declarations, definitions and externals

I've discussed this somewhat in the C++ part of the forum as well, but I want to discuss the subject further in C, so I made a thread here.

The question is as simple as this:
If I want to share a global variable over 2 source files, could I just include this headerfile in both my files?

Code:

#ifndef MOFF
    #define MOFF
    int test; // Global variable to be used in 2 files
#endif

I have compiled it, and it works.
The question is, is it legal, and more over, is it considered ok as a programming practice?

I guess part of the question is, is this a declaration or definition?

Code:

int test;

I tried to compile the following program:

Code:

int test;
int test=50;
int test;


int main(int argc, char *argv[]) {
  printf("%d\n", test);
  system("PAUSE");	
  return 0;
}

And this compiled, linked and ran..and gave me output "50".

Tentative Definitions

Thanks. That helps a lot.

And now I have to ask, why in the lords name does C allow this kind of code?
It looks straight out messy.(yeah, I come from C++)