glibc strerrorname_np

[Adhemerval Zanella]

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).