How to correctly use the extern keyword in C

Question

My question is about when a function should be referenced with the extern keyword in C.

I am failing to see when this should be used in practice. As I

Answer 1

extern tells the compiler that this data is defined somewhere and will be connected with the linker.

With the help of the responses here and talking to a few friends here is the practical example of a use of extern.

Example 1 - to show a pitfall:

File stdio.h:

int errno;
/* other stuff...*/

---

myCFile1.c:
#include 

Code...

---

myCFile2.c:
#include 

Code...

If myCFile1.o and myCFile2.o are linked, each of the c files have separate copies of errno. This is a problem as the same errno is supposed to be available in all linked files.

Example 2 - The fix.

File stdio.h:

extern int errno;
/* other stuff...*/

---

File stdio.c

int errno;

---

myCFile1.c:
#include 

Code...

---

myCFile2.c:
#include 

Code...

Now if both myCFile1.o and MyCFile2.o are linked by the linker they will both point to the same errno. Thus, solving the implementation with extern.