ros2-interfaces

# ROS 2 Interfaces ## Static-size arrays (verbatim rules of thumb) In ROS 2, a static size array is defined in message files (`.msg`) using the syntax `Type[N]`, where `N` is a fixed, non-zero integer representing the exact number of elements. When generated into C++, these are mapped to `std::array<T, N>`, and their default value is an array of `N` zero-initialized elements. Key characteristics and limitations include: - **IDL Mapping:** Static arrays map to `array` in the Interface Definition Language (IDL) and `T[N]` in DDS types. - **Visualization Limits:** Tools like `rqt_plot` may have bugs or limitations when plotting fixed-size array members (e.g., covariance in `NavSatFix`), sometimes requiring workarounds or specific indexing. - **CLI Constraints:** The `ros2 topic pub` command may fail or truncate arrays if the total number of elements exceeds 32 (specifically noted in older Dashing/Crystal versions for `geometry_msgs` covariance arrays), whereas `ros2 topic echo` truncates output at 128 elements by default for usability. - **Simulation Integration:** In Simulink, variable-length arrays are converted to fixed-length arrays with a default maximum of 128 for primitive types; this limit must be manually adjusted if the static array size or expected dynamic data exceeds this bound. ## Field syntax cheat sheet | `.msg` syntax | Meaning | C++ | Python | |---------------|---------|-----|--------| | `float64 x` | scalar | `double` | `float` | | `float64[9]` | **static** array length 9 | `std::array<double, 9>` | `array('d')` / list len 9 | | `float64[]` | **unbounded** sequence | `std::vector<double>` | `list` | | `float64[<=N]` | bounded sequence (IDL) | vector with max | list (runtime check) | | `string name` | UTF-8 string | `std::string` | `str` | | `string<=N name` | bounded string | string | str | Examples: ``` # Static (PoseWithCovariance-style) float64[36] covariance # Dynamic geometry_msgs/Point[] points # Bounded (prefer when wire size must be capped) uint8[<=1024] payload ``` ## Interface package checklist 1. Separate package (or clear `rosidl_generate_interfaces` target) for msgs/srvs/actions. 2. `package.xml`: `rosidl_default_generators` (build), `rosidl_default_runtime` (exec), `member_of_group` `rosidl_interface_packages`. 3. List dependencies of nested types in `rosidl_generate_interfaces(... DEPENDENCIES ...)`. 4. After rebuild: `source install/setup.bash` before Python/C++ consumers see new types. 5. Prefer **static** arrays only when length is part of the contract (covariance 36, IMU 9). Prefer **unbounded/bounded sequences** for variable payloads. ## Common pitfalls - Changing `Type[N]` ↔ `Type[]` is an **ABI/API break** — bump interface version / coordinate all publishers. - Nested messages with large static arrays blow up DDS sample size; watch MTU / Cyclone buffer sizing. - `ros2 interface show <pkg>/msg/<Name>` to confirm what was generated. - Do not hand-edit generated `*__struct.hpp` / Python stubs — change `.msg` and regenerate. ## Related - QoS / large topics: skill `ros2-qos-dds` - Build after interface changes: skill `ros2-workspace-ops` - Pub/echo truncation: skill `ros2-cli-debug`