flac dev - Oct 2024 - C API: How to get a seektable for very long files?

Hello,

I'm using flac to compress s signal data during capture. The sample rate
is almost a thousands time higher compared to audio (40 MHz), resulting
in a lot of data very quickly.
I'm using the C API in my capturing application (mostly copy&paste
directly from the example) and it works so far, but unfortunately for
longer captures there is no seeking information. How can I tell flac to
write seeking information? I assume it's not reserving enough space for
the seek table (as a 2 hour capture at 40 MHz is almost 3 month at 40kHz
(flac samplerate field is limited, so I set 40kHz)) and it won't rewrite
a file that is several hundred GBs.
I cannot find any clues in the documentation how to write a
seektable/reserve space for it.
Can someone help me out?

Best regards
Stefan

Op zo 13 okt 2024 om 02:16 schreef Stefan Oltmanns <stefan-oltmanns at
gmx.net>:
>
> I cannot find any clues in the documentation how to write a
> seektable/reserve space for it.
> Can someone help me out?
>
There's actually quite a lot of documentation for this.

Please review
https://xiph.org/flac/api/group__flac__stream__encoder.html#ga80d57f9069e354cbf1a15a3e3ad9ca78

I quote:
> SEEKTABLE blocks are handled specially. Since you will not know the
> values for the seek point stream offsets, you should pass in a
> SEEKTABLE 'template', that is, a SEEKTABLE object with the required
> sample numbers (or placeholder points), with 0 for the frame_samples
> and stream_offset fields for each point. If the client has specified
> that it supports seeking by providing a seek callback to
> FLAC__stream_encoder_init_stream() or both seek AND read callback to
> FLAC__stream_encoder_init_ogg_stream() (or by using
> FLAC__stream_encoder_init*_file() or
> FLAC__stream_encoder_init*_FILE()), then while it is encoding the
> encoder will fill the stream offsets in for you and when encoding is
> finished, it will seek back and write the real values into the
> SEEKTABLE block in the stream. There are helper routines for
> manipulating seektable template blocks; see metadata.h:
> FLAC__metadata_object_seektable_template_*(). If the client does not
> support seeking, the SEEKTABLE will have inaccurate offsets which will
> slow down or remove the ability to seek in the FLAC stream.

Also, take a look at this:
https://xiph.org/flac/api/group__flac__metadata__object.html#gab91c8b020a1da37d7524051ae82328cb

Hope that helps.

Also, as I'm always extra curious when FLAC is used for non-audio
purposes: could you perhaps say a little bit about what kind of
signals you're compressing?

Kind regards,

Martijn van Beurden