rcutils is a C API consisting of macros, functions, and data structures used through out the ROS 2 code base.

The API is a combination of parts:

Allocator concept, used to inject the allocating and deallocating methods into a function or type.
- rcutils_allocator_t
- rcutils/allocator.h
Command line interface utilities:
- rcutils/cmdline_parser.h
Utilities for setting error states (error message, file, and line number) like strerror for errno:
- rcutils/error_handling.h
Some basic filesystem utilities like checking for path existence, getting the cwd, etc...:
- rcutils/filesystem.h
A C string find method:
A convenient string formatting function, which takes a custom allocator:
- rcutils_format_string()
- rcutils/format_string.h
A function to get an environment variable's value:
- rcutils_get_env()
- rcutils/get_env.h
Extensible logging macros:
- Some examples (not exhaustive):
- rcutils/logging_macros.h
- rcutils/logging.h
A string replacement function which takes an allocator, based on http://creativeandcritical.net/str-replace-c:
- rcutils_repl_str()
- rcutils/repl_str.h
String splitting functions which take a custom allocator:
A version of strdup which takes an allocator:
- rcutils_strdup()
- rcutils/strdup.h
Portable implementations of "get system time" and "get steady time":
Some useful data structures:
- A "string array" data structure (analogous to std::vector<std::string>):
  - rcutils_string_array_t
  - rcutils/types/string_array.h
- A "string-string map" data structure (analogous to std::map<std::string, std::string>)
  - rcutils_string_map_t
  - rcutils/types/string_map.h
Macros for controlling symbol visibility and linkage for this library:
- rcutils/visibility_control.h