linking时报错

linking...
d:/keil_v5/arm/5.2_2015q4/bin/../lib/gcc/arm-none-eabi/5.2.1/../../../../arm-none-eabi/bin/ld.exe: warning: cannot find entry symbol _start; defaulting to 00008000
./objects/startup_STM32f0xx.o: (.text+0x0): undefined reference to `_sidata'
./objects/startup_stm32f0xx.o: (.text+0x4): undefined reference to `_sdata'
./objects/startup_stm32f0xx.o: (.text+0xc): undefined reference to `_***ss'
./objects/startup_stm32f0xx.o: (.text+0x10): undefined reference to `_ebss'
./objects/startup_stm32f0xx.o: In function `LoopFillZerobss':
E:STM32F0project/../Libraries/CMSIS/Device/ST/STM32F0xx/Source/Templates/gcc_ride7/startup_stm32f0xx.s:101: undefined reference to `SystemInit'
E:STM32F0project/../Libraries/CMSIS/Device/ST/STM32F0xx/Source/Templates/gcc_ride7/startup_stm32f0xx.s:76: undefined reference to `_sidata'
E:STM32F0project/../Libraries/CMSIS/Device/ST/STM32F0xx/Source/Templates/gcc_ride7/startup_stm32f0xx.s:82: undefined reference to `_sdata'
E:STM32F0project/../Libraries/CMSIS/Device/ST/STM32F0xx/Source/Templates/gcc_ride7/startup_stm32f0xx.s:87: undefined reference to `_***ss'
E:STM32F0project/../Libraries/CMSIS/Device/ST/STM32F0xx/Source/Templates/gcc_ride7/startup_stm32f0xx.s:97: undefined reference to `_ebss'
./objects/startup_stm32f0xx.o: (.isr_vector+0x0): undefined reference to `_estack'
collect2.exe: error: ld returned 1 exit status
".ObjectsSTM32F051.elf" - 1 Error(s), 0 Warning(s).
Target not created.
Build Time Elapsed:  00:00:05

以上是linking是出现的报错。
我试过禁止编译Thume指令；clean整个工程，然后重新编译连接。都没成功。
目前使用的IDE是Keil V5+gcc。startup文件用STM32F0xx_StdPeriph_Lib_V1.0.0LibrariesCMSISDeviceSTSTM32F0xxSourceTemplatesgcc_ride7的文件。
请问有朋友知道是怎么回事么？请多指教，谢谢。