//file1.cpp
#include 

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
}

//file2.cpp
#include 
extern int x;
void func2(void)
{
        x = x + 300;
        cout<<"x in file2 = "<< x << endl;
}

Output:

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

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