Re: [PATCH 2/3]: C++20 P0482R6 and C2X N2653: Implement mbrtoc8, c8rtomb, char8_t

[Tom Honermann <tom@honermann.net>]

On 5/18/22 12:17 PM, Adhemerval Zanella wrote:
>
> On 18/05/2022 12:32, Tom Honermann wrote:
>> On 5/17/22 5:33 PM, Florian Weimer wrote:
>>> * Adhemerval Zanella via Libc-alpha:
>>>
>>>> On 17/05/2022 15:12, Joseph Myers wrote:
>>>>> On Tue, 17 May 2022, Adhemerval Zanella wrote:
>>>>>
>>>>>>> +/* This is the private state used if PS is NULL.  */
>>>>>>> +static mbstate_t state;
>>>>>> Although it was done for other convertion interfaces, I wonder if we should
>>>>>> keep supporting this mt-unsafe usage for newer ones.  It was not clear from
>>>>> In C23 it's implementation-defined whether the internal state for such
>>>>> functions has static or thread storage duration (see the general
>>>>> introduction to the uchar.h functions).
>>>>>
>>>> Right, so glibc still need to support either mode.
>>> No, I think we can and should switch.  Maybe with new symbol versions
>>> (but the same implementation) if we want to play it conservative.
>>>
>>> The intent in POSIX and C has been for a long time that thread-local
>>> state is permitted for these functions, only the wording did not
>>> technically allow it.  The phrase was “not required to avoid data
>>> races”.  The problem is that it's possible to tell the difference
>>> without data races (with external synchronization).  Many libcs already
>>> use thread-local state for these functions, without any apparent ill
>>> effects.
>> I would err on the side of maintaining consistency across these functions for now and then transition them all at once if there is a desire to do so.
> Do we need to keep distinct states for each one or can use the a shared
> one?

Each is required to maintain its own state. The relevant wording from 
C17 7.28.1 (Restartable multibyte/wide character conversion functions) 
paragraph 1 states:

These functions have a parameter, ps, of type pointer to mbstate_t that 
points to an object that can completely describe the current conversion 
state of the associated multibyte character sequence, which the 
functions alter as necessary. *If ps is a null pointer, each function 
uses its own internal **mbstate_t**object instead, which is initialized 
at program startup to the initial conversion state; the****functions are 
not required to avoid data races with other calls to the same function 
in this case. The **implementation behaves as if no library function 
calls these functions with a null pointer for ps.*

Tom.