These are user-provided functions. There are WEAK definitions of these functions in the REXIS library. A user may override any of these functions by providing their own definitions. See the Doxygen document for function descriptions and code fragments implemented in the library. In the simplest case, a user does not need to override any of the functions and may just use the ones in the library. However, note that printf is used in many of the default routines, so the user code MUST provide a definition for putchar for the functions to link and to work.