Gzip processing¶

Attempting to compute a delta between compressed input files usually results in poor compression. This is because small differences between the original contents causes changes in the compression of whole blocks of data. To simplify things, Xdelta implements a special case for gzip(1) compressed files. If any version input to the delta command is recognized as having gzip compression, it will be automatically decompressed into a temporary location prior to comparison. This temporary location is either the value of the TMPDIR environment variable, if set, otherwise "/tmp".

The Xdelta patch header contains a flag indicating that the reconstructed version should be recompressed after applying the patch. In general, this allows Xdelta to operate transparently on gzip compressed inputs.

There is one potential problem when automatically processing gzip compressed files, which is that the recompressed content does not always match byte-for-byte with the original compressed content. The uncompressed content still matches, but if there is an external integrity check such as cryptographic signature verification, it may fail. To prevent this from happening, the --pristine option disables automatic gzip processing.

MD5 integrity check¶

By default, Xdelta always verifies the MD5 checksum of the files it reconstructs. This prevents you from supplying an incorrect input during patch, which would result in corrupt output. Because of this feature, you can feel confident that patch has produced valid results. The --noverify option disables MD5 verification, but this is only recommended for performance testing.

Compressed patch format¶

Xdelta uses a fairly simple encoding for its delta, then applies zlib compression to the result. You should not have to post-compress an Xdelta delta.

Delta¶

The delta subcommand has the following synopsis:

xdelta delta [ option... ] fromfile tofile patchout

Computes a delta from fromfile to tofile and writes it to patchout

Patch¶

The patch subcommand has the following synopsis:

xdelta patch [ option... ] patchin [ fromfile [ tofile ]]

Applies patchin to fromfile and produces a reconstructed version of tofile.

If fromfile was omitted, Xdelta attempts to use the original fromfile name, which is stored in the delta. The from file must be identical to the one used to create the delta. If its length or MD5 checksum differs, patch will abort with an error message.

If tofile was omitted, Xdelta attempts to use the original tofile name, which is also stored in the delta. If the original tofile name already exists, a unique filename extension will be added to avoid destroying any existing data.

Info¶

The info subcommand has the following synopsis:

xdelta info patchinfo

Prints information about patchinfo and the version it reconstructs, including file names, lengths, and MD5 checksums.

Options¶

-0..9

Set the zlib compression level. Zero indicates no compression. Nine indicates maximum compression.

-h, --help

Print a short help message and exit.

-q, --quiet

Quiet. Surpresses several warning messages.

-v, --version

Print the Xdelta version number and exit.

-V, --verbose

Verbose. Prints a bit of extra information.

-n, --noverify

No verify. Turns off MD5 checksum verification of the input and output files.

-m=SIZE, --maxmem=SIZE

Set an upper bound on the size of an in-memory page cache. For example, --maxmem=32M will use a 32 megabyte page cache.

-s=BLOCK_SIZE

Set the block size, unless it was hard coded (20% speed improvement). Should be a power of 2.

-p, --pristine

Disable the automatic decompression of gzipped inputs, to prevent unexpected differences in the re-compressed content.