tar/doc/intern.texi

@c This is part of the paxutils manual.
@c Copyright (C) 2006--2022 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{} does not treat missing end-of-file marker as an
error and silently ignores the fact.  You can instruct it to issue
a warning, however, by using the @option{--warning=missing-zero-blocks}
option (@pxref{General Warnings, missing-zero-blocks}).

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.
(In the extended @acronym{GNU} format, the numeric fields can take
other forms.)

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 represents the data modification time of the file at
the time it was archived.  It represents the integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.

The @code{chksum} field represents
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
used to handle a sparse file:

The @code{sp} is an array of @code{struct sparse}.  Each @code{struct
sparse} contains two 12-character strings which represent an offset
into the file and a number of bytes to be written at that offset.
The offset is absolute, and not relative to the offset in preceding
array element.

The header can hold four of these @code{struct sparse} at the moment;
if more are needed, they are not stored in the header.

The @code{isextended} flag is set when an @code{extended_header}
is needed to deal with a file.  Note that this means that this flag
can only be set when dealing with a sparse file, and it is only set
in the event that the description of the file will not fit in the
allotted room for sparse structures in the header.  In other words,
an extended_header is needed.

The @code{extended_header} structure is used for sparse files which
need more sparse structures than can fit in the header.  The header can
fit 4 such structures; if more are needed, the flag @code{isextended}
gets set and the next block is an @code{extended_header}.

Each @code{extended_header} structure contains an array of 21
sparse structures, along with a similar @code{isextended} flag
that the header had.  There can be an indeterminate number of such
@code{extended_header}s to describe a sparse file.

@table @asis

@item @code{REGTYPE}
@itemx @code{AREGTYPE}
These flags represent a regular file.  In order to be compatible
with older versions of @command{tar}, a @code{typeflag} value of
@code{AREGTYPE} should be silently recognized as a regular file.
New archives should be created using @code{REGTYPE}.  Also, for
backward compatibility, @command{tar} treats a regular file whose name
ends with a slash as a directory.

@item @code{LNKTYPE}
This flag represents a file linked to another file, of any type,
previously archived.  Such files are identified in Unix by each
file having the same device and inode number.  The linked-to name is
specified in the @code{linkname} field with a trailing null.

@item @code{SYMTYPE}
This represents a symbolic link to another file.  The linked-to name
is specified in the @code{linkname} field with a trailing null.

@item @code{CHRTYPE}
@itemx @code{BLKTYPE}
These represent character special files and block special files
respectively.  In this case the @code{devmajor} and @code{devminor}
fields will contain the major and minor device numbers respectively.
Operating systems may map the device specifications to their own
local specification, or may ignore the entry.

@item @code{DIRTYPE}
This flag specifies a directory or sub-directory.  The directory
name in the @code{name} field should end with a slash.  On systems where
disk allocation is performed on a directory basis, the @code{size} field
will contain the maximum number of bytes (which may be rounded to
the nearest disk block allocation unit) which the directory may
hold.  A @code{size} field of zero indicates no such limiting.  Systems
which do not support limiting in this manner should ignore the
@code{size} field.

@item @code{FIFOTYPE}
This specifies a FIFO special file.  Note that the archiving of a
FIFO file archives the existence of this file and not its contents.

@item @code{CONTTYPE}
This specifies a contiguous file, which is the same as a normal
file except that, in operating systems which support it, all its
space is allocated contiguously on the disk.  Operating systems
which do not allow contiguous allocation should silently treat this
type as a normal file.

@item @code{A} @dots{} @code{Z}
These are reserved for custom implementations.  Some of these are
used in the @acronym{GNU} modified format, as described below.

@end table

Other values are reserved for specification in future revisions of
the P1003 standard, and should not be used by any @command{tar} program.

The @code{magic} field indicates that this archive was output in
the P1003 archive format.  If this field contains @code{TMAGIC},
the @code{uname} and @code{gname} fields will contain the ASCII
representation of the owner and group of the file respectively.
If found, the user and group @acronym{ID}s are used rather than the values in
the @code{uid} and @code{gid} fields.

For references, see ISO/IEC 9945-1:1990 or IEEE Std 1003.1-1990, pages
169-173 (section 10.1) for @cite{Archive/Interchange File Format}; and
IEEE Std 1003.2-1992, pages 380-388 (section 4.48) and pages 936-940
(section E.4.48) for @cite{pax - Portable archive interchange}.

@node Extensions
@unnumberedsec @acronym{GNU} Extensions to the Archive Format
@UNREVISED{}

The @acronym{GNU} format uses additional file types to describe new types of
files in an archive.  These are listed below.

@table @code
@item GNUTYPE_DUMPDIR
@itemx 'D'
This represents a directory and a list of files created by the
@option{--incremental} (@option{-G}) option.  The @code{size} field gives the total
size of the associated list of files.  Each file name is preceded by
either a @samp{Y} (the file should be in this archive) or an @samp{N}.
(The file is a directory, or is not stored in the archive.)  Each file
name is terminated by a null.  There is an additional null after the
last file name.

@item GNUTYPE_MULTIVOL
@itemx 'M'
This represents a file continued from another volume of a multi-volume
archive created with the @option{--multi-volume} (@option{-M}) option.  The original
type of the file is not given here.  The @code{size} field gives the
maximum size of this piece of the file (assuming the volume does
not end before the file is written out).  The @code{offset} field
gives the offset from the beginning of the file where this part of
the file begins.  Thus @code{size} plus @code{offset} should equal
the original size of the file.

@item GNUTYPE_SPARSE
@itemx 'S'
This flag indicates that we are dealing with a sparse file.  Note
that archiving a sparse file requires special operations to find
holes in the file, which mark the positions of these holes, along
with the number of bytes of data to be found after the hole.

@item GNUTYPE_VOLHDR
@itemx 'V'
This file type is used to mark the volume header that was given with
the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option when the archive was created.  The @code{name}
field contains the @code{name} given after the @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) option.
The @code{size} field is zero.  Only the first file in each volume
of an archive should have this type.

@end table

For fields containing numbers or timestamps that are out of range for
the basic format, the @acronym{GNU} format uses a base-256
representation instead of an ASCII octal number.  If the leading byte
is 0xff (255), all the bytes of the field (including the leading byte)
are concatenated in big-endian order, with the result being a negative
number expressed in two's complement form.  If the leading byte is
0x80 (128), the non-leading bytes of the field are concatenated in
big-endian order, with the result being a positive number expressed in
binary form.  Leading bytes other than 0xff, 0x80 and ASCII octal
digits are reserved for future use, as are base-256 representations of
values that would be in range for the basic format.

You may have trouble reading a @acronym{GNU} format archive on a
non-@acronym{GNU} system if the options @option{--incremental} (@option{-G}),
@option{--multi-volume} (@option{-M}), @option{--sparse} (@option{-S}), or @option{--label=@var{archive-label}} (@option{-V @var{archive-label}}) were
used when writing the archive.  In general, if @command{tar} does not
use the @acronym{GNU}-added fields of the header, other versions of
@command{tar} should be able to read the archive.  Otherwise, the
@command{tar} program will give an error, the most likely one being a
checksum error.

@node Sparse Formats
@unnumberedsec Storing Sparse Files
@include sparse.texi

@node Snapshot Files
@unnumberedsec Format of the Incremental Snapshot Files
@include snapshot.texi

@node Dumpdir
@unnumberedsec Dumpdir
@include dumpdir.texi