extern arrays in c without size

Question

Not compiling:

int a[];

Compiling:

extern int a[];

Not compiling (it can't find sizeof):

printf("%lu\n", sizeof(a)/sizeof(int));

So questions:

1. why it is possible to create extern array.
2. how much memory allocated?

The reason of question - http://users.dcc.uchile.cl/~rbaeza/handbook/algs/4/444.sort.c

extern int maxfiles, maxruns[], actruns[];
for (i=0; i<=maxfiles; i++) maxruns[i] = actruns[i] = 0;

Is it correct?

Answer 1

1.) You have not created an extern array. Rather, you have declared that an array exists, and it was created somewhere else (externally). And where it was created, it does have a specific size.

2.) Because you have not created an array, no size is allocated for it. (see #1). Rather, the array was created in another file/module/component (externally), and this is a declaration that the array exists, and may be accessed here.