[FFmpeg-devel] [PATCH v3 2/3] avormat/av1dec: add low-overhead bitstream format

[Xu, Guangxin]

Hi Martin, 
Thanks for your detailed explaining. 
It's a good education to me. 

thanks

> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-bounces at ffmpeg.org> On Behalf Of Martin
> Storsjö
> Sent: Friday, August 14, 2020 3:25 AM
> To: FFmpeg development discussions and patches <ffmpeg-devel at ffmpeg.org>
> Subject: Re: [FFmpeg-devel] [PATCH v3 2/3] avormat/av1dec: add low-
> overhead bitstream format
> 
> On Thu, 13 Aug 2020, James Almer wrote:
> 
> > On 8/13/2020 3:51 AM, Xu Guangxin wrote:
> 
> >> +
> >> +    ret = obu_prefetch(s, header, MAX_OBU_HEADER_SIZE);
> >> +    if (!ret)
> >> +        return AVERROR(EOF);
> >
> > We use AVERROR_EOF rather than AVERROR(EOF) (Afair, it was done
> > because EOF is not portable, but don't quote me on it).
> 
> Actually, it's a more severe thing than that.
> 
> AVERROR() is used for mapping an errno style error code, EINVAL etc, which can
> be either positive or negative numbers depending on platforms, to the AVERROR
> range (which is negative numbers).
> 
> So AVERROR() actually is shorthand for ERRNO_CODE_TO_AVERROR(). On
> platforms where errno codes are positive (most common modern platforms
> except BeOS/Haiku, iirc), it's essentially defined as (-(x)).
> 
> Now EOF isn't an errno error code, and is defined to be a negative value (often -
> 1). So if you do AVERROR(EOF) on a system where errno codes are positive,
> AVERROR(EOF) evalues to +1, which most caller would identify as not an error
> at all.
> 
> TL;DR: AVERROR(EOF) is never correct.
> 
> // Martin
> 
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