POSIX Thread-Safe Functions option.
In Zephyr, _POSIX_THREAD_SAFE_FUNCTIONS is defined when CONFIG_POSIX_C_LANG_SUPPORT_R, CONFIG_POSIX_FILE_SYSTEM_R, and CONFIG_POSIX_FILE_LOCKING are enabled.
_POSIX_THREAD_SAFE_FUNCTIONS
CONFIG_POSIX_C_LANG_SUPPORT_R
CONFIG_POSIX_FILE_SYSTEM_R
CONFIG_POSIX_FILE_LOCKING