Re: Struct Error

On 1/23/2025 4:54 AM, bart wrote:

On 23/01/2025 01:05, James Kuyper wrote:

On 2025-01-22, bart <bc@freeuk.com> wrote:

Gcc 14.1 gives me an error compiling this code:
>
    struct vector;
    struct scenet;
>
    struct vector {
        double x;
        double y;
        double z;
    };
>
    struct scenet {
        struct vector center;
        double radius;
        struct scenet (*child)[];
    };

>
6.7.6.2p2: "The element type shall not be an incomplete or function
type."
>
I have many draft versions of the C standard. n2912.pdf, dated
2022-06-08, says in 6.7.2.1.p3 about struct types that "... the type is
incomplete144) until immediately after the closing brace of the list
defining the content, and complete thereafter."
>
Therefore, struct scenet is not a complete type until the closing brace
of it's declaration.

 
Wouldn't this also be the case here:
 
    struct scenet *child;
   };
 
The struct is incomplete, but it still knows how to do pointer
arithmetic with that member. The calculation is not that different from
the array version (actually, the code from my compiler is identical).

The difference is that there's an explicit requirement that the element
type of an array type be complete. As far as I know, there's no such
requirement that applies when you have a pointer to incomplete type,
rather than an array of an incomplete type. If you think otherwise,
please identify the requirement. I started reviewing all the places
where the standard says something about complete and incomplete types,
but there's way too many of them.

The reason, I think, is the following:
"A pointer to void shall have the same representation and alignment
requirements as a pointer to a character type.53) Similarly, pointers to
qualified or unqualified versions of compatible types shall have the
same representation and alignment requirements. All pointers to
structure types shall have the same representation and alignment
requirements as each other. All pointers to union types shall have the
same representation and alignment requirements as each other. Pointers
to other types need not have the same representation or alignment
requirements." (6.2.5p33)

This means that, in principle, the representation and alignment of a
pointer to an array of an incomplete struct type might depend upon the
unknown content of that struct type, if only through the size of the
type. So a pointer to an array of an incomplete type presents a possible
challenge that isn't a problem for a pointer to an object of that
incomplete type.
I'm sure that you're used to platforms where all pointers to object
types have the same representation and alignment requirements - most
developers are. However, there are very real platforms where that isn't
the case, and the C standard goes out of its way to permit conforming
implementations of C on such platforms.