Metadata

[IdfbAn]

I'm looking at the TODO, and I'm a bit confused on what some stuff means.

First, by "metadata", I assume you mean stuff like EXIF? If I download this, for example, and then compress it, the EXIF is gone. Again, I assume this is what you mean? CMIIW, though.

Second, this:

When both metadata and data are wanted in the same .xz file, two or more Streams would be concatenated.

Going back to my image example, if an .xz file has just one Stream, does that mean it's only either just the image without the EXIF, or just the EXIF without the actual image?

Thanks.

[Larhzu]

Metadata means an ability to add something extra that doesn't affect normal decompression of the file. For example, one could store freeform file description or comment, or cryptographic signature of the contents. There are different possible uses. The .gz format has special fields for the original filename and timestamp, and the format supports also custom metadata.

I don't understand your EXIF example. EXIF is metadata about the image, so in that sense the comparison is spot on. But compression doesn't delete the EXIF from the file. If EXIF is missing after some operation, it's because the operation removed it.

When both metadata and data are wanted in the same .xz file, two or more Streams would be concatenated.

Going back to my image example, if an .xz file has just one Stream, does that mean it's only either just the image without the EXIF, or just the EXIF without the actual image?

If metadata was implemented as its own Stream type, then it's as you wrote. Normal .xz files can have more than one Stream, so such a metadata implementation would allow .xz files to have both data and metadata at the same time.

[IdfbAn]

Metadata means an ability to add something extra that doesn't affect normal decompression of the file. For example, one could store freeform file description or comment, or cryptographic signature of the contents.

How would this metadata be added? A command-line argument, I assume?

[Larhzu]

A new command line argument is one way. For example, --metadata=KEY=VALUE so one could say xz --metadata=mime_type=text/plain foo.txt or something like that. I haven't thought about it much even though the metadata idea existed well before XZ Utils 5.0.0. It's just one of the missing features.