path: root/doc/tarlz.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename tarlz.info
@documentencoding ISO-8859-15
@settitle Tarlz Manual
@finalout
@c %**end of header

@set UPDATED 16 December 2018
@set VERSION 0.8

@dircategory Data Compression
@direntry
* Tarlz: (tarlz).               Archiver with multimember lzip compression
@end direntry


@ifnothtml
@titlepage
@title Tarlz
@subtitle Archiver with multimember lzip compression
@subtitle for Tarlz version @value{VERSION}, @value{UPDATED}
@author by Antonio Diaz Diaz

@page
@vskip 0pt plus 1filll
@end titlepage

@contents
@end ifnothtml

@node Top
@top

This manual is for Tarlz (version @value{VERSION}, @value{UPDATED}).

@menu
* Introduction::              Purpose and features of tarlz
* Invoking tarlz::            Command line interface
* File format::               Detailed format of the compressed archive
* Amendments to pax format::  The reasons for the differences with pax
* Examples::                  A small tutorial with examples
* Problems::                  Reporting bugs
* Concept index::             Index of concepts
@end menu

@sp 1
Copyright @copyright{} 2013-2018 Antonio Diaz Diaz.

This manual is free documentation: you have unlimited permission
to copy, distribute and modify it.


@node Introduction
@chapter Introduction
@cindex introduction

@uref{http://www.nongnu.org/lzip/tarlz.html,,Tarlz} is a small and simple
implementation of the tar archiver. By default tarlz creates, lists and
extracts archives in a simplified posix pax format compressed with
@uref{http://www.nongnu.org/lzip/lzip.html,,lzip} on a per file basis. Each
tar member is compressed in its own lzip member, as well as the end-of-file
blocks. This method is fully backward compatible with standard tar tools
like GNU tar, which treat the resulting multimember tar.lz archive like any
other tar.lz archive. Tarlz can append files to the end of such compressed
archives.

Tarlz can create tar archives with four levels of compression
granularity; per file, per directory, appendable solid, and solid.

@noindent
Of course, compressing each file (or each directory) individually is
less efficient than compressing the whole tar archive, but it has the
following advantages:

@itemize @bullet
@item
The resulting multimember tar.lz archive can be decompressed in
parallel with plzip, multiplying the decompression speed.

@item
New members can be appended to the archive (by removing the EOF
member) just like to an uncompressed tar archive.

@item
It is a safe posix-style backup format. In case of corruption,
tarlz can extract all the undamaged members from the tar.lz
archive, skipping over the damaged members, just like the standard
(uncompressed) tar. Moreover, the option @code{--keep-damaged} can be
used to recover as much data as possible from each damaged member,
and lziprecover can be used to recover some of the damaged members.

@item
A multimember tar.lz archive is usually smaller than the
corresponding solidly compressed tar.gz archive, except when
individually compressing files smaller than about 32 KiB.
@end itemize

Tarlz protects the extended records with a CRC in a way compatible with
standard tar tools. @xref{crc32}.

Tarlz does not understand other tar formats like @samp{gnu}, @samp{oldgnu},
@samp{star} or @samp{v7}.

Tarlz is intended as a showcase project for the maintainers of real tar
programs to evaluate the format and perhaps implement it in their tools.


@node Invoking tarlz
@chapter Invoking tarlz
@cindex invoking
@cindex options
@cindex usage
@cindex version

The format for running tarlz is:

@example
tarlz [@var{options}] [@var{files}]
@end example

@noindent
On archive creation or appending, tarlz removes leading and trailing
slashes from filenames, as well as filename prefixes containing a
@samp{..} component. On extraction, archive members containing a
@samp{..} component are skipped. Tarlz detects when the archive being
created or enlarged is among the files to be dumped, appended or
concatenated, and skips it.

On extraction and listing, tarlz removes leading @samp{./} strings from
member names in the archive or given in the command line, so that
@w{@code{tarlz -xf foo ./bar baz}} extracts members @samp{bar} and
@samp{./baz} from archive @samp{foo}.

tarlz supports the following options:

@table @code
@item -h
@itemx --help
Print an informative help message describing the options and exit.

@item -V
@itemx --version
Print the version number of tarlz on the standard output and exit.
This version number should be included in all bug reports.

@item -A
@itemx --concatenate
Append tar.lz archives to the end of a tar.lz archive. All the archives
involved must be regular (seekable) files compressed as multimember lzip
files, and the two end-of-file blocks plus any zero padding must be
contained in the last lzip member of each archive. The intermediate
end-of-file blocks are removed as each new archive is concatenated. Exit
with status 0 without modifying the archive if no @var{files} have been
specified. Tarlz can't concatenate uncompressed tar archives.

@item -c
@itemx --create
Create a new archive from @var{files}.

@item -C @var{dir}
@itemx --directory=@var{dir}
Change to directory @var{dir}. When creating or appending, the position
of each @code{-C} option in the command line is significant; it will
change the current working directory for the following @var{files} until
a new @code{-C} option appears in the command line. When extracting, all
the @code{-C} options are executed in sequence before starting the
extraction. Listing ignores any @code{-C} options specified. @var{dir}
is relative to the then current working directory, perhaps changed by a
previous @code{-C} option.

@item -f @var{archive}
@itemx --file=@var{archive}
Use archive file @var{archive}. @samp{-} used as an @var{archive}
argument reads from standard input or writes to standard output.

@item -q
@itemx --quiet
Quiet operation. Suppress all messages.

@item -r
@itemx --append
Append files to the end of a tar.lz archive. The archive must be a
regular (seekable) file compressed as a multimember lzip file, and the
two end-of-file blocks plus any zero padding must be contained in the
last lzip member of the archive. First this last member is removed, then
the new members are appended, and then a new end-of-file member is
appended to the archive. Exit with status 0 without modifying the
archive if no @var{files} have been specified. Tarlz can't append files
to an uncompressed tar archive.

@item -t
@itemx --list
List the contents of an archive. If @var{files} are given, list only the
given @var{files}.

@item -v
@itemx --verbose
Verbosely list files processed.

@item -x
@itemx --extract
Extract files from an archive. If @var{files} are given, extract only
the given @var{files}. Else extract all the files in the archive.

@item -0 .. -9
Set the compression level. The default compression level is @samp{-6}.
Like lzip, tarlz also minimizes the dictionary size of the lzip members
it creates, reducing the amount of memory required for decompression.

@item --asolid
When creating or appending to a compressed archive, use appendable solid
compression. All the files being added to the archive are compressed
into a single lzip member, but the end-of-file blocks are compressed
into a separate lzip member. This creates a solidly compressed
appendable archive.

@item --dsolid
When creating or appending to a compressed archive, use solid
compression for each directory especified in the command line. The
end-of-file blocks are compressed into a separate lzip member. This
creates a compressed appendable archive with a separate lzip member for
each top-level directory.

@item --no-solid
When creating or appending to a compressed archive, compress each file
separately. The end-of-file blocks are compressed into a separate lzip
member. This creates a compressed appendable archive with a separate
lzip member for each file. This option allows tarlz revert to default
behavior if, for example, tarlz is invoked through an alias like
@code{tar='tarlz --solid'}.

@item --solid
When creating or appending to a compressed archive, use solid
compression. The files being added to the archive, along with the
end-of-file blocks, are compressed into a single lzip member. The
resulting archive is not appendable. No more files can be later appended
to the archive.

@item --anonymous
Equivalent to @code{--owner=root --group=root}.

@item --owner=@var{owner}
When creating or appending, use @var{owner} for files added to the
archive. If @var{owner} is not a valid user name, it is decoded as a
decimal numeric user ID.

@item --group=@var{group}
When creating or appending, use @var{group} for files added to the
archive. If @var{group} is not a valid group name, it is decoded as a
decimal numeric group ID.

@item --keep-damaged
Don't delete partially extracted files. If a decompression error happens
while extracting a file, keep the partial data extracted. Use this
option to recover as much data as possible from each damaged member.

@item --missing-crc
Exit with error status 2 if the CRC of the extended records is missing.
When this option is used, tarlz detects any corruption in the extended
records (only limited by CRC collisions). But note that a corrupt
@samp{GNU.crc32} keyword, for example @samp{GNU.crc33}, is reported as a
missing CRC instead of as a corrupt record. This misleading
@samp{Missing CRC} message is the consequence of a flaw in the posix pax
format; i.e., the lack of a mandatory check sequence in the extended
records. @xref{crc32}.

@ignore
@item --permissive
Allow some violations of the archive format, like consecutive extended
headers preceding a ustar header, or several records with the same
keyword appearing in the same block of extended records.
@end ignore

@item --uncompressed
With @code{--create}, don't compress the created tar archive. Create an
uncompressed tar archive instead.

@end table

Exit status: 0 for a normal exit, 1 for environmental problems (file not
found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or
invalid input file, 3 for an internal consistency error (eg, bug) which
caused tarlz to panic.


@node File format
@chapter File format
@cindex file format

In the diagram below, a box like this:
@verbatim
+---+
|   | <-- the vertical bars might be missing
+---+
@end verbatim

represents one byte; a box like this:
@verbatim
+==============+
|              |
+==============+
@end verbatim

represents a variable number of bytes or a fixed but large number of
bytes (for example 512).

@sp 1
A tar.lz file consists of a series of lzip members (compressed data sets).
The members simply appear one after another in the file, with no
additional information before, between, or after them.

Each lzip member contains one or more tar members in a simplified posix
pax interchange format; the only pax typeflag value supported by tarlz
(in addition to the typeflag values defined by the ustar format) is
@samp{x}. The pax format is an extension on top of the ustar format that
removes the size limitations of the ustar format.

Each tar member contains one file archived, and is represented by the
following sequence:

@itemize @bullet
@item
An optional extended header block with extended header records. This
header block is of the form described in pax header block, with a
typeflag value of @samp{x}. The extended header records are included as
the data for this header block.

@item
A header block in ustar format that describes the file. Any fields
defined in the preceding optional extended header records override the
associated fields in this header block for this file.

@item
Zero or more blocks that contain the contents of the file.
@end itemize

At the end of the archive file there are two 512-byte blocks filled with
binary zeros, interpreted as an end-of-archive indicator. These EOF
blocks are either compressed in a separate lzip member or compressed
along with the tar members contained in the last lzip member.

The diagram below shows the correspondence between each tar member
(formed by one or two headers plus optional data) in the tar archive and
each
@uref{http://www.nongnu.org/lzip/manual/lzip_manual.html#File-format,,lzip member}
in the resulting multimember tar.lz archive:
@ifnothtml
@xref{File format,,,lzip}.
@end ifnothtml

@verbatim
tar
+========+======+=================+===============+========+======+========+
| header | data | extended header | extended data | header | data |   EOF  |
+========+======+=================+===============+========+======+========+

tar.lz
+===============+=================================================+========+
|     member    |                      member                     | member |
+===============+=================================================+========+
@end verbatim

@ignore
When @code{--permissive} is used, the following violations of the
archive format are allowed:@*
If several extended headers precede an ustar header, only the last
extended header takes effect. The other extended headers are ignored.
Similarly, if several records with the same keyword appear in the same
block of extended records, only the last record for the repeated keyword
takes effect. The other records for the repeated keyword are ignored.
@end ignore

@sp 1
@section Pax header block

The pax header block is identical to the ustar header block described below
except that the typeflag has the value @samp{x} (extended). The size field
is the size of the extended header data in bytes. Most other fields in the
pax header block are zeroed on archive creation to prevent trouble if the
archive is read by an ustar tool, and are ignored by tarlz on archive
extraction. @xref{flawed-compat}.

The pax extended header data consists of one or more records, each of
them constructed as follows:@*
@code{"%d %s=%s\n", <length>, <keyword>, <value>}

The <length>, <blank>, <keyword>, <equals-sign>, and <newline> in the
record must be limited to the portable character set. The <length> field
contains the decimal length of the record in bytes, including the
trailing <newline>. The <value> field is stored as-is, without
conversion to UTF-8 nor any other transformation.

These are the <keyword> fields currently supported by tarlz:

@table @code
@item linkpath
The pathname of a link being created to another file, of any type,
previously archived. This record overrides the linkname field in the
following ustar header block. The following ustar header block
determines the type of link created. If typeflag of the following header
block is 1, it will be a hard link. If typeflag is 2, it will be a
symbolic link and the linkpath value will be used as the contents of the
symbolic link.

@item path
The pathname of the following file. This record overrides the name and
prefix fields in the following ustar header block.

@item size
The size of the file in bytes, expressed as a decimal number using
digits from the ISO/IEC 646:1991 (ASCII) standard. This record overrides
the size field in the following ustar header block. The size record is
used only for files with a size value greater than 8_589_934_591
@w{(octal 77777777777)}. This is 2^33 bytes or larger.

@anchor{key_crc32}
@item GNU.crc32
CRC32-C (Castagnoli) of the extended header data excluding the 8 bytes
representing the CRC <value> itself. The <value> is represented as 8
hexadecimal digits in big endian order,
@w{@samp{22 GNU.crc32=00000000\n}}. The keyword of the CRC record is
protected by the CRC to guarante that corruption is always detected
(except in case of CRC collision). A CRC was chosen because a checksum
is too weak for a potentially large list of variable sized records. A
checksum can't detect simple errors like the swapping of two bytes.
@end table

@sp 1
@section Ustar header block

The ustar header block has a length of 512 bytes and is structured as
shown in the following table. All lengths and offsets are in decimal.

@multitable {Field Name} {Offset} {Length (in bytes)}
@item Field Name @tab Offset @tab Length (in bytes)
@item name     @tab   0 @tab 100
@item mode     @tab 100 @tab   8
@item uid      @tab 108 @tab   8
@item gid      @tab 116 @tab   8
@item size     @tab 124 @tab  12
@item mtime    @tab 136 @tab  12
@item chksum   @tab 148 @tab   8
@item typeflag @tab 156 @tab   1
@item linkname @tab 157 @tab 100
@item magic    @tab 257 @tab   6
@item version  @tab 263 @tab   2
@item uname    @tab 265 @tab  32
@item gname    @tab 297 @tab  32
@item devmajor @tab 329 @tab   8
@item devminor @tab 337 @tab   8
@item prefix   @tab 345 @tab 155
@end multitable

All characters in the header block are coded using the ISO/IEC 646:1991
(ASCII) standard, except in fields storing names for files, users, and
groups. For maximum portability between implementations, names should
only contain characters from the portable filename character set. But if
an implementation supports the use of characters outside of @samp{/} and
the portable filename character set in names for files, users, and
groups, tarlz will use the byte values in these names unmodified.

The fields name, linkname, and prefix are null-terminated character
strings except when all characters in the array contain non-null
characters including the last character.

The name and the prefix fields produce the pathname of the file. A new
pathname is formed, if prefix is not an empty string (its first
character is not null), by concatenating prefix (up to the first null
character), a <slash> character, and name; otherwise, name is used
alone. In either case, name is terminated at the first null character.
If prefix begins with a null character, it is ignored. In this manner,
pathnames of at most 256 characters can be supported. If a pathname does
not fit in the space provided, an extended record is used to store the
pathname.

The linkname field does not use the prefix to produce a pathname. If the
linkname does not fit in the 100 characters provided, an extended record
is used to store the linkname.

The mode field provides 12 access permission bits. The following table
shows the symbolic name of each bit and its octal value:

@multitable {Bit Name} {Bit value}
@item Bit Name @tab Bit value
@item S_ISUID @tab 04000
@item S_ISGID @tab 02000
@item S_ISVTX @tab 01000
@item S_IRUSR @tab 00400
@item S_IWUSR @tab 00200
@item S_IXUSR @tab 00100
@item S_IRGRP @tab 00040
@item S_IWGRP @tab 00020
@item S_IXGRP @tab 00010
@item S_IROTH @tab 00004
@item S_IWOTH @tab 00002
@item S_IXOTH @tab 00001
@end multitable

The uid and gid fields are the user and group ID of the owner and group
of the file, respectively.

The size field contains the octal representation of the size of the file
in bytes. If the typeflag field specifies a file of type '0' (regular
file) or '7' (high performance regular file), the number of logical
records following the header is @w{(size / 512)} rounded to the next
integer. For all other values of typeflag, tarlz either sets the size
field to 0 or ignores it, and does not store or expect any logical
records following the header. If the file size is larger than
8_589_934_591 bytes @w{(octal 77777777777)}, an extended record is used
to store the file size.

The mtime field contains the octal representation of the modification
time of the file at the time it was archived, obtained from the stat()
function.

The chksum field contains the octal representation of the value of the
simple sum of all bytes in the header logical record. Each byte in the
header is treated as an unsigned value. When calculating the checksum,
the chksum field is treated as if it were all <space> characters.

The typeflag field contains a single character specifying the type of
file archived:

@table @code
@item '0'
Regular file.

@item '1'
Hard link to another file, of any type, previously archived.

@item '2'
Symbolic link.

@item '3', '4'
Character special file and block special file respectively. In this case
the devmajor and devminor fields contain information defining the
device in unspecified format.

@item '5'
Directory.

@item '6'
FIFO special file.

@item '7'
Reserved to represent a file to which an implementation has associated
some high-performance attribute. Tarlz treats this type of file as a
regular file (type 0).

@end table

The magic field contains the ASCII null-terminated string "ustar". The
version field contains the characters "00" (0x30,0x30). The fields
uname, and gname are null-terminated character strings. Each numeric
field contains a leading zero-filled, null-terminated octal number using
digits from the ISO/IEC 646:1991 (ASCII) standard.


@node Amendments to pax format
@chapter The reasons for the differences with pax
@cindex Amendments to pax format

Tarlz is meant to reliably detect invalid or corrupt metadata during
extraction and to not create safety risks in the archives it creates. In
order to achieve these goals, tarlz makes some changes to the variant of the
pax format that it uses. This chapter describes these changes and the
concrete reasons to implement them.

@sp 1
@anchor{crc32}
@section Add a CRC of the extended records

The posix pax format has a serious flaw. The metadata stored in pax extended
records are not protected by any kind of check sequence. Corruption in a
long filename may cause the extraction of the file in the wrong place
without warning. Corruption in a long file size may cause the truncation of
the file or the appending of garbage to the file, both followed by a
spurious warning about a corrupt header far from the place of the undetected
corruption.

Metadata like filename and file size must be always protected in an archive
format because of the adverse effects of undetected corruption in them,
potentially much worse that undetected corruption in the data. Even more so
in the case of pax because the amount of metadata it stores is potentially
large, making undetected corruption more probable.

Because of the above, tarlz protects the extended records with a CRC in
a way compatible with standard tar tools. @xref{key_crc32}.

@sp 1
@anchor{flawed-compat}
@section Remove flawed backward compatibility

In order to allow the extraction of pax archives by a tar utility conforming
to the POSIX-2:1993 standard, POSIX.1-2008 recommends selecting extended
header field values that allow such tar to create a regular file containing
the extended header records as data. This approach is broken because if the
extended header is needed because of a long filename, the name and prefix
fields will be unable to contain the full pathname of the file. Therefore
the files corresponding to both the extended header and the overridden ustar
header will be extracted using truncated filenames, perhaps overwriting
existing files or directories. It may be a security risk to extract a file
with a truncated filename.

To avoid this problem, tarlz writes extended headers with all fields zeroed
except size, chksum, typeflag, magic and version. This prevents old tar
programs from extracting the extended records as a file in the wrong place.
Tarlz also sets to zero those fields of the ustar header overridden by
extended records.

If the extended header is needed because of a file size larger than
@w{8 GiB}, the size field will be unable to contain the full size of the
file. Therefore the file may be partially extracted, and the tool will issue
a spurious warning about a corrupt header at the point where it thinks the
file ends. Setting to zero the overridden size in the ustar header at least
prevents the partial extraction and makes obvious that the file has been
truncated.

@sp 1
@section As simple as possible (but not simpler)

The tarlz format is mainly ustar. Extended pax headers are used only when
needed because the length of a filename or link name, or the size of a file
exceed the limits of the ustar format. Adding extended headers to each
member just to record subsecond timestamps seems wasteful for a backup
format.

@sp 1
@section Avoid misconversions to/from UTF-8

There is no portable way to tell what charset a text string is coded into.
Therefore, tarlz stores all fields representing text strings as-is, without
conversion to UTF-8 nor any other transformation. This prevents accidental
double UTF-8 conversions. If the need arises this behavior will be adjusted
with a command line option in the future.


@node Examples
@chapter A small tutorial with examples
@cindex examples

@noindent
Example 1: Create a multimember compressed archive @samp{archive.tar.lz}
containing files @samp{a}, @samp{b} and @samp{c}.

@example
tarlz -cf archive.tar.lz a b c
@end example

@sp 1
@noindent
Example 2: Append files @samp{d} and @samp{e} to the multimember
compressed archive @samp{archive.tar.lz}.

@example
tarlz -rf archive.tar.lz d e
@end example

@sp 1
@noindent
Example 3: Create a solidly compressed appendable archive
@samp{archive.tar.lz} containing files @samp{a}, @samp{b} and @samp{c}.
Then append files @samp{d} and @samp{e} to the archive.

@example
tarlz --asolid -cf archive.tar.lz a b c
tarlz --asolid -rf archive.tar.lz d e
@end example

@sp 1
@noindent
Example 4: Create a compressed appendable archive containing directories
@samp{dir1}, @samp{dir2} and @samp{dir3} with a separate lzip member per
directory. Then append files @samp{a}, @samp{b}, @samp{c}, @samp{d} and
@samp{e} to the archive, all of them contained in a single lzip member.
The resulting archive @samp{archive.tar.lz} contains 5 lzip members
(including the EOF member).

@example
tarlz --dsolid -cf archive.tar.lz dir1 dir2 dir3
tarlz --asolid -rf archive.tar.lz a b c d e
@end example

@sp 1
@noindent
Example 5: Create a solidly compressed archive @samp{archive.tar.lz}
containing files @samp{a}, @samp{b} and @samp{c}. Note that no more
files can be later appended to the archive.

@example
tarlz --solid -cf archive.tar.lz a b c
@end example

@sp 1
@noindent
Example 6: Extract all files from archive @samp{archive.tar.lz}.

@example
tarlz -xf archive.tar.lz
@end example

@sp 1
@noindent
Example 7: Extract files @samp{a} and @samp{c} from archive
@samp{archive.tar.lz}.

@example
tarlz -xf archive.tar.lz a c
@end example

@sp 1
@noindent
Example 8: Copy the contents of directory @samp{sourcedir} to the
directory @samp{targetdir}.

@example
tarlz -C sourcedir -c . | tarlz -C targetdir -x
@end example


@node Problems
@chapter Reporting bugs
@cindex bugs
@cindex getting help

There are probably bugs in tarlz. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
you don't, no one will ever know about them and they will remain unfixed
for all eternity, if not longer.

If you find a bug in tarlz, please send electronic mail to
@email{lzip-bug@@nongnu.org}. Include the version number, which you can
find by running @w{@code{tarlz --version}}.


@node Concept index
@unnumbered Concept index

@printindex cp

@bye