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

[Andrew Sayers <ffmpeg-devel@pileofstuff.org>]

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.
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