README

rcl_logging_implementation

Overview

The rcl_logging_implementation package provides dynamic loading capabilities for rcl logging implementations, similar to how rmw_implementation works for middleware layers. This allows users to switch logging backends at runtime without rebuilding rcl.

[!NOTE] This package is only used when dynamic loading is enabled at build time. When RCL is built with a specific logging implementation (e.g., rcl_logging_spdlog), it links directly to that implementation without this abstraction layer, eliminating function call overhead.

Features

Dynamic Loading: Load logging implementations (e.g., rcl_logging_spdlog, rcl_logging_noop) at runtime
Runtime Configuration: Use RCL_LOGGING_IMPLEMENTATION environment variable to select implementation
Fallback Default: Defaults to rcl_logging_spdlog if no implementation is specified

Architecture

Dynamic Loading (Default)

When rcl is built without specifying RCL_LOGGING_IMPLEMENTATION, it uses this package for runtime selection:

rcl application
    ↓
rcl (core library)
    ↓
rcl_logging_implementation (this package) ← Dynamic loading happens here
    ├─ Runtime Loading → rcl_logging_spdlog
    ├─ Runtime Loading → rcl_logging_noop
    └─ Runtime Loading → custom implementations

Static Linking (Opt-in)

When rcl is built with RCL_LOGGING_IMPLEMENTATION=rcl_logging_spdlog, it bypasses this package entirely:

rcl application
    ↓
rcl (core library)
    ↓
rcl_logging_spdlog ← Direct link, no abstraction layer

See more details for basic design.

Usage

Runtime Configuration (Dynamic Loading)

Set the RCL_LOGGING_IMPLEMENTATION environment variable to select a logging implementation at runtime:

# Use spdlog (default)
export RCL_LOGGING_IMPLEMENTATION=rcl_logging_spdlog
ros2 run my_package my_node

# Use noop
export RCL_LOGGING_IMPLEMENTATION=rcl_logging_noop
ros2 run my_package my_node

# Use custom implementation
export RCL_LOGGING_IMPLEMENTATION=my_custom_logging
ros2 run my_package my_node

If not set, the implementation defaults to rcl_logging_spdlog that you can also set via DEFAULT_RCL_LOGGING_IMPLEMENTATION.

# Set the default to noop (still allows runtime override via environment variable)
colcon build --packages-select rcl_logging_implementation --cmake-args -DDEFAULT_RCL_LOGGING_IMPLEMENTATION=rcl_logging_noop

Build-Time Configuration (Static Linking)

For applications that require static linking (e.g., embedded systems, strict deployment requirements), configure rcl at build time to link directly to a specific implementation:

# Build RCL with static linking to spdlog (bypasses rcl_logging_implementation)
colcon build --packages-select rcl --cmake-args -DRCL_LOGGING_IMPLEMENTATION=rcl_logging_spdlog

# Build RCL with static linking to noop (bypasses rcl_logging_implementation)
colcon build --packages-select rcl --cmake-args -DRCL_LOGGING_IMPLEMENTATION=rcl_logging_noop