External Delaration for An Array?

Question

I have an array defined in a file and in another I have to use it, for e.g-

/* a.c - defines an array */  int a[] = {1,2,3,4,5,6,7,8,9};    /* b.c - declare and use it. */  #define COUNT ((sizeof a)/(sizeof int)) extern int a[];  //size of array  . . .  int i; for(i=0; i<COUNT; i++)   printf("%d", a[i]); . . . 

Now when I try to compile it it gave me error saying that sizeof cann't be used on incomplete type.

Can anybody tell me how to handle such case in C/C++? I don't want to array subscript in a.c

Thanks in advance

Answer 1

You might put something like the following into a.c and then extern it from b.c.

In a.c:

int a[] = {1, 2, 3}; const int lengthofa = sizeof( a ) / sizeof( a[0] ); 

And then in b.c:

extern int a[]; // the extern (thanks Tim Post) declaration means the actual storage is in another  // module and fixed up at link time.  The const (thanks Jens Gustedt) prevents it // from being modified at runtime (and thus rendering it incorrect). extern const int lengthofa;  void somefunc() {   int i;   for ( i = 0; i < lengthofa; i++ )     printf( "%d\n", a[i] ); } 

Answer 2

If you want your array size to be accessible as a compile-time constant, then you have no other choice but to specify array size explicitly in the extern declaration of the array

extern int a[9]; 

In this case it becomes your responsibility to make sure that array size is consistent between the extern declaration and definition. You can use a manifest constant for that, but still it is going to be your responsibility to make sure that the number of initializers between the {} and the declared size are the same.

If you don't care to have the array size as a compile-time constant, then you can do what Mark Wilkins suggests in his answer.