve as their "member functions". Sometimes, these types are opaque and not related to other types in the system in a user-visible way: they should have a _create and _destroy function. See time_state, time_state_create, time_state_destroy

In other cases, an interface will have function pointers defined as fields in the interface structure. (A type implementing these may be opaque, but would begin with a member of the interface/base type.) These interface function pointers must still take in a self pointer as their first parameter, because there is no implied this pointer in C. This would result in awkward calls with repeated, error-prone mentions of the object pointer, such as this example calling the xrt_device::update_inputs interface: xdev->update_inputs(xdev). These are typically wrapped by inline free functions that make the call through the function pointer. Considering again the xrt_device example, the way you would call xrt_device::update_inputs is actually xrt_device_update_inputs().

Destroy takes a pointer to a pointer, nulls it out

Destroy free functions should take a pointer to a pointer, performing null checks before destruction, and setting null. They always succeed (void return): a failure when destroying an object has little meaning in most cases. For a sample, see xrt_images_destroy. It would be used like this:

struct xrt_image_native_allocator *xina = /* created and initialized, or maybe NULL */;
/* ... */
/* here, xina is NULL in all cases, and if it wasn't NULL before, it has been freed. */
static void xrt_images_destroy(struct xrt_image_native_allocator **xina_ptr)
Destroy the image allocator.
Definition: xrt_compositor.h:2062
Allocator for system native images, in general you do not need to free the images as they will be con...
Definition: xrt_compositor.h:2001

Note that this pattern is used in most cases but not all in the codebase: we are gradually migrating those that don't fit this pattern. If you call a destroy function that does not take a pointer-to-a-pointer, make sure to do null checks before calling and set it to null after it returns.

Also note: when an interface includes a "destroy" function pointer, it takes the normal pointer to an object: The free function wrapper is the one that takes a pointer-to-a-pointer and handles the null checks. See for example xrt_instance_destroy takes the pointer-to-a-pointer, while the interface method xrt_instance::destroy takes the single pointer.