[PATCH 2/2] Documentation Update.

[Richard Biener]

On Wed, 1 Feb 2023, Siddhesh Poyarekar wrote:

> On 2023-02-01 13:24, Qing Zhao wrote:
> > 
> > 
> >> On Feb 1, 2023, at 11:55 AM, Siddhesh Poyarekar <siddhesh@gotplt.org>
> >> wrote:
> >>
> >> On 2023-01-31 09:11, Qing Zhao wrote:
> >>> Update documentation to clarify a GCC extension on structure with
> >>> flexible array member being nested in another structure.
> >>> gcc/ChangeLog:
> >>>  * doc/extend.texi: Document GCC extension on a structure containing
> >>>  a flexible array member to be a member of another structure.
> >>
> >> Should this resolve pr#77650 since the proposed action there appears to be
> >> to document these semantics?
> > 
> > My understanding of pr77650 is specifically for documentation on the
> > following case:
> > 
> > The structure with a flexible array member is the middle field of another
> > structure.
> > 
> > Which I added in the documentation as the 2nd situation.
> > However, I am still not very comfortable on my current clarification on this
> > situation: how should we document on
> > the expected gcc behavior to handle such situation?
> 
> I reckon wording that dissuades programmers from using this might be
> appropriate, i.e. don't rely on this and if you already have such nested flex
> arrays, change code to remove them.
> 
> >>> +In the above, @code{flex_data.data[]} is allowed to be extended flexibly
> >>> to
> >>> +the padding. E.g, up to 4 elements.
> 
> """
> ... Relying on space in struct padding is bad programming practice and any
> code relying on this behaviour should be modified to ensure that flexible
> array members only end up at the ends of arrays.  The `-pedantic` flag should
> help identify such uses.
> """
> 
> Although -pedantic will also flag on flex arrays nested in structs even if
> they're at the end of the parent struct, so my suggestion on the warning is
> not really perfect.

Wow, so I checked and we indeed accept

struct X { int n; int data[]; };
struct Y { struct X x; int end; };

and -pedantic says

t.c:2:21: warning: invalid use of structure with flexible array member 
[-Wpedantic]
    2 | struct Y { struct X x; int end; };
      |           

and clang reports

t.c:2:21: warning: field 'x' with variable sized type 'struct X' not at 
the end of a struct or class is a GNU extension 
[-Wgnu-variable-sized-type-not-at-end]
struct Y { struct X x; int end; };
                    ^

looking at PR77650 what seems missing there is the semantics of this
extension as expected/required by the glibc use.  comment#5 seems
to suggest that for my example above its expected that
Y.x.data[0] aliases Y.end?!  There must be a better way to write
the glibc code and IMHO it would be best to deprecate this extension.
Definitely the middle-end wouldn't consider this aliasing for
my example - maybe it "works" when wrapped inside a union but
then for sure only when the union is visible in all accesses ...

typedef union
{
  struct __gconv_info __cd;
  struct
  {
    struct __gconv_info __cd;
    struct __gconv_step_data __data;
  } __combined;
} _G_iconv_t;

could be written as

typedef union
{
  struct __gconv_info __cd;
  char __dummy[sizeof(struct __gconv_info) + sizeof(struct 
__gconv_step_data)];
} _G_iconv_t;

in case the intent is to provide a complete type with space for
a single __gconv_step_data.

Richard.