[Flac-dev] [BUG] --sector-align zero padding is not entirely zero

Dave Chapman dave at dchapman.com
Mon Jul 11 16:09:23 PDT 2005


A user at the www.thetradersden.org website reported a difference when
fixing "sector boundary errors" in CD-quality WAV files with shntool and
flac.  As as result of investigating this, I think I've found a bug in
the zero-ising of the buffer used to zero-pad the last file when
encoding with the --sector-align option.

My test consisted of the creation of four "non-aligned" CD quality WAV
files.  I then "fixed" these files using both the "shntool fix" command
and the "flac --sector-align" command.

I then compared the WAV files created by shntool and the WAV files
created by uncompressing the FLAC files, and the MD5 checksums did not
match for the final track.

On comparing the contents of the WAV files, the "shntool" fixed version
correctly contained 448 zero bytes at the end (112 samples).  However,
the FLAC version contained 224 zero bytes, followed by 224 non-zero bytes.

I've tracked the bug to the calculation of the number of bytes to fill
the input_ buffer with, which is calculated in line 778 of encode.c as

data_bytes = wide_samples * (bps >> 3);

However, my understanding is that the input_buffer is always an array of
32-bit integers, and therefore the calculation should always be:

data_bytes = wide_samples * 4;

The expression (bps >> 3) is also used elsewhere in encode.c so you may
wish to examine those instances as well for similar bugs.

Best wishes, and thanks for FLAC.


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