Code that used to compile will not compile in Fedora.
it gives error: 'array type has incomplete element type' on the first line in file A.h.
I've understood why, but still I have no workaround.
How can I fix the following code with minimum changes?

file: A.c
#include B.h
#include A.h

struct B a[3];
...

file A.h
extern struct B a[3];
...

HELP PLEASE!

Tx,
Dudu Arbel

It sounds like 'struct B' is not defined anywhere - did you define it in b.h?

Yes - B is a simple struct defined at B.h

Do you really need the extern array in A.h ? If not just leave it in the c file and remove it from the header file. Otherwise, you're going to need knowledge about B inside the A.h file (include or forward declaration).

I want to use 'a' variable inside file A.h, hence I must have some kind of extern to it, no?
Adding forward declaration does not help:

file A.h
struct B;
extern struct B a[3];
...

Try including the file B.h in A.h

Sorry to say but you're missing the point.
The new gcc compiler refuses to compile lines like:
extern struct B a[3];

I have found lots of information and explanations why this line is illegal, and I'm cool with that.

My question is: How can I change this code, so it will compile?
One possible solution is to change the definition of a:
struct B *a = new B[3];

extern B *a; // This is ok

The above code is legal, but this change is unexceptable.
Any other ideas?

The problem is that the type of the structure is not known and therefore a forward declaration of the type will not work unless the instance is a reference, in C a pointer and in C++ also a reference.

"The above code is legal, but this change is unexceptable." If you can not include the definition of the type in the header the normal path to take is to use the pimpl idiom or a reference. The fault maybe from your design if this is your code, I do not know it is but will assume so and assume C++ as you are using new.
Why does your design require a global instance of an array?
Why an array and not a vector?
Why not local or in a namespace other than global?
Why does the definition need to be made available to header which can not use it or operate on the type?

How new? This seems to work fine on my version of gcc. As long as you include B.h before declaring the array I don't see any reason why it shouldn't work. In this code it's done by including B.h before A.h in the c file which is probably not a great idea, but the compiler doesn't care.