Defining an extern variable in multiple files in C

0.c

extern int num;

int main(){
 return num;
}

1.c

int num;

2.c

int num = 5;

Above compiles fine with gcc 0.c 1.c 2.c. Why don't I get a multiple definition error during linking?

Isn't int num; under 1.c a full definition?:

tentative definition becomes a full definition if the end of the translation unit is reached and no definition has appeared with an initializer for the identifier.

Or is this undefined behaviour?