解决的办法是：勾选Use MicroLIB
Keil µVision的详细信息：MDK-ARM Plus Version：5.37.0.0
µVision User's Guide: Target (Arm Compiler)
- Use MicroLIB to change the C run-time library set for smallest code application code. MicroLIB is not fully ANSI compatible but sufficient for most small embedded applications.
Compiler Getting Started Guide: Avoid linking in the Arm C library
- C standardlib.
- C microlib.
The standard C library (standardlib) is the default C library that projects are likely to use. The micro library (microlib) is an alternative to the standard C library. Microlib focuses in particular on smaller code size, but with some documented limitations and restrictions.
这是一个编译器相关的选项，可以选用不同的lib。
暂时只要知道这些就够了。
记录以免遗忘。
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