@c This is part of the paxutils manual.
-@c Copyright (C) 2006 Free Software Foundation, Inc.
+@c Copyright (C) 2006, 2014 Free Software Foundation, Inc.
@c This file is distributed under GFDL 1.1 or any later version
@c published by the Free Software Foundation.
+@cindex sparse formats
+@cindex sparse versions
The notion of sparse file, and the ways of handling it from the point
of view of @GNUTAR{} user have been described in detail in
@ref{sparse}. This chapter describes the internal format @GNUTAR{}
earliest version featuring this support that I was able to find was 1.09,
released in November, 1990. The format introduced back then is called
@dfn{old GNU} sparse format and in spite of the fact that its design
-contained many flaws, it was the only format @GNUTAR{} supported
+contained many flaws, it was the only format @GNUTAR{} supported
until version 1.14 (May, 2004), which introduced initial support for
sparse archives in @acronym{PAX} archives (@pxref{posix}). This
-format was not free from design flows, either and it was subsequently
+format was not free from design flaws, either and it was subsequently
improved in versions 1.15.2 (November, 2005) and 1.15.92 (June,
-2006).
+2006).
In addition to GNU sparse format, @GNUTAR{} is able to read and
extract sparse files archived by @command{star}.
@node Old GNU Format
@appendixsubsec Old GNU Format
-The format introduced some time around 1990 (v. 1.09). It was
+@cindex sparse formats, Old GNU
+@cindex Old GNU sparse format
+The format introduced in November 1990 (v. 1.09) was
designed on top of standard @code{ustar} headers in such an
unfortunate way that some of its fields overwrote fields required by
POSIX.
@end multitable
Each of @code{sparse_header} object at offset 386 describes a single
-data chunk. It has the following structure:
+data chunk. It has the following structure:
@multitable @columnfractions 0.10 0.10 0.20 0.60
@headitem Offset @tab Size @tab Data type @tab Contents
@multitable @columnfractions 0.10 0.10 0.20 0.20 0.40
@headitem Offset @tab Size @tab Name @tab Data type @tab Contents
@item 0 @tab 21 @tab sp @tab @code{sparse_header} @tab
-(21 entires) File map.
+(21 entries) File map.
@item 504 @tab 1 @tab isextended @tab Bool @tab @code{1} if an
extension sparse header follows, or @code{0} otherwise.
@end multitable
@node PAX 0
@appendixsubsec PAX Format, Versions 0.0 and 0.1
-@UNREVISED{}
+@cindex sparse formats, v.0.0
There are two formats available in this branch. The version @code{0.0}
is the initial version of sparse format used by @command{tar}
versions 1.14--1.15.1. The sparse file map is kept in extended
(@code{x}) PAX header variables:
@table @code
+@vrindex GNU.sparse.size, extended header variable
@item GNU.sparse.size
-Real size of the stored file
+Real size of the stored file;
@item GNU.sparse.numblocks
-Number of blocks in the sparse map
+@vrindex GNU.sparse.numblocks, extended header variable
+Number of blocks in the sparse map;
@item GNU.sparse.offset
-Offset of the data block
+@vrindex GNU.sparse.offset, extended header variable
+Offset of the data block;
@item GNU.sparse.numbytes
-Size of the data block
+@vrindex GNU.sparse.numbytes, extended header variable
+Size of the data block.
@end table
The latter two variables repeat for each data block, so the overall
@smallexample
@group
-GNU.sparse.size=@var{size}
-GNU.sparse.numblocks=@var{numblocks}
+GNU.sparse.size=@var{size}
+GNU.sparse.numblocks=@var{numblocks}
repeat @var{numblocks} times
- GNU.sparse.offset=@var{offset}
- GNU.sparse.numbytes=@var{numbytes}
+ GNU.sparse.offset=@var{offset}
+ GNU.sparse.numbytes=@var{numbytes}
end repeat
@end group
@end smallexample
@item
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 ocurrences of @code{GNU.sparse.offset} and
-@code{GNU.sparse.numbytes} are conficting with the POSIX specs.
+meaningful. Thus, multiple occurrences of @code{GNU.sparse.offset} and
+@code{GNU.sparse.numbytes} are conflicting with the POSIX specs.
@item
-Attempting to extract such archives using a third-party @command{tar}s
-results in extraction of sparse files in @emph{compressed form}. If
+Attempting to extract such archives using a third-party's @command{tar}
+results in extraction of sparse files in @emph{condensed form}. If
the @command{tar} 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-aware @command{tar}s will usually ignore the
-unknown variables, which makes restoring the file much more
-difficult@FIXME-xref{how to extract sparse file using third-party @command{tar}s}.
+unknown variables, which makes restoring the file more
+difficult. @xref{extracting sparse v.0.x, Extraction of sparse
+members in v.0.0 format}, for the detailed description of how to
+restore such members using non-GNU @command{tar}s.
@end enumerate
+@cindex sparse formats, v.0.1
@GNUTAR{} 1.15.2 introduced sparse format version @code{0.1}, which
attempted to solve these problems. As its predecessor, this format
stores sparse map in the extended POSIX header. It retains
@table @code
@item GNU.sparse.map
+@vrindex GNU.sparse.map, extended header variable
Map of non-null data chunks. It is a string consisting of
-comma-separated values "@var{offset},@var{size}[,@var{offset-1},@var{size-1}...]"
+comma-separated values "@var{offset},@var{size}[,@var{offset-1},@var{size-1}...]"
@end table
To address the 2nd problem, the @code{name} field in @code{ustar}
%d/GNUSparseFile.%p/%f
@end smallexample
+@vrindex GNU.sparse.name, extended header variable
The real name of the sparse file is stored in the variable
@code{GNU.sparse.name}. Thus, those @command{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 @FIXME-ref{how to extract sparse file using third-party
-@command{tar}s}.
+afterwards. @xref{extracting sparse v.0.x, Extraction of sparse
+members in v.0.1 format}, for the detailed description of how to
+restore such members using non-GNU @command{tar}s.
The resulting @code{GNU.sparse.map} string can be @emph{very} long.
Although POSIX does not impose any limit on the length of a @code{x}
-header variable, this possibly can confuse some tars.
+header variable, this possibly can confuse some @command{tar}s.
@node PAX 1
@appendixsubsec PAX Format, Version 1.0
-@UNREVISED{}
+@cindex sparse formats, v.1.0
The version @code{1.0} of sparse format was introduced with @GNUTAR{}
1.15.92. Its main objective was to make the resulting file
extractable with little effort even by non-posix aware @command{tar}
@table @code
@item GNU.sparse.major
+@vrindex GNU.sparse.major, extended header variable
Major version
@item GNU.sparse.minor
+@vrindex GNU.sparse.minor, extended header variable
Minor version
@end table
%d/GNUSparseFile.%p/%f
@end smallexample
+@vrindex GNU.sparse.name, extended header variable, in v.1.0
+@vrindex GNU.sparse.realsize, extended header variable
The real name of the sparse file is stored in the variable
@code{GNU.sparse.name}. The real size of the file is stored in the
variable @code{GNU.sparse.realsize}.
The sparse map itself is stored in the file data block, preceding the actual
-file data. It consists of a series of octal numbers of arbitrary length, delimited
+file data. It consists of a series of octal numbers of arbitrary length, delimited
by newlines. The map is padded with nulls to the nearest block boundary.
The first number gives the number of entries in the map. Following are map entries,
each one consisting of two numbers giving the offset and size of the
data block it describes.
-The format is designed in such a way that non-posix aware tars and tars not
+The format is designed in such a way that non-posix aware @command{tar}s and @command{tar}s not
supporting @code{GNU.sparse.*} keywords will extract each sparse file
in its condensed form with the file map prepended and will place it
into a separate directory. Then, using a simple program it would be
-possible to expand the file to its original form even without GNU tar.
-@FIXME-xref{how to extract sparse file using third-party
-@command{tar}s}. @FIXME{Write the program and give its URL here}.
-
+possible to expand the file to its original form even without @GNUTAR{}.
+@xref{Sparse Recovery}, for the detailed information on how to extract
+sparse members without @GNUTAR{}.
projects / chaz / tar / blobdiff