intern.texi

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@c This is part of the paxutils manual.
@c Copyright (C) 2006 Free Software Foundation, Inc.
@c This file is distributed under GFDL 1.1 or any later version
@c published by the Free Software Foundation.

@menu
* Standard::           Basic Tar Format
* Extensions::         @acronym{GNU} Extensions to the Archive Format
* Sparse Formats::     Storing Sparse Files
* Snapshot Files::
* Dumpdir::
@end menu

@node Standard
@unnumberedsec Basic Tar Format
@UNREVISED

While an archive may contain many files, the archive itself is a
single ordinary file.  Like any other file, an archive file can be
written to a storage device such as a tape or disk, sent through a
pipe or over a network, saved on the active file system, or even
stored in another archive.  An archive file is not easy to read or
manipulate without using the @command{tar} utility or Tar mode in
@acronym{GNU} Emacs.

Physically, an archive consists of a series of file entries terminated
by an end-of-archive entry, which consists of two 512 blocks of zero
bytes.  A file
entry usually describes one of the files in the archive (an
@dfn{archive member}), and consists of a file header and the contents
of the file.  File headers contain file names and statistics, checksum
information which @command{tar} uses to detect file corruption, and
information about file types.

Archives are permitted to have more than one member with the same
member name.  One way this situation can occur is if more than one
version of a file has been stored in the archive.  For information
about adding new versions of a file to an archive, see @ref{update}.

In addition to entries describing archive members, an archive may
contain entries which @command{tar} itself uses to store information.
@xref{label}, for an example of such an archive entry.

A @command{tar} archive file contains a series of blocks.  Each block
contains @code{BLOCKSIZE} bytes.  Although this format may be thought
of as being on magnetic tape, other media are often used.

Each file archived is represented by a header block which describes
the file, followed by zero or more blocks which give the contents
of the file.  At the end of the archive file there are two 512-byte blocks
filled with binary zeros as an end-of-file marker.  A reasonable system
should write such end-of-file marker at the end of an archive, but
must not assume that such a block exists when reading an archive.  In
particular @GNUTAR{} always issues a warning if it does not encounter it.

The blocks may be @dfn{blocked} for physical I/O operations.
Each record of @var{n} blocks (where @var{n} is set by the
@option{--blocking-factor=@var{512-size}} (@option{-b @var{512-size}}) option to @command{tar}) is written with a single
@w{@samp{write ()}} operation.  On magnetic tapes, the result of
such a write is a single record.  When writing an archive,
the last record of blocks should be written at the full size, with
blocks after the zero block containing all zeros.  When reading
an archive, a reasonable system should properly handle an archive
whose last record is shorter than the rest, or which contains garbage
records after a zero block.

The header block is defined in C as follows.  In the @GNUTAR{}
distribution, this is part of file @file{src/tar.h}:

@smallexample
@include header.texi
@end smallexample

All characters in header blocks are represented by using 8-bit
characters in the local variant of ASCII.  Each field within the
structure is contiguous; that is, there is no padding used within
the structure.  Each character on the archive medium is stored
contiguously.

Bytes representing the contents of files (after the header block
of each file) are not translated in any way and are not constrained
to represent characters in any character set.  The @command{tar} format
does not distinguish text files from binary files, and no translation
of file contents is performed.

The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and
@code{gname} are null-terminated character strings.  All other fields
are zero-filled octal numbers in ASCII.  Each numeric field of width
@var{w} contains @var{w} minus 1 digits, and a null.

The @code{name} field is the file name of the file, with directory names
(if any) preceding the file name, separated by slashes.

@FIXME{how big a name before field overflows?}

The @code{mode} field provides nine bits specifying file permissions
and three bits to specify the Set @acronym{UID}, Set @acronym{GID}, and Save Text
(@dfn{sticky}) modes.  Values for these bits are defined above.
When special permissions are required to create a file with a given
mode, and the user restoring files from the archive does not hold such
permissions, the mode bit(s) specifying those special permissions
are ignored.  Modes which are not supported by the operating system
restoring files from the archive will be ignored.  Unsupported modes
should be faked up when creating or updating an archive; e.g., the
group permission could be copied from the @emph{other} permission.

The @code{uid} and @code{gid} fields are the numeric user and group
@acronym{ID} of the file owners, respectively.  If the operating system does
not support numeric user or group @acronym{ID}s, these fields should
be ignored. 

The @code{size} field is the size of the file in bytes; linked files
are archived with this field specified as zero. 

The @code{mtime} field is the data modification time of the file at
the time it was archived.  It is the ASCII representation of the octal
value of the last time the file's contents were modified, represented
as an integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.

The @code{chksum} field is the ASCII representation of the octal value
of the simple sum of all bytes in the header block.  Each 8-bit
byte in the header is added to an unsigned integer, initialized to
zero, the precision of which shall be no less than seventeen bits.
When calculating the checksum, the @code{chksum} field is treated as
if it were all blanks.

The @code{typeflag} field specifies the type of file archived.  If a
particular implementation does not recognize or permit the specified
type, the file will be extracted as if it were a regular file.  As this
action occurs, @command{tar} issues a warning to the standard error.

The @code{atime} and @code{ctime} fields are used in making incremental
backups; they store, respectively, the particular file's access and
status change times.

The @code{offset} is used by the @option{--multi-volume} (@option{-M}) option, when
making a multi-volume archive.  The offset is number of bytes into
the file that we need to restart at to continue the file on the next
tape, i.e., where we store the location that a continued file is
continued at.

The following fields were added to deal with sparse files.  A file
is @dfn{sparse} if it takes in unallocated blocks which end up being
represented as zeros, i.e., no useful data.  A test to see if a file
is sparse is to look at the number blocks allocated for it versus the
number of characters in the file; if there are fewer blocks allocated
for the file than would normally be allocated for a file of that
size, then the file is sparse.  This is the method @command{tar} uses to
detect a sparse file, and once such a file is detected, it is treated
differently from non-sparse files.

Sparse files are often @code{dbm} files, or other database-type files
which have data at some points and emptiness in the greater part of
the file.  Such files can appear to be very large when an @samp{ls
-l} is done on them, when in truth, there may be a very small amount
of important data contained in the file.  It is thus undesirable
to have @command{tar} think that it must back up this entire file, as
great quantities of room are wasted on empty blocks, which can lead
to running out of room on a tape far earlier than is necessary.
Thus, sparse files are dealt with so that these empty blocks are
not written to the tape.  Instead, what is written to the tape is a
description, of sorts, of the sparse file: where the holes are, how
big the holes are, and how much data is found at the end of the hole.
This way, the file takes up potentially far less room on the tape,
and when the file is extracted later on, it will look exactly the way
it looked beforehand.  The following is a description of the fields