[Flac-dev] FLAC__metadata_simple_iterator_set_block corrupting Flac (ogg) stream?

[Josh Coalson]

two unfortunate problems:

1. ID3 is illegal on Ogg files; that will probably always be
2. the metadata editing API currently only works for native FLAC,
not Ogg FLAC yet; eventually I will get to fixing that

Josh


--- Marc Wan <marcwanlists at gmail.com> wrote:

> Hey!
> 
>   So, I'm clearly doing something wrong, because I've written a
> program to
> view and modify the comment tags in a FLAC file, which appears to be
> embedded an Ogg stream.  This particular file also has ID3 headers at
> the
> beginning, but I was planning on ignoring those and fixing them up
> using
> some ID3 code later on ...
> 
>   Now, to read the comments is quite simple:
> 
>   FLAC__metadata_get_tags(fileName, &tagData);
>   ...
>   ..
>   .
>   FLAC__metadata_object_delete(tagData);
> 
> 
> 
>   For writing to the file, however, I do something along the
> following lines
> (error code stripped out to keep things shorter), this is pretty much
> as the
> flac documentation suggests:
> 
>         result = FLAC__metadata_simple_iterator_init(iterator,
> fileName,
> FALSE, FALSE);
> 
>         /**
>          * Zip through the file for the vorbis comment section.
>          */
>         while
> (FLAC__metadata_simple_iterator_get_block_type(iterator) !=
> FLAC__METADATA_TYPE_VORBIS_COMMENT)
>         {
>             result = FLAC__metadata_simple_iterator_next(iterator);
>         }
> 
>         /**
>          * Get the current block and write out the new values for it.
>          */
>         metaData =
> FLAC__metadata_simple_iterator_get_block(iterator);
> 
>         /**
>          * Save the strings out.
>          */
>         [self saveCommentsToIterator: metaData];
> 
>         /**
>          * Now, save out the block.
>          */
>         result = FLAC__metadata_simple_iterator_set_block(iterator,
> metaData, TRUE);
> 
>         FLAC__metadata_simple_iterator_delete(metaData);
> 
> 
>    the saveCommentsToIterator method just updates the "entry" and
> "length"
> fields of those comment entries in the vorbis_comments that have
> changed.
> (is that perhaps not correct?) .... hexdumping the file after writing
> seems
> to show a new comment block with the appropriate new values in it ...
> 
>   The problem, which the docs hinted at:
> 
>   - if the comments are shorter or not much longer:  everything works
> great
>   - if the comments are much longer:  the file appears to be
> rewritten.
> hexdump -C shows that the ID3 header is destroyed (not a problem,
> really)
> and the file now has a 'fLaC' binary signature at the top.  (again,
> no
> problem -- other FLAC files appear to have this).
> 
>   This new file, however, cannot be read in by the above code using
> FLAC__metadata_get_tags.  It barfs internally while trying to read
> some
> metadata chunks from the file (not the first one, either).
> 
>   Any idea why the newly written file cannot be read by my program
> any
> more?  Programs that play audio, such as VLC, can still play the FLAC
> file
> just fine ....
> 
>   Any pointers to what I'm doing wrong would be greatly appreciated.
> 
>   Thanks,
>   marc.
> > _______________________________________________
> Flac-dev mailing list
> Flac-dev at xiph.org
> http://lists.xiph.org/mailman/listinfo/flac-dev
> 


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