[FFmpeg-devel] [PATCH] lavu/opt: Mention that AVOptions is not reentrant

[Andreas Rheinhardt]

Andrew Sayers:
> On Thu, Jun 06, 2024 at 04:24:11PM +0200, Michael Niedermayer wrote:
>> On Thu, Jun 06, 2024 at 09:29:24AM +0100, Andrew Sayers wrote:
>>> On Thu, Jun 06, 2024 at 01:17:48AM +0200, Michael Niedermayer wrote:
>>> [...]
>>>> AVOption simply provides light weight access to the struct fields.
>>>> Calling AVOption non re-entrant in modifying a field you arent even allowed
>>>> to modify from 2 threads is confusing
>>>
>>> I think you're saying there's already a rule about modifying AVOptions from
>>> 2 threads.  Could you explain that in more detail?
>>
>> Well, one way this can be argued is this:
>> Latest C draft: (but i doubt this is different) ISO/IEC 9899:2017   C17 ballot N2176
>>
>> "Two expression evaluations conflict if one of them modifies a memory location and the other one
>>  reads or modifies the same memory location"
>>
>> so if you have 2 threads and one writes into a int and another reads it at the
>> same time, the code is broken.
>> The code doing said act through some API doesnt become less broken
>>
>> Calling AVOption non re-rentrant because of that is false thats as if one executed
>> strtok_r(a,b,c) with the VERY same a,b,c from 2 threads and then said
>> its not thread safe
>>
>> strtok_r() is thread safe and reentrant if its used correctly, so is AVOption
> [...]
> 
> Ok, how about if the patch avoided the word "reentrant" and just said:
> 
> + * Note: AVOptions values should not be modified after a struct is initialized.

This is wrong either. As Paul has already pointed out to you, some
options are allowed to be modified lateron.

- Andreas