[FFmpeg-devel] [PATCH 25/35] avutil/opt: add av_opt_to_string

[Diederick C. Niehorster]

On Thu, Jun 10, 2021 at 11:39 AM Diederick C. Niehorster
<dcnieho at gmail.com> wrote:
> So in av_opt_get(), I'd do something like this:
> AVBPrint buf;
> av_bprint_init(&buf, 0, AV_BPRINT_SIZE_AUTOMATIC);
> // 1. call internal print function, with &buf
> // ...
> // 2. provide output to caller
> if (!av_bprint_is_complete(&buf))
> {
>     av_bprint_finalize(&buf,NULL);
>     return AVERROR(ENOMEM);
> }
> ret = av_bprint_finalize(&buf,out_val);
> return ret<0 ? ret : 0;
>
> I'll experiment with that.

Ok, i have experimented with this, and hit some problems quickly.
There are at least the following issues:
1. how to deal with AV_OPT_ALLOW_NULL in av_opt_get, if its guts use a
move over to AVBPrint? leaving AVBPrint empty is not the same as a
null, and does not signal null.
2. some cases forward to other functions, most importantly
AV_OPT_TYPE_DICT, which uses av_dict_get_string. These do not play
with AVBPrint, but just plain char** or similar. Should i use a local
buffer in that case and then copy/print its contents into the
AVBPrint? That seems to not improve the situation.
This is assuming we'd want to reuse the guts of av_opt_get for this
new function, which i think we would. Not doing so would mean a lot of
code duplication.

And there is also that this would be the only av_opt_* function
returning output through a AVBPrint instead of plain arrays. Is that
inconsistency in the API worth usually avoiding a dynamic allocation
in code that'll not be used in a critical path?

Cheers,
Dee