undefined symble error about stdlib in nRF5 SDK Keil project

Hi everyone,

I'm in trouble with the stdlib of Keil SDK , which is nRF5_SDK_15.0.0_a53641a, while compliing.

.\_build\nrf52840_xxaa.axf: Error: L6218E: Undefined symbol exit (referred from loslib.o).
.\_build\nrf52840_xxaa.axf: Error: L6218E: Undefined symbol system (referred from loslib.o).
.\_build\nrf52840_xxaa.axf: Error: L6218E: Undefined symbol time (referred from loslib.o).

The code what I add in the demo project causes Undefined symble error reported as above.

However, I notice that exit() and system() are both declared in stdlib.h

extern _ARMABI_NORETURN void exit(int /*status*/);

extern _ARMABI int  system(const char * /*string*/);

while time() is declared in time.h.

extern _ARMABI time_t time(time_t * /*timer*/);

Why errors occoured? Aren't they already realizaed in the stdlib of SDK?

Someone help me, pls.