Tailoring the C library to a new execution environment involves re-implementing functions to produce an application for a new execution environment, for example, embedded in ROM or used with an RTOS.

Symbols that start with a single or double underscore name functions that are used as part of the low-level implementation. You can re-implement some of these functions. Additional information on these library functions is available in the rt_heap.h, rt_locale.h, rt_misc.h, and rt_sys.h include files and the rt_memory.s assembler file.