--- /dev/null
+@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
+* 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}.
+@FIXME-xref{To learn more about having more than one archive member with the
+same name, see -backup node, when it's written.}
+
+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 UID, Set 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
+ID of the file owners, respectively. If the operating system does
+not support numeric user or group IDs, 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. @FIXME-xref{Modifiers, in
+particular the @option{--incremental} (@option{-G}) option.}
+
+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
+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 IDs 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 Snapshot Files
+@unnumberedsec Format of the Incremental Snapshot Files
+@include snapshot.texi
+
+@node Dumpdir
+@unnumberedsec Dumpdir
+@include dumpdir.texi
projects / chaz / tar / commitdiff
