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TextFile

.TH TAR 5 "31 October 1986"
.SH NAME
tar \- tape (or other media) archive file format
.SH DESCRIPTION
A ``tar tape'' or file contains a series of records.  Each record contains
TRECORDSIZE bytes (see below).  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 indicator.  A conforming
system must write a record of zeros at the end, but must not assume that
an end-of-file record exists when reading an archive.

The records may be blocked for physical I/O operations.  Each block of
\fIN\fP records (where \fIN\fP is set by the \fB\-b\fP option to \fItar\fP)
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 shall be written
at the full size, with records after the zero record containing
undefined data.  When reading an archive, a confirming system shall
properly handle an archive whose last block is shorter than the rest.

The header record is defined in the header file <tar.h> as follows:
.nf
.sp .5v
.DT
/*
 * Standard Archive Format - Standard TAR - USTAR
 */
#define	RECORDSIZE	512
#define	NAMSIZ	100
#define	TUNMLEN	32
#define	TGNMLEN	32

union record {
	char		charptr[RECORDSIZE];
	struct header {
		char	name[NAMSIZ];
		char	mode[8];
		char	uid[8];
		char	gid[8];
		char	size[12];
		char	mtime[12];
		char	chksum[8];
		char	linkflag;
		char	linkname[NAMSIZ];
		char	magic[8];
		char	uname[TUNMLEN];
		char	gname[TGNMLEN];
		char	devmajor[8];
		char	devminor[8];
	} header;
};

/* 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 if uname and gname are valid. */
#define	TMAGIC	"ustar  "		/* 7 chars and a null */

/* The linkflag defines the type of file */
#define	LF_OLDNORMAL '\\0'		/* Normal disk file, Unix compatible */
#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. */

/* 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 */
.fi
.LP
All characters in header records
are represented 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 or to be in any character set.
The \fItar\fP(5) format does not distinguish text files from binary
files, and no translation of file contents should be performed.

The fields \fIname, linkname, magic, uname\fP, and \fIgname\fP are
null-terminated
character strings.  All other fields are zero-filled octal numbers in
ASCII.  Each numeric field (of width \fIw\fP) contains \fIw\fP-2 digits, a space, and
a null, except \fIsize\fP and \fImtime\fP,
which do not contain the trailing null.

The \fIname\fP field is the pathname of the file, with directory names
(if any) preceding the file name, separated by slashes.

The \fImode\fP field provides nine bits specifying file permissions and three
bits to specify the Set UID, Set GID and Save Text (TSVTX) 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 an archive; e.g.
the group permission could be copied from the `other' permission.

The \fIuid\fP and \fIgid\fP fields are the user and group ID of the file owners,
respectively.

The \fIsize\fP field is the size of the file in bytes; linked files are archived
with this field specified as zero.

The \fImtime\fP 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 in integer number of
seconds since January 1, 1970, 00:00 Coordinated Universal Time.

The \fIchksum\fP field is the ASCII representaion of the octal value of the
simple sum of all bytes in the header record.  Each 8-bit byte in the
header is treated as an unsigned value.  These values are added to an
unsigned integer, initialized to zero, the precision of which shall be no
less than seventeen bits.  When calculating the checksum, the \fIchksum\fP
field is treated as if it were all blanks.

The \fItypeflag\fP 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,
\fItar\fP issues a warning to the standard error.
.IP "LF_NORMAL or LF_OLDNORMAL"
represents a regular file.
For backward compatibility, a \fItypeflag\fP value of LF_OLDNORMAL
should be silently recognized as a regular file.  New archives should
be created using LF_NORMAL.
Also, for backward
compatability, \fItar\fP treats a regular file whose name ends
with a slash as a directory.
.IP LF_LINK
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 \fIlinkname\fP field with a trailing null.
.IP LF_SYMLINK
represents a symbolic link to another file.  The linked-to
name is specified in the \fIlinkname\fP field with a trailing null.
.IP "LF_CHR or LF_BLK"
represent character special files and block
special files respectively.
In this case the \fIdevmajor\fP and \fIdevminor\fP
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.
.IP LF_DIR
specifies a directory or sub-directory.  The directory name
in the \fIname\fP field should end with a slash.
On systems where
disk allocation is performed on a directory basis the \fIsize\fP
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 \fIsize\fP field of zero indicates no such
limiting.  Systems which do not support limiting in this
manner should ignore the \fIsize\fP field.
.IP LF_FIFO
specifies a FIFO special file.  Note that the archiving of
a FIFO file archives the existence of this file and not its
contents.
.IP LF_CONTIG
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.
.IP "`A' \- `Z'"
are reserved for custom implementations.  None are used by this
version of the \fItar\fP program.
.IP \fIother\fP
values are reserved for specification in future revisions of the
P1003 standard, and should not be used by any \fItar\fP program.
.LP
The \fImagic\fP field indicates that this archive was output in the P1003
archive format.  If this field contains TMAGIC, then the
\fIuname\fP and \fIgname\fP
fields will contain the ASCII representation of the owner and group of the
file respectively.  If found, the user and group ID represented by these
names
will be used rather than the values contained
within the \fIuid\fP and \fIgid\fP fields.
User names longer than TUNMLEN-1 or group
names longer than TGNMLEN-1 characters will be truncated.
.SH "SEE ALSO"
tar(1), ar(5), cpio(5)
.SH BUGS
Names or link names longer than NAMSIZ-1 characters cannot be archived.

This format does not address multi-volume archives.
.SH NOTES
This manual page was adapted by John Gilmore
from Draft 6 of the P1003 specification
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