Function giving precedence to GLOBAL variable instead of LOCAL - (Anomalous behaviour)

I have:

1. string s = "global" [global scope]
2. string s = "local" [local scope (main function)]

I want function f1() to print out the local s when it is called from main, but the function is printing the global s instead.

#include <iostream>
using namespace std;

//global variables & functions .h

string s = "global";    void f1();

//main begins
int main()
{
    string s = "local";
    f1();

    return 0;
}

//function definitions .cpp

void f1()
{
    cout << s;
}

Output is:

global
Process returned 0 (0x0)   execution time : 0.281 s
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