Re: [FFmpeg-devel] [PATCH] avformat/dhav: Do not evaluate avio_size() multiple times

[Kieran Kunhya via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On Sat, 7 Jun 2025, 00:12 Michael Niedermayer, <michael@niedermayer.cc>
wrote:

> Code like FFMIN(MAX_DURATION_BUFFER_SIZE, avio_size(s->pb)) is not safe
> as FFMIN() is a macro and avio_size() is thus evaluated multiple
> times
>
> CC: Justin Ruggles <justinr@vimeo.com>
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavformat/dhav.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/libavformat/dhav.c b/libavformat/dhav.c
> index 5a83a8aea9d..c7e5371636a 100644
> --- a/libavformat/dhav.c
> +++ b/libavformat/dhav.c
> @@ -246,11 +246,12 @@ static int64_t get_duration(AVFormatContext *s)
>      int64_t end_buffer_pos;
>      int64_t offset;
>      unsigned date;
> +    int64_t size = avio_size(s->pb);
>
>      if (!s->pb->seekable)
>          return 0;
>
> -    if (start_pos + 16 > avio_size(s->pb))
> +    if (start_pos + 16 > size)
>          return 0;
>
>      avio_skip(s->pb, 16);
> @@ -258,13 +259,13 @@ static int64_t get_duration(AVFormatContext *s)
>      get_timeinfo(date, &timeinfo);
>      start = av_timegm(&timeinfo) * 1000LL;
>
> -    end_buffer_size = FFMIN(MAX_DURATION_BUFFER_SIZE, avio_size(s->pb));
> +    end_buffer_size = FFMIN(MAX_DURATION_BUFFER_SIZE, size);
>      end_buffer = av_malloc(end_buffer_size);
>      if (!end_buffer) {
>          avio_seek(s->pb, start_pos, SEEK_SET);
>          return 0;
>      }
> -    end_buffer_pos = avio_size(s->pb) - end_buffer_size;
> +    end_buffer_pos = size - end_buffer_size;
>      avio_seek(s->pb, end_buffer_pos, SEEK_SET);
>      avio_read(s->pb, end_buffer, end_buffer_size);
>
> --
> 2.49.0
>

Can you explain what "not safe" means?
I assume it means avio_size() causes a seek to the end to get the length
but it's not obvious.

Kieran

>
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