What is the use of extern in c/c++?

The extern keyword is used to tell the compiler that a data object is declared in a different *.cpp or *.c file. So there wont be any new memory is allocated for these variables. Check the below example and understand the working.

#include <iostream.h>
int x;
extern void func2();
void func1(void)
        x = x + 100;
        cout<<"x in func1 = "<< x << endl;
int main(void)
        x = 10;
        cout<<"x in main function = "<< x << endl;
        func1(); // calls func1 
        func2(); // calls func2 from file2
#include <iostream.h>
extern int x;
void func2(void)
        x = x + 300;
        cout<<"x in file2 = "<< x << endl;


x in main function = 10
x in func1 = 110
x in file2 = 410

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