编译simpe_test时出错??

limon

+ linking o file ... : start.o at91_aic.o at91_usart.o at91_tc.o at91_init.o skyeye_stdio.o skyeye_printf.o main_entry.o -> simple_test.elf
arm-elf-ld -p -X -T../../samples/samples.lds start.o --start-group ../ulib/libuc.a at91_aic.o at91_usart.o at91_tc.o at91_init.o skyeye_stdio.o skyeye_printf.o main_entry.o --end-group -o simple_test.elf
skyeye_printf.o: In function `getnum':
/home/limon/ucosii4skyeye/samples/simple_test/../../lib/skyeye_printf.c:110: undefined reference to `isdigit'
skyeye_printf.o: In function `skyeye_printf':
/home/limon/ucosii4skyeye/samples/simple_test/../../lib/skyeye_printf.c:156: undefined reference to `isdigit'
make[1]: *** [simple_test.elf] Error 1
make[1]: Leaving directory `/home/limon/ucosii4skyeye/samples/simple_test'
make: *** [all] Error 2