Usage of extern in C Language

In the C language, the extern specifier is used before the declaration of a variable or function to indicate that "this variable/function is defined elsewhere and needs to be referenced here."

1. extern for variable declarations. For example, if file a.c needs to reference the variable int v from b.c, you can declare extern int v in a.c, and then you can reference v. It's important to note that the linkage property of the referenced variable v must be external. This means that for a.c to reference v, it doesn't just depend on declaring extern int v in a.c, but also on v itself being accessible. This touches upon another topic in C language — variable scope. Variables that can be referenced by other modules using the extern specifier are usually global variables. Another very important point is that extern int v can be placed anywhere in a.c. For instance, you can declare extern int v at the beginning of the fun function definition in a.c, and then v can be referenced, though only within the scope of the fun function. This is still a matter of variable scope. Many people have concerns about this, as if extern declarations can only be used at file scope.

2. extern for function declarations. Essentially, there is no difference between variables and functions. A function name is a pointer to the beginning of the function's binary block. If file a.c needs to reference a function from b.c, for example, if its prototype in b.c is int fun(int mu), then you can declare extern int fun(int mu) in a.c, and then use fun for anything. Just like variable declarations, extern int fun(int mu) can be placed anywhere in a.c, not necessarily only within the file scope of a.c. The most common way to reference functions from other modules is to include header files containing their declarations. What's the difference between using extern and including header files to reference functions? The extern method of referencing is much more concise than including header files! The usage of extern is straightforward: declare whichever function you want to reference with extern. This is probably an embodiment of the KISS principle! A clear advantage of doing this is that it speeds up the compilation (more precisely, the pre-processing) process, saving time. In the compilation of large C programs, this difference is very noticeable.

3. Furthermore, the extern specifier can be used to indicate the calling convention for C or C++ functions. For example, when calling C library functions in C++, you need to declare the functions to be referenced using extern "C" in the C++ program. This is for the linker, telling it to use the C function calling convention during linking. The main reason is that C++ and C programs have different naming conventions in the object code after compilation.