GNU tar 1.34: E.0.2 PAX Format, Versions 0.0 and 0.1
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E.0.2 PAX Format, Versions 0.0 and 0.1
There are two formats available in this branch. The version 0.0
is the initial version of sparse format used by tar
versions 1.14–1.15.1. The sparse file map is kept in extended (x
) PAX header variables:
GNU.sparse.size
Real size of the stored file;
GNU.sparse.numblocks
Number of blocks in the sparse map;
GNU.sparse.offset
Offset of the data block;
GNU.sparse.numbytes
Size of the data block.
The latter two variables repeat for each data block, so the overall structure is like this:
GNU.sparse.size=size GNU.sparse.numblocks=numblocks repeat numblocks times GNU.sparse.offset=offset GNU.sparse.numbytes=numbytes end repeat
This format presented the following two problems:
- Whereas the POSIX specification allows a variable to appear multiple times in a header, it requires that only the last occurrence be meaningful. Thus, multiple occurrences of
GNU.sparse.offset
andGNU.sparse.numbytes
are conflicting with the POSIX specs. - Attempting to extract such archives using a third-party’s
tar
results in extraction of sparse files in condensed form. If thetar
implementation in question does not support POSIX format, it will also extract a file containing extension header attributes. This file can be used to expand the file to its original state. However, posix-awaretar
s will usually ignore the unknown variables, which makes restoring the file more difficult. See Extraction of sparse members in v.0.0 format, for the detailed description of how to restore such members using non-GNUtar
s.
GNU tar
1.15.2 introduced sparse format version 0.1
, which
attempted to solve these problems. As its predecessor, this format
stores sparse map in the extended POSIX header. It retains
GNU.sparse.size
and GNU.sparse.numblocks
variables, but
instead of GNU.sparse.offset
/GNU.sparse.numbytes
pairs
it uses a single variable:
GNU.sparse.map
Map of non-null data chunks. It is a string consisting of comma-separated values "
offset
,size
[,offset-1
,size-1
...]"
To address the 2nd problem, the name
field in ustar
is replaced with a special name, constructed using the following pattern:
%d/GNUSparseFile.%p/%f
The real name of the sparse file is stored in the variable GNU.sparse.name
. Thus, those tar
implementations that are not aware of GNU extensions will at least extract the files into separate directories, giving the user a possibility to expand it afterwards. See Extraction of sparse members in v.0.1 format, for the detailed description of how to restore such members using non-GNU tar
s.
The resulting GNU.sparse.map
string can be very long. Although POSIX does not impose any limit on the length of a x
header variable, this possibly can confuse some tar
s.
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