最近在做一个移植UCOS-II的任务，遇到了semihosting的问题，现在把这个问题的解决办法总结一下。
Step 1 : 用#pragma import(__use_no_semihosting_swi)保证用户程序不调用semihostSWI；
此后，link时会有
Error :L6200E:Symbol __semihosting_swi_guard multiply defined (by use_semi.o and use_no_semi.o)
这是因为，还有compiler helper functions 和 initialization code在调用semihostSWI (This error is reported when functions that use semihosting SWIs are linked in from the Clibrary, in the presence of the __use_no_semihosting_swi guard)
Step 2 : armlink -verbose 结果中，查出调用semihostSWI的外部库函数，如：
Loading member sys_exit.o from c_a__un.l.
definition: _sys_exit
reference : __I_use_semihosting_swi
在报错的窗口中寻找__I_use_semihosting_swi很麻烦，我们可以再在连接器中修改一下参数如下：
Link with 'ARMlink -verbose -errors err.txt'
----------------------------------------
For example:
Loading member sys_exit.o from c_a__un.l.
reference : __I_use_semihosting_swi
definition: _sys_exit
:This shows that the SWI-using function _sys_exit is being linked-in from the C library. To
prevent this, you will need to provide your own implementation of this function.
----------------------------------------
Step 3 : 在用户程序中重写这些函数，如：
AREA ||.text||, CODE, READONLY
__user_initial_stackheap
LDR r0, =0x20000 ; HB
LDR r1, =0x40000 ; SB
; r2 not used (HL)
; r3 not used (SL)
MOV PC,LR
EXPORT __user_initial_stackheap
END
Step 4 : compile and link OK！
