Extern "C"

Extern "C"
* It is used for C/C++ mix programming. Two aspects it has:
- All the variables and functions it is with are extern
- All of these are compiled and linked with C methods

* C and C++ Object Name
- Functions and variables in C are identified by their names
- Functions and variables in C++ are identified by their name, input arguments, return value and class name associated, etc. by combined, due to the overloading and other features in C++.

* Usage
It is only used in C++ header file and can not be included in C files.
- When C++ uses C header files, use the following in the C++ header file:
extern "C"
{
     #include "xxx.h"
}
The C++ files must include this header file instead of using extern directly for C variables and functions.
- When C uses C++ header files, use extern instead of including the C++ header file:
cppfile.h:
extern "C"
{
     void xxx(void);
}
cfile.c
extern void xxx(void);

* _cplusplus
In order to share this header file by C and C++, the macro _cplusplus is used to wrap the extern "C":
#ifdef _cplusplus
extern "C"{
#endif
... ... ...
#ifdef _cplusplus
}
#endif