Re: [FFmpeg-devel] [PATCH v2 1/2] avformat: add avformat_query_seekable

[Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org>]

> On Apr 15, 2025, at 18:39, Gyan Doshi <ffmpeg@gyani.pro> wrote:
> 
> Utility function to report seekability features for a given input.
> 
> Useful for ffprobe and to extend seek possibilities in fftools.
> ---
> v2:
>   made constants more descriptive
>   add exception for rtsp false negative seekability
> 
> doc/APIchanges         |  3 +++
> libavformat/avformat.h | 22 ++++++++++++++++++
> libavformat/seek.c     | 53 ++++++++++++++++++++++++++++++++++++++++++
> libavformat/version.h  |  2 +-
> 4 files changed, 79 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/APIchanges b/doc/APIchanges
> index 65bf5a9419..879f56b572 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -2,6 +2,9 @@ The last version increases of all libraries were on 2025-03-28
> 
> API changes, most recent first:
> 
> +2025-04-xx - xxxxxxxxxx - lavf 62.1.100 - avformat.h
> +  Add avformat_query_seekable().
> +
> 2025-04-07 - 19e9a203b7 - lavu 60.01.100 - dict.h
>   Add AV_DICT_DEDUP.
> 
> diff --git a/libavformat/avformat.h b/libavformat/avformat.h
> index 498c3020a5..f9da5e9e79 100644
> --- a/libavformat/avformat.h
> +++ b/libavformat/avformat.h
> @@ -2338,6 +2338,28 @@ int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp,
>  */
> int avformat_seek_file(AVFormatContext *s, int stream_index, int64_t min_ts, int64_t ts, int64_t max_ts, int flags);
> 
> +#define AVSEEKABLE_IO_NORMAL       0x00000001 ///< I/O is seekable like a local file
> +#define AVSEEKABLE_IO_PROTOCOL     0x00000002 ///< I/O seek is through protocol request via avio_seek_time
> +#define AVSEEKABLE_VIA_DEMUXER     0x00000004 ///< demuxer has a seek function
> +#define AVSEEKABLE_VIA_PKTSCAN     0x00000008 ///< seek is performed by consuming and scanning packet timestamps
> +#define AVSEEKABLE_BY_TIME         0x00000100 ///< seek target can be a timestamp
> +#define AVSEEKABLE_BY_BYTE         0x00000200 ///< seek target can be in bytes
> +#define AVSEEKABLE_BY_FRAME        0x00000400 ///< seek target can be a frame index
> +#define AVSEEKABLE_PROP_PTS        0x00010000 ///< seek target timestamp is expected to be PTS
> +#define AVSEEKABLE_PROP_FAST       0x00020000 ///< demuxer allows fast but inaccurate seeking
> +#define AVSEEKABLE_PROP_FWDONLY    0x00040000 ///< set seek will be equal or forward of specified seek point
> +
> +/**
> + * Report if and how a seek can be performed in a given input.
> + *
> + * @param s            media file handle
> + *
> + * @return 0 if no seek can be performed on input,
> + *         -1 if the fmt ctx is NULL or is not an input
> + *         else return AVSEEKABLE_ bitflags indicating seekability features.
> + */
> +int avformat_query_seekable(AVFormatContext *s);
> +
> /**
>  * Discard all internally buffered data. This can be useful when dealing with
>  * discontinuities in the byte stream. Generally works only with formats that
> diff --git a/libavformat/seek.c b/libavformat/seek.c
> index c0d94371e6..8be1bdec30 100644
> --- a/libavformat/seek.c
> +++ b/libavformat/seek.c
> @@ -712,6 +712,59 @@ int avformat_seek_file(AVFormatContext *s, int stream_index, int64_t min_ts,
>     return ret;
> }
> 
> +int avformat_query_seekable(AVFormatContext *s)
> +{
> +    int ret = 0;
> +
> +    if (!s || !s->iformat)
> +        return -1;
> +
> +    if ( strcmp(s->iformat->name, "rtsp") && (!(s->pb && s->pb->seekable) || s->ctx_flags & AVFMTCTX_UNSEEKABLE) )
> +        return 0;

dvdvideo is also seekable. How about check AVFMT_NOFILE flag and seek method in FFInputFormat.

For rtsp, I think there is no way to query whether it’s really seeable or not until try to do that, at least no such information is exported for now.

> +
> +    if (s->pb->seekable & AVIO_SEEKABLE_NORMAL)
> +        ret |= AVSEEKABLE_IO_NORMAL;
> +
> +    if (s->pb->seekable & AVIO_SEEKABLE_TIME)
> +        ret |= AVSEEKABLE_IO_PROTOCOL;
> +
> +    if (ffifmt(s->iformat)->read_seek || ffifmt(s->iformat)->read_seek2)
> +        ret |= AVSEEKABLE_VIA_DEMUXER;
> +
> +    if (ffifmt(s->iformat)->read_timestamp && !(s->iformat->flags & AVFMT_NOBINSEARCH))
> +        ret |= AVSEEKABLE_VIA_PKTSCAN;
> +
> +    if (!(s->iformat->flags & AVFMT_NOTIMESTAMPS))
> +        ret |= AVSEEKABLE_BY_TIME;
> +
> +    // TODO: incomplete, a few demuxers don't set flag but error out on byte seek
> +    if (!(s->iformat->flags & AVFMT_NO_BYTE_SEEK))
> +        ret |= AVSEEKABLE_BY_BYTE;
> +
> +    // TODO: no flag for frame seeking. Add flag and enable this check
> +    if (s->iformat->flags & 0)
> +        ret |= AVSEEKABLE_BY_FRAME;
> +
> +    if (s->iformat->flags & AVFMT_SEEK_TO_PTS)
> +        ret |= AVSEEKABLE_PROP_PTS;
> +
> +    // TODO: flag exists but not added to the demuxers which support it
> +    if (s->iformat->flags & AVFMT_FLAG_FAST_SEEK)
> +        ret |= AVSEEKABLE_PROP_FAST;
> +
> +    if (!(ret & AVSEEKABLE_VIA_DEMUXER) && ret & AVSEEKABLE_VIA_PKTSCAN)
> +        ret |= AVSEEKABLE_PROP_FWDONLY;
> +
> +    if ( !(ret & AVSEEKABLE_VIA_DEMUXER) &&
> +         !(ret & AVSEEKABLE_VIA_PKTSCAN) &&
> +         !(ret & AVSEEKABLE_BY_BYTE) &&
> +         !(ret & AVSEEKABLE_IO_PROTOCOL) &&
> +         (s->iformat->flags & AVFMT_NOGENSEARCH) )
> +        ret = 0;
> +
> +    return ret;
> +}
> +
> /** Flush the frame reader. */
> void ff_read_frame_flush(AVFormatContext *s)
> {
> diff --git a/libavformat/version.h b/libavformat/version.h
> index 752aac16f7..a7c80dc564 100644
> --- a/libavformat/version.h
> +++ b/libavformat/version.h
> @@ -31,7 +31,7 @@
> 
> #include "version_major.h"
> 
> -#define LIBAVFORMAT_VERSION_MINOR   0
> +#define LIBAVFORMAT_VERSION_MINOR   1
> #define LIBAVFORMAT_VERSION_MICRO 100
> 
> #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \
> -- 
> 2.49.0
> 
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