Manual Reference Pages - XDELTA (1)
xdelta - Invoke Xdelta
MD5 integrity check
Compressed patch format
option... ] [
Xdelta provides the ability to generate deltas between a pair of files
and later apply those deltas. It operates similar to the
patch commands, but works on binary files and does not produce a human
Xdelta has three subcommands, delta, patch, and info. Delta accepts
two file versions and produces a delta, while patch accepts the
original file version and delta and produces the second version. The
info command prints useful information about a delta. Each subcommand
will be detailed seperately.
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
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
The delta subcommand has the following synopsis:
Computes a delta from
tofile and writes it to
The patch subcommand has the following synopsis:
fromfile and produces a reconstructed version of
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.
The info subcommand has the following synopsis:
Prints information about
patchinfo and the version it reconstructs, including file names, lengths, and
Set the zlib compression level. Zero indicates no compression. Nine
indicates maximum compression.
Print a short help message and exit.
Quiet. Surpresses several warning messages.
Print the Xdelta version number and exit.
Verbose. Prints a bit of extra information.
No verify. Turns off MD5 checksum verification of the input and
Set an upper bound on the size of an in-memory page cache. For
example, --maxmem=32M will use a 32 megabyte page cache.
Set the block size, unless it was hard coded (20% speed improvement).
Should be a power of 2.
Disable the automatic decompression of gzipped inputs, to prevent
unexpected differences in the re-compressed content.
The delta command exits with status 0 to indicate that no differences
were found, with status 1 to indicate that some differences were
found, and with status 2 to indicate an error of some kind. The patch
and info commands exit with status 0 on success and 2 on failure.
Author: Joshua P. MacDonald, firstname.lastname@example.org
Manual Page Revision: 1.6; Release Date: Sun, 28 Jan 2007 12:21:11 -0800.
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