用RTC功能时打开这个宏RT_USING_LIBC有什么作用

Searching for 'RT_USING_LIBC'...
D: tt t-thread - 副本\include tdebug.h(18) : #if !defined(RT_USING_LIBC)
D: tt t-thread - 副本\include tdebug.h(19) : #error "POSIX layer need standard C library(RT_USING_LIBC)"
D: tt t-thread - 副本\bsp\stm32\stm32l475-atk-pandora tconfig.h(92) : #define RT_USING_LIBC
Lines matched: 3      Files matched: 2      Total files searched: 173

/* settings depend check */
#ifdef RT_USING_POSIX
#if !defined(RT_USING_DFS) || !defined(RT_USING_DFS_DEVFS)
#error "POSIX poll/select, stdin need file system(RT_USING_DFS) and device file system(RT_USING_DFS_DEVFS)"
#endif
#if !defined(RT_USING_LIBC)
#error "POSIX layer need standard C library(RT_USING_LIBC)"
#endif
#endif
#ifdef RT_USING_POSIX_TERMIOS
#if !defined(RT_USING_POSIX)
#error "termios need POSIX layer(RT_USING_POSIX)"
#endif
#endif