如果在函数内部使用extern声明怎么办？

Question

I've tried the following code and got an error.

int main()
{
    //this will cause redefinition error
    extern int x;
    int x=2; 
}

I've seen some answers about extern such as

When to use extern in C++

Defining extern variable in main() vs. globally

and got an concept,but I am still wondering what does the compiler do in this case. Can extern be used(legal) inside some function?

update:

More specifically, since extern int x is just a declaration,why can't I define int x? Does the compiler take extern int x as a definition?

Answer 1

Of course it can be used, don't define another x inside the function:

int main()
{
    extern int x;
    x=2; 
}

Answer 2

but I am still wondering what does the compiler do in this case. Can extern be used(legal) inside some function?

it can, but you must not redeclare variable as you have in your code. So this is a valid example:

int main()
{
    //this will cause redefinition error
    extern int x;
    x=2; 
}

int x; 

Answer 3

As the others have answered, yes, you can use it in your function as long as you don't declare another variable with that name.

To your question of what the compiler does, dreamlax's answer to the question you linked, handles it pretty well. The compiler doesn't need to do/know anything other than what it's type is so that it knows how it can be used. The linker will see that it's an extern and know that it needs to go find where it is actually declared.

This MSDN link provides more general info on externs and what Microsoft does in VS 2015.