Re: glibc strerrorname_np

["Zack Weinberg" <zack@owlfolio.org>]

On Mon, Nov 8, 2021, at 8:56 AM, Adhemerval Zanella wrote:
> On 07/11/2021 14:37, Zack Weinberg via Libc-alpha wrote:
>> On Sat, Nov 6, 2021, at 8:51 AM, Adhemerval Zanella via Libc-alpha wrote:
>>> On 05/11/2021 19:23, Jonny Grant wrote:
>>>> On 05/11/2021 13:01, Adhemerval Zanella wrote:
>>>>> On 05/11/2021 08:51, Jonny Grant wrote:
>>>>>> Thank you for your reply. Personally I understood an ABI break would be
>>>>>> the return type, the name, or the parameters. But the proposed change is
>>>>>> not so. Changing to return a string, should be fine.
>>>>>
>>>>> It is still an ABI break, code that checks NULL for invalid input will
>>>>> stop to work.
>> ...
>>> As I said before it is an ABI break, since users that check for invalid 
>>> errno against NULL will start to fail.  For such change we *do need* all 
>>> the trouble of adding a compat symbol with current semantic.
>> 
>> A compat symbol doesn't do any good here, though.  As soon as the program is recompiled it will start getting the new semantics, and since that doesn't cause a compile-time error, the break will go unnoticed -- particularly in this case, since this is likely to affect only error-handling paths that test suites tend not to exercise.
>> 
>> If we want to preserve backward compatibility we need a whole new function name.
>
> This is the burden of any semantic change on exported symbols: new users
> will need to adapt to it, since the idea is to keep older programs 
> expecting the new semantic (since either rebuilding is not possible or
> troublesome).

I think you misunderstand.  I'm saying that a compat symbol is *not enough* backward compatibility, since it only protects old binaries, not old sources that have been recompiled with a newer libc.  Compat symbols, in my view, are only sufficient when either a recompilation will fix whatever the problem was (e.g. changing the size of a FILE) or when old code will not *compile* until corrected for the new semantics.

I know we have done compat symbols as the only backward compatibility net for runtime-only semantic changes in the past, but I think that was wrong and we shouldn't do it anymore.

zw