c++——Use of extern “C” (cpp file calls c file)

1. The concept of extern “C”

extern "C" is a syntax feature provided by C++ for calling and using C language-style functions and variables in C++ code. C++ and C have some different compilation and linking conventions at the bottom level, which will cause the compiled function name of C++ to carry extra information when linking, which does not match the function name of the C code. By using extern "C", we can tell the compiler to process function names and links according to the conventions of the C language, thereby achieving mixed programming of C and C++.

2. Use of extern “C”

① Calling C language functions in C++
Normally, the function name in C++ will be modified by some names to include information such as parameter types. But if we want to call a function written in C language in C++, we need to ensure that the function name and linkage method are compatible with C language.

extern "C" {
    
    
    void cFunction(int a, int b);  // 在 C++ 代码中声明 C 语言风格的函数
}

int main() {
    
    
    cFunction(5, 3);  // 调用 C 语言风格的函数
    return 0;
}

②Use C language variables in C++
Similarly, if we want to use C language global variables in C++, we also need to use extern "C".

extern "C" {
    
    
    int cGlobalVariable;  // 在 C++ 代码中声明 C 语言风格的全局变量
}

int main() {
    
    
    cGlobalVariable = 10;  // 使用 C 语言风格的全局变量
    return 0;
}

③Including C language header files in C++
If you include a C language style header file in C++ code, you need to use extern "C" to ensure correct linking.

extern "C" {
    
    
    #include "c_header.h"  // 包含 C 语言风格的头文件
}

int main() {
    
    
    cFunction(5, 3);  // 调用 C 语言风格的函数
    return 0;
}