Re: [PATCH 2/2] Documentation Update.

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

Okay, thanks all for the comments and suggestions.

Based on the discussion so far, I have the following plan for resolving this issue:

In GCC13:

1. Add documentation in extend.texi to include all the following 3 cases as GCC extension:

Case 1: The structure with a flexible array member is the last field of another
structure, for example:

struct flex  { int length; char data[]; }
struct out_flex { int m; struct flex flex_data; }

In the above, flex_data.data[] is considered as a flexible array too.

Case 2: The structure with a flexible array member is the field of another union, for example:

struct flex1  { int length1; char data1[]; }
struct flex2  { int length2; char data2[]; }
union out_flex { struct flex1 flex_data1; struct flex2 flex_data2; }

In the above, flex_data1.data1[] or flex_data2.data2[] is considered as flexible arrays too.

Case 3: The structure with a flexible array member is the middle field of another
structure, for example:

struct flex  { int length; char data[]; }
struct out_flex { int m; struct flex flex_data; int n; }

In the above, flex_data.data[] is allowed to be extended flexibly to
the padding. E.g, up to 4 elements.

However, relying on space in struct padding is a bad programming practice,  compilers do not 
handle such extension consistently, and any code relying on this behavior should be modified
to ensure that flexible array members only end up at the ends of structures.

Please use warning option -Wgnu-variable-sized-type-not-at-end (to be consistent with CLANG) 
to identify all such cases in the source code and modify them. This extension will be deprecated
from gcc in the next release.

2. Add a new warning option -Wgnu-varaible-sized-type-not-at-end to warn such usage.

In GCC14:

1. Include this new warning -Wgnu-varaible-sized-type-not-at-end to -Wall
2. Deprecate this extension from GCC. (Or delay this to next release?).


Let me know any comments and suggestions?

thanks.

Qing



> On Feb 3, 2023, at 3:55 PM, Joseph Myers <joseph@codesourcery.com> wrote:
> 
> On Thu, 2 Feb 2023, Siddhesh Poyarekar wrote:
> 
>> I dug into this on the glibc end and it looks like this commit:
>> 
>> commit 63fb8f9aa9d19f85599afe4b849b567aefd70a36
>> Author: Zack Weinberg <zackw@panix.com>
>> Date:   Mon Feb 5 14:13:41 2018 -0500
>> 
>>    Post-cleanup 2: minimize _G_config.h.
>> 
>> ripped all of that gunk out.  AFAICT there's no use of struct __gconv_info
>> anywhere else in the code.
>> 
>> I reckon it is safe to say now that glibc no longer needs this misfeature.
> 
> It would be worth testing whether any change warns anywhere else in glibc 
> (not necessarily in installed headers).  And to have fixincludes for the 
> installed _G_config.h from old glibc if we start rejecting such code.
> 
> -- 
> Joseph S. Myers
> joseph@codesourcery.com