intern.texi

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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

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