I have got 2 cpp files & a header file, which I have included in both cpp files. It's like this:

abc.h

extern uint32_t key;

a.cpp

#include "abc.h"
uint32_t key;
int main
{
.............
}

b.cpp #include "abc.h"

int main
{
printf("Key: %.8x\n", key);
.............
}

Now when I compile a.cpp, there is no error. but when i compile b.cpp it gives error "undefined reference to `key'". Please help me in finding the problem in this code.