Forward referencing with typedef

[David Brown]

On 09/12/2011 08:26, Divyesh wrote:
>
> Hi,
>
> Please refer to the following code snippet.
>
> struct strArr;
>
> typedef struct strArr myStAr[17];
>
> typedef struct
> {
> 	unsigned short DISPLAY_:1;
> } structDisp;
>
> typedef structDisp disp;
>
> struct strArr
> {
> 	disp dispBit;
> } ;
>
> int f()
> {
> 	return 0;
> }
>
>
> In the code we have a typedef to an array of struct.
>
> Now since the definition of that struct is given after the use, gcc cribs.
>
> The error reported by gcc is : array type has incomplete element type.
>
> Interesting thing is gcc 3.4.4 compiles the code where as gcc 4.3.4 and 4.6
> doesn't.
>
> I think this is a gcc bug.
>
> Request assistance for the same.
>
> Thanks and Regards,
> Divyesh

It is not a bug.  It is interesting that gcc 3.4.4 accepted the code, as 
it is (AFAIK) invalid C.

It is easy to see what is going on when you think about how the compiler 
will interpret this - starting from the top and moving down.  The usual 
use of forward struct declarations is so that you can make pointers to 
them in a circular type definition - that works fine, because the 
compiler can conceive of a "pointer to struct" without knowing the 
details of the struct.  But the compiler cannot work with an instance of 
the forward-declared struct, or an array of them, or use it as an 
element in another struct (only pointers are allowed) - all these things 
require a knowledge of the size of the struct.