Extern "C" overview

1 Intention
extern "C" yes C++ Unique instructions （C Cannot use this command ）, The purpose is to support C++ And C Mixed programming .

2 effect
extern “C” To tell C++ For compiler C The rule compiles the specified code （ In addition to function overloading ,extern “C” No effect C++ Other features ）.

3 reason
Why use C Rule compilation C++ What about code? ？

because C and C++ The Compilation Rules are different , The main difference is that the rules for generating function symbols during compilation are inconsistent .

You can see ,MyFunction.s（ The source file is .c file ） As defined in Function, and main.s（ The source file is .cpp file ） What is called is _Z8Functionii, Different function names , So the function implementation cannot be found when connecting . Here we know C and C++ Get different symbols during compilation , therefore C++ Code and C Code cannot call each other .

If you want to achieve C、C++ What about mixed programming ？ Make the compiled function symbols consistent ;

How consistent ？ use extern "C"!

therefore ,extern “C” The purpose of this is to tell C++ compiler , Use the specified function as C Rule compilation （ Be careful , In addition to function overloading ,extern “C” No effect C++ Other features ）, Then the next thing is logical .

aaa.h

#ifdef __cplusplus
extern "C" {
#endif

#ifdef __cplusplus
}
#endif