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Archive Format
The Standard Format
===================
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A `tar' archive file contains a series of records. Each record
contains `RECORDSIZE' 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 record which describes
the file, followed by zero or more records which give the contents of
the file. At the end of the archive file there may be a record filled
with binary zeros as an end-of-file marker. A reasonable system should
write a record of zeros at the end, but must not assume that such a
record exists when reading an archive.
The records may be "blocked" for physical I/O operations. Each
block of N records (where N is set by the `--block-size=512-SIZE' (`-b
512-SIZE') option to `tar') is written with a single `write ()'
operation. On magnetic tapes, the result of such a write is a single
tape record. When writing an archive, the last block of records should
be written at the full size, with records after the zero record
containing all zeroes. When reading an archive, a reasonable system
should properly handle an archive whose last block is shorter than the
rest, or which contains garbage records after a zero record.
The header record is defined in C as follows. In the GNU `tar'
distribution, this is part of file `src/tar.h':
/* Standard Archive Format - Standard TAR - USTAR. */
/* Header block on tape.
We use traditional DP naming conventions here. A "block" is a big chunk
of stuff that we do I/O on. A "record" is a piece of info that we care
about. Typically many "record"s fit into a "block". */
#define RECORDSIZE 512
#define NAMSIZ 100
#define TUNMLEN 32
#define TGNMLEN 32
#define SPARSE_EXT_HDR 21
#define SPARSE_IN_HDR 4
struct sparse
{
char offset[12];
char numbytes[12];
};
union record
{
char charptr[RECORDSIZE];
struct header
{
char arch_name[NAMSIZ];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char chksum[8];
char linkflag;
char arch_linkname[NAMSIZ];
char magic[8];
char uname[TUNMLEN];
char gname[TGNMLEN];
char devmajor[8];
char devminor[8];
/* The following fields were added for GNU and are not standard. */
char atime[12];
char ctime[12];
char offset[12];
char longnames[4];
/* Some compilers would insert the pad themselves, so pad was
once autoconfigured. It is simpler to always insert it! */
char pad;
struct sparse sp[SPARSE_IN_HDR];
char isextended;
char realsize[12]; /* true size of the sparse file */
#if 0
char ending_blanks[12]; /* number of nulls at the end of the file,
if any */
#endif
}
header;
struct extended_header
{
struct sparse sp[21];
char isextended;
}
ext_hdr;
};
/* The checksum field is filled with this while the checksum is computed. */
#define CHKBLANKS " " /* 8 blanks, no null */
/* The magic field is filled with this value if uname and gname are valid,
marking the archive as being in standard POSIX format (though GNU tar
itself is not POSIX conforming). */
#define TMAGIC "ustar " /* 7 chars and a null */
/* The magic field is filled with this if this is a GNU format dump entry.
But I suspect this is not true anymore. */
#define GNUMAGIC "GNUtar " /* 7 chars and a null */
/* The linkflag defines the type of file. */
#define LF_OLDNORMAL '\0' /* normal disk file, Unix compat */
#define LF_NORMAL '0' /* normal disk file */
#define LF_LINK '1' /* link to previously dumped file */
#define LF_SYMLINK '2' /* symbolic link */
#define LF_CHR '3' /* character special file */
#define LF_BLK '4' /* block special file */
#define LF_DIR '5' /* directory */
#define LF_FIFO '6' /* FIFO special file */
#define LF_CONTIG '7' /* contiguous file */
/* Further link types may be defined later. */
/* Note that the standards committee allows only capital A through
capital Z for user-defined expansion. This means that defining
something as, say '8' is a *bad* idea. */
/* This is a dir entry that contains the names of files that were in the
dir at the time the dump was made. */
#define LF_DUMPDIR 'D'
/* Identifies the NEXT file on the tape as having a long linkname. */
#define LF_LONGLINK 'K'
/* Identifies the NEXT file on the tape as having a long name. */
#define LF_LONGNAME 'L'
/* This is the continuation of a file that began on another volume. */
#define LF_MULTIVOL 'M'
/* For storing filenames that didn't fit in 100 characters. */
#define LF_NAMES 'N'
/* This is for sparse files. */
#define LF_SPARSE 'S'
/* This file is a tape/volume header. Ignore it on extraction. */
#define LF_VOLHDR 'V'
#if 0
/* The following two blocks of #define's are unused in GNU tar. */
/* Bits used in the mode field - values in octal */
#define TSUID 04000 /* set UID on execution */
#define TSGID 02000 /* set GID on execution */
#define TSVTX 01000 /* save text (sticky bit) */
/* File permissions */
#define TUREAD 00400 /* read by owner */
#define TUWRITE 00200 /* write by owner */
#define TUEXEC 00100 /* execute/search by owner */
#define TGREAD 00040 /* read by group */
#define TGWRITE 00020 /* write by group */
#define TGEXEC 00010 /* execute/search by group */
#define TOREAD 00004 /* read by other */
#define TOWRITE 00002 /* write by other */
#define TOEXEC 00001 /* execute/search by other */
#endif
/* End of Standard Archive Format description. */
All characters in header records 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 record of
each file) are not translated in any way and are not constrained to
represent characters in any character set. The `tar' format does not
distinguish text files from binary files, and no translation of file
contents is performed.
The `name', `linkname', `magic', `uname', and `gname' are
null-terminated character strings. All other fileds are zero-filled
octal numbers in ASCII. Each numeric field of width W contains W minus
2 digits, a space, and a null, except `size', and `mtime', which do not
contain the trailing null.
The `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 `mode' field provides nine bits specifying file permissions and
three bits to specify the Set UID, Set GID, and Save Text ("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 *other* permission.
The `uid' and `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 `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 `--incremental' (`-G') option.
The `mtime' field is the 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 was modified, represented as an integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.
The `chksum' field is the ASCII representation of the octal value of
the simple sum of all bytes in the header record. 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 `chksum' field is treated as if it were
all blanks.
The `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, `tar' issues a warning to the standard error.
The `atime' and `ctime' fields are used in making incremental
backups; they store, respectively, the particular file's access time
and last inode-change time.
The `offset' is used by the `--multi-volume' (`-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 "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 `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 `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 `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 `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 `sp' is an array of `struct sparse'. Each `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 `struct sparse' at the moment; if
more are needed, they are not stored in the header.
The `isextended' flag is set when an `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 alloted room for sparse
structures in the header. In other words, an extended_header is needed.
The `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 `isextended' gets set
and the next record is an `extended_header'.
Each `extended_header' structure contains an array of 21 sparse
structures, along with a similar `isextended' flag that the header had.
There can be an indeterminate number of such `extended_header's to
describe a sparse file.
`LF_NORMAL'
`LF_OLDNORMAL'
These flags represent a regular file. In order to be compatible
with older versions of `tar', a `typeflag' value of `LF_OLDNORMAL'
should be silently recognized as a regular file. New archives
should be created using `LF_NORMAL'. Also, for backward
compatibility, `tar' treats a regular file whose name ends with a
slash as a directory.
`LF_LINK'
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 `linkname' field with a trailing null.
`LF_SYMLINK'
This represents a symbolic link to another file. The linked-to
name is specified in the `linkname' field with a trailing null.
`LF_CHR'
`LF_BLK'
These represent character special files and block special files
respectively. In this case the `devmajor' and `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.
`LF_DIR'
This flag specifies a directory or sub-directory. The directory
name in the `name' field should end with a slash. On systems where
disk allocation is performed on a directory basis, the `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 `size' field of zero indicates no such limiting. Systems
which do not support limiting in this manner should ignore the
`size' field.
`LF_FIFO'
This specifies a FIFO special file. Note that the archiving of a
FIFO file archives the existence of this file and not its contents.
`LF_CONTIG'
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.
`A' ... `Z'
These are reserved for custom implementations. Some of these are
used in the GNU modified format, as described below.
Other values are reserved for specification in future revisions of
the P1003 standard, and should not be used by any `tar' program.
The `magic' field indicates that this archive was output in the
P1003 archive format. If this field contains `TMAGIC', the `uname' and
`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 `uid' and `gid' fields.
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