Fix C Scope Issues with Extern

In C language, the issue of non-qualified identifiers typically refers to referencing a function or variable outside of its scope. This problem can be resolved by using the following methods:

1. To reference a variable defined in another file within a function, use the extern keyword to declare the external variable. This way, the compiler will recognize that the variable is defined elsewhere and can reference it properly.
2. Put variable or function declarations in the header file: By putting the declarations of variables or functions in the header file and then including this header file in the files that need to reference those variables or functions, you can ensure that the compiler can correctly recognize them when they are referenced.
3. Utilize global or static variables: Define variables that need to be shared between different functions as global or static variables. Global variables are visible throughout the entire program, while static variables are only visible in the file where they are defined.

By using the above methods, the issue of non-qualified identifiers in C language can be effectively resolved, ensuring that the program can correctly reference functions and variables.