Why is creating a variable using 'extern' a declaration and not a definition?

Question

I came across the following problem while reading ...just cant get the logic behind this.

auto int c;
static int c;
register int c;
extern int c;

Answer 1

The last one with extern does not define storage for c. It merely indicates that c exists somewhere and the linker should be able to resolve it to some global c defined elsewhere.

If you compiled and linked a single .c file and tried to use the last c you'd have a linker error. With the the first 3 cs you would not as they have substance (they've been defined) in the current compilation unit.

If you'd like to learn more about extern and declaration vs definition here's a good article on the topic. To quote from that article:

Declaration of a variable/function simply declares that the variable/function exists somewhere in the program but the memory is not allocated for them