-fno-common flag for GCC

Functions and initialized global variables get strong symbols. Uninitialized global variables get weak symbols

Rule 1: Multiple strong symbols (with same variable name) are not allowed.
Rule 2: Given a strong symbol and multiple weak symbols, choose the strong symbol.
Rule 3: Given multiple weak symbols, choose any of the weak symbols.

/*a.c*/
#include stdio.h
void b(void);

int x;
int main()
{
    x = 2016;
    b();
    printf("x = %d ",x);
    return 0;
}

/*b.c*/
#include stdio.h

int x;

void b()
{
    x = 2017;
}