If you want only the compiler support functions for division, structure copy, and floating-point arithmetic, you must provide __rt_raise(). This also enables very simple library functions such as those in errno.h, setjmp.h, and most of string.h to work.

If you call setlocale() explicitly, locale-dependent functions are activated. This enables you to use the atoi family, sprintf(), sscanf(), and the functions in ctype.h.

Programs that use floating-point must call _fp_init(). If you select software floating-point in --fpmode=ieee_fixed or --fpmode=ieee_full mode, the program must also provide __rt_fp_status_addr().

Implementing high-level input/output support is necessary for functions that use fprintf() or fputs(). The high-level output functions depend on fputc() and ferror(). The high-level input functions depend on fgetc() and __backspace().