Re: [PATCH v8 1/5] Provide counted_by attribute to flexible array member field (PR108896)

[Qing Zhao <qing.zhao@oracle.com>]

> On Apr 10, 2024, at 14:44, Joseph Myers <josmyers@redhat.com> wrote:
> 
> On Wed, 10 Apr 2024, Qing Zhao wrote:
> 
>> A stupid question first, the same scope means the same file? (Or same function)
> 
> struct X { int a; };
> struct X { int a; };
> 
> is an example of the same scope (file scope, in this case).  The 
> structures must have the same contents (in an appropriate sense) and are 
> then considered the same type.
> 
> struct X { int a; };
> void f() { struct X { int a; }; }
> 
> is not the same scope - but C23 makes the types compatible (not the same).  
> It's OK to have incompatible types with the same tag in different scopes 
> as well
> 
> struct X { int a; };
> void f() { struct X { long b; }; }
> 
> but if you use them in a way requiring compatibility, then the contents 
> must be compatible
> 
> struct X { int a; } v;
> void f() { struct X { int a; } *p = &v; }

Okay, the above is very clear, thanks a lot for the explanation.
So, basically, for “counted-by” attribute:
**The following is good:
struct f { 
  int b;
  int c;
  int a[]  __attribute__ ((counted_by (b))) };
struct f {
  int b;
  int c;
  int a[] __attribute__ ((counted_by (b))) };

**The following should error:

struct f { 
  int b;
  int c;
  int a[]  __attribute__ ((counted_by (b))) };
struct f {
  int b;
  int c;
  int a[] __attribute__ ((counted_by (c))) };  /* error here */

For the same tag in different scopes case:

struct f { 
  int b;
  int c;
  int a[]  __attribute__ ((counted_by (b))) }  y0;

void test1(void) 
{   
struct f {
  int b;
  int c;
  int a[] __attribute__ ((counted_by (c))) } x;

  y0 = x;  /* will report incompatible type error here */
}

Are the above complete?

> 
>> Is there a testing case for this feature in current GCC source tree I can take a look? (and
>> Then I can use it to construct the new testing case for the counted-by attribute).
> 
> See gcc.dg/c23-tag-*.c for many tests of different cases involving the tag 
> compatibility rules (and gcc.dg/gnu23-tag-* where GNU extensions are 
> involved).

Got it. Will take a look on them.

thanks.

Qing

> 
> -- 
> Joseph S. Myers
> josmyers@redhat.com
>