C Programming: Understanding the 'extern' Keyword - Usage, Examples, and Benefits

'extern' is a keyword in C used to declare a variable or function defined in another source file or module. It informs the compiler that the variable or function is defined elsewhere in the program, preventing multiple definitions.

To use 'extern', declare the variable or function in a header file or at the top of a source file using the 'extern' keyword followed by the data type and name. For example:

extern int my_variable;
extern void my_function();

In another source file where you want to use the variable or function, simply include the header file or declare the variable or function again using 'extern':

#include "my_header.h"

int main() {
    extern int my_variable;
    my_variable = 10;

    extern void my_function();
    my_function();
    return 0;
}

Use 'extern' when variables or functions are defined in one source file and used in another. This separates code into smaller modules, avoids code duplication, shares variables/functions across multiple source files, and facilitates library creation.