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<p>TAR(5) BSD File Formats Manual TAR(5)</p>

<p style="margin-top: 1em"><b>NAME</b></p>

<p style="margin-left:6%;"><b>tar</b> &mdash; format of
tape archive files</p>

<p style="margin-top: 1em"><b>DESCRIPTION</b></p>

<p style="margin-left:6%;">The <b>tar</b> archive format
collects any number of files, directories, and other file
system objects (symbolic links, device nodes, etc.) into a
single stream of bytes. The format was originally designed
to be used with tape drives that operate with fixed-size
blocks, but is widely used as a general packaging
mechanism.</p>

<p style="margin-left:6%; margin-top: 1em"><b>General
Format</b> <br>
A <b>tar</b> archive consists of a series of 512-byte
records. Each file system object requires a header record
which stores basic metadata (pathname, owner, permissions,
etc.) and zero or more records containing any file data. The
end of the archive is indicated by two records consisting
entirely of zero bytes.</p>

<p style="margin-left:6%; margin-top: 1em">For
compatibility with tape drives that use fixed block sizes,
programs that read or write tar files always read or write a
fixed number of records with each I/O operation. These
&ldquo;blocks&rdquo; are always a multiple of the record
size. The maximum block size supported by early
implementations was 10240 bytes or 20 records. This is still
the default for most implementations although block sizes of
1MiB (2048 records) or larger are commonly used with modern
high-speed tape drives. (Note: the terms &ldquo;block&rdquo;
and &ldquo;record&rdquo; here are not entirely standard;
this document follows the convention established by John
Gilmore in documenting <b>pdtar</b>.)</p>

<p style="margin-left:6%; margin-top: 1em"><b>Old-Style
Archive Format</b> <br>
The original tar archive format has been extended many times
to include additional information that various implementors
found necessary. This section describes the variant
implemented by the tar command included in Version&nbsp;7
AT&amp;T UNIX, which seems to be the earliest widely-used
version of the tar program.</p>

<p style="margin-left:6%; margin-top: 1em">The header
record for an old-style <b>tar</b> archive consists of the
following:</p>

<p style="margin-left:14%; margin-top: 1em">struct
header_old_tar {</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char name[100];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char mode[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char uid[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char gid[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char size[12];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char mtime[12];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char checksum[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char linkflag[1];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char linkname[100];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char pad[255];</p></td>
<td width="65%">
</td></tr>
</table>

<p style="margin-left:14%;">};</p>

<p style="margin-left:6%;">All unused bytes in the header
record are filled with nulls.</p>

<p style="margin-top: 1em"><i>name</i></p>

<p style="margin-left:17%; margin-top: 1em">Pathname,
stored as a null-terminated string. Early tar
implementations only stored regular files (including
hardlinks to those files). One common early convention used
a trailing &quot;/&quot; character to indicate a directory
name, allowing directory permissions and owner information
to be archived and restored.</p>

<p style="margin-top: 1em"><i>mode</i></p>

<p style="margin-left:17%; margin-top: 1em">File mode,
stored as an octal number in ASCII.</p>

<p style="margin-top: 1em"><i>uid</i>, <i>gid</i></p>

<p style="margin-left:17%;">User id and group id of owner,
as octal numbers in ASCII.</p>

<p style="margin-top: 1em"><i>size</i></p>

<p style="margin-left:17%; margin-top: 1em">Size of file,
as octal number in ASCII. For regular files only, this
indicates the amount of data that follows the header. In
particular, this field was ignored by early tar
implementations when extracting hardlinks. Modern writers
should always store a zero length for hardlink entries.</p>

<p style="margin-top: 1em"><i>mtime</i></p>

<p style="margin-left:17%; margin-top: 1em">Modification
time of file, as an octal number in ASCII. This indicates
the number of seconds since the start of the epoch, 00:00:00
UTC January 1, 1970. Note that negative values should be
avoided here, as they are handled inconsistently.</p>

<p style="margin-top: 1em"><i>checksum</i></p>

<p style="margin-left:17%;">Header checksum, stored as an
octal number in ASCII. To compute the checksum, set the
checksum field to all spaces, then sum all bytes in the
header using unsigned arithmetic. This field should be
stored as six octal digits followed by a null and a space
character. Note that many early implementations of tar used
signed arithmetic for the checksum field, which can cause
interoperability problems when transferring archives between
systems. Modern robust readers compute the checksum both
ways and accept the header if either computation
matches.</p>

<p style="margin-top: 1em"><i>linkflag</i>,
<i>linkname</i></p>

<p style="margin-left:17%;">In order to preserve hardlinks
and conserve tape, a file with multiple links is only
written to the archive the first time it is encountered. The
next time it is encountered, the <i>linkflag</i> is set to
an ASCII &rsquo;1&rsquo; and the <i>linkname</i> field holds
the first name under which this file appears. (Note that
regular files have a null value in the <i>linkflag</i>
field.)</p>

<p style="margin-left:6%; margin-top: 1em">Early tar
implementations varied in how they terminated these fields.
The tar command in Version&nbsp;7 AT&amp;T UNIX used the
following conventions (this is also documented in early BSD
manpages): the pathname must be null-terminated; the mode,
uid, and gid fields must end in a space and a null byte; the
size and mtime fields must end in a space; the checksum is
terminated by a null and a space. Early implementations
filled the numeric fields with leading spaces. This seems to
have been common practice until the IEEE Std 1003.1-1988
(&ldquo;POSIX.1&rdquo;) standard was released. For best
portability, modern implementations should fill the numeric
fields with leading zeros.</p>

<p style="margin-left:6%; margin-top: 1em"><b>Pre-POSIX
Archives</b> <br>
An early draft of IEEE Std 1003.1-1988
(&ldquo;POSIX.1&rdquo;) served as the basis for John
Gilmore&rsquo;s <b>pdtar</b> program and many system
implementations from the late 1980s and early 1990s. These
archives generally follow the POSIX ustar format described
below with the following variations:</p>

<p><b>&bull;</b></p>

<p style="margin-left:17%;">The magic value consists of the
five characters &ldquo;ustar&rdquo; followed by a space. The
version field contains a space character followed by a
null.</p>

<p><b>&bull;</b></p>

<p style="margin-left:17%;">The numeric fields are
generally filled with leading spaces (not leading zeros as
recommended in the final standard).</p>

<p><b>&bull;</b></p>

<p style="margin-left:17%;">The prefix field is often not
used, limiting pathnames to the 100 characters of old-style
archives.</p>

<p style="margin-left:6%; margin-top: 1em"><b>POSIX ustar
Archives</b> <br>
IEEE Std 1003.1-1988 (&ldquo;POSIX.1&rdquo;) defined a
standard tar file format to be read and written by compliant
implementations of tar(1). This format is often called the
&ldquo;ustar&rdquo; format, after the magic value used in
the header. (The name is an acronym for &ldquo;Unix Standard
TAR&rdquo;.) It extends the historic format with new
fields:</p>

<p style="margin-left:14%; margin-top: 1em">struct
header_posix_ustar {</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char name[100];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char mode[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char uid[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char gid[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char size[12];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char mtime[12];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char checksum[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char typeflag[1];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char linkname[100];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char magic[6];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char version[2];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char uname[32];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char gname[32];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char devmajor[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char devminor[8];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char prefix[155];</p></td>
<td width="65%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char pad[12];</p></td>
<td width="65%">
</td></tr>
</table>

<p style="margin-left:14%;">};</p>

<p style="margin-top: 1em"><i>typeflag</i></p>

<p style="margin-left:17%;">Type of entry. POSIX extended
the earlier <i>linkflag</i> field with several new type
values:</p>

<p>&ldquo;0&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">Regular file.
NUL should be treated as a synonym, for compatibility
purposes.</p>

<p>&ldquo;1&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">Hard link.</p>

<p>&ldquo;2&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">Symbolic
link.</p>

<p>&ldquo;3&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">Character
device node.</p>

<p>&ldquo;4&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">Block device
node.</p>

<p>&ldquo;5&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">Directory.</p>

<p>&ldquo;6&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">FIFO node.</p>

<p>&ldquo;7&rdquo;</p>

<p style="margin-left:27%; margin-top: 1em">Reserved.</p>

<p>Other</p>

<p style="margin-left:27%; margin-top: 1em">A
POSIX-compliant implementation must treat any unrecognized
typeflag value as a regular file. In particular, writers
should ensure that all entries have a valid filename so that
they can be restored by readers that do not support the
corresponding extension. Uppercase letters &quot;A&quot;
through &quot;Z&quot; are reserved for custom extensions.
Note that sockets and whiteout entries are not
archivable.</p>

<p style="margin-left:17%;">It is worth noting that the
<i>size</i> field, in particular, has different meanings
depending on the type. For regular files, of course, it
indicates the amount of data following the header. For
directories, it may be used to indicate the total size of
all files in the directory, for use by operating systems
that pre-allocate directory space. For all other types, it
should be set to zero by writers and ignored by readers.</p>

<p style="margin-top: 1em"><i>magic</i></p>

<p style="margin-left:17%; margin-top: 1em">Contains the
magic value &ldquo;ustar&rdquo; followed by a NUL byte to
indicate that this is a POSIX standard archive. Full
compliance requires the uname and gname fields be properly
set.</p>

<p style="margin-top: 1em"><i>version</i></p>

<p style="margin-left:17%;">Version. This should be
&ldquo;00&rdquo; (two copies of the ASCII digit zero) for
POSIX standard archives.</p>

<p style="margin-top: 1em"><i>uname</i>, <i>gname</i></p>

<p style="margin-left:17%;">User and group names, as
null-terminated ASCII strings. These should be used in
preference to the uid/gid values when they are set and the
corresponding names exist on the system.</p>

<p style="margin-top: 1em"><i>devmajor</i>,
<i>devminor</i></p>

<p style="margin-left:17%;">Major and minor numbers for
character device or block device entry.</p>

<p style="margin-top: 1em"><i>name</i>, <i>prefix</i></p>

<p style="margin-left:17%;">If the pathname is too long to
fit in the 100 bytes provided by the standard format, it can
be split at any <i>/</i> character with the first portion
going into the prefix field. If the prefix field is not
empty, the reader will prepend the prefix value and a
<i>/</i> character to the regular name field to obtain the
full pathname. The standard does not require a trailing
<i>/</i> character on directory names, though most
implementations still include this for compatibility
reasons.</p>

<p style="margin-left:6%; margin-top: 1em">Note that all
unused bytes must be set to NUL.</p>

<p style="margin-left:6%; margin-top: 1em">Field
termination is specified slightly differently by POSIX than
by previous implementations. The <i>magic</i>, <i>uname</i>,
and <i>gname</i> fields must have a trailing NUL. The
<i>pathname</i>, <i>linkname</i>, and <i>prefix</i> fields
must have a trailing NUL unless they fill the entire field.
(In particular, it is possible to store a 256-character
pathname if it happens to have a <i>/</i> as the 156th
character.) POSIX requires numeric fields to be zero-padded
in the front, and requires them to be terminated with either
space or NUL characters.</p>

<p style="margin-left:6%; margin-top: 1em">Currently, most
tar implementations comply with the ustar format,
occasionally extending it by adding new fields to the blank
area at the end of the header record.</p>

<p style="margin-left:6%; margin-top: 1em"><b>Numeric
Extensions</b> <br>
There have been several attempts to extend the range of
sizes or times supported by modifying how numbers are stored
in the header.</p>

<p style="margin-left:6%; margin-top: 1em">One obvious
extension to increase the size of files is to eliminate the
terminating characters from the various numeric fields. For
example, the standard only allows the size field to contain
11 octal digits, reserving the twelfth byte for a trailing
NUL character. Allowing 12 octal digits allows file sizes up
to 64 GB.</p>

<p style="margin-left:6%; margin-top: 1em">Another
extension, utilized by GNU tar, star, and other newer
<b>tar</b> implementations, permits binary numbers in the
standard numeric fields. This is flagged by setting the high
bit of the first byte. The remainder of the field is treated
as a signed twos-complement value. This permits 95-bit
values for the length and time fields and 63-bit values for
the uid, gid, and device numbers. In particular, this
provides a consistent way to handle negative time values.
GNU tar supports this extension for the length, mtime,
ctime, and atime fields. Joerg Schilling&rsquo;s star
program and the libarchive library support this extension
for all numeric fields. Note that this extension is largely
obsoleted by the extended attribute record provided by the
pax interchange format.</p>

<p style="margin-left:6%; margin-top: 1em">Another early
GNU extension allowed base-64 values rather than octal. This
extension was short-lived and is no longer supported by any
implementation.</p>

<p style="margin-left:6%; margin-top: 1em"><b>Pax
Interchange Format</b> <br>
There are many attributes that cannot be portably stored in
a POSIX ustar archive. IEEE Std 1003.1-2001
(&ldquo;POSIX.1&rdquo;) defined a &ldquo;pax interchange
format&rdquo; that uses two new types of entries to hold
text-formatted metadata that applies to following entries.
Note that a pax interchange format archive is a ustar
archive in every respect. The new data is stored in
ustar-compatible archive entries that use the
&ldquo;x&rdquo; or &ldquo;g&rdquo; typeflag. In particular,
older implementations that do not fully support these
extensions will extract the metadata into regular files,
where the metadata can be examined as necessary.</p>

<p style="margin-left:6%; margin-top: 1em">An entry in a
pax interchange format archive consists of one or two
standard ustar entries, each with its own header and data.
The first optional entry stores the extended attributes for
the following entry. This optional first entry has an
&quot;x&quot; typeflag and a size field that indicates the
total size of the extended attributes. The extended
attributes themselves are stored as a series of text-format
lines encoded in the portable UTF-8 encoding. Each line
consists of a decimal number, a space, a key string, an
equals sign, a value string, and a new line. The decimal
number indicates the length of the entire line, including
the initial length field and the trailing newline. An
example of such a field is:</p>

<p style="margin-left:14%;">25 ctime=1084839148.1212\n</p>

<p style="margin-left:6%;">Keys in all lowercase are
standard keys. Vendors can add their own keys by prefixing
them with an all uppercase vendor name and a period. Note
that, unlike the historic header, numeric values are stored
using decimal, not octal. A description of some common keys
follows:</p>

<p style="margin-top: 1em"><b>atime</b>, <b>ctime</b>,
<b>mtime</b></p>

<p style="margin-left:17%;">File access, inode change, and
modification times. These fields can be negative or include
a decimal point and a fractional value.</p>

<p style="margin-top: 1em"><b>hdrcharset</b></p>

<p style="margin-left:17%;">The character set used by the
pax extension values. By default, all textual values in the
pax extended attributes are assumed to be in UTF-8,
including pathnames, user names, and group names. In some
cases, it is not possible to translate local conventions
into UTF-8. If this key is present and the value is the
six-character ASCII string &ldquo;BINARY&rdquo;, then all
textual values are assumed to be in a platform-dependent
multi-byte encoding. Note that there are only two valid
values for this key: &ldquo;BINARY&rdquo; or
&ldquo;ISO-IR&nbsp;10646&nbsp;2000&nbsp;UTF-8&rdquo;. No
other values are permitted by the standard, and the latter
value should generally not be used as it is the default when
this key is not specified. In particular, this flag should
not be used as a general mechanism to allow filenames to be
stored in arbitrary encodings.</p>

<p style="margin-top: 1em"><b>uname</b>, <b>uid</b>,
<b>gname</b>, <b>gid</b></p>

<p style="margin-left:17%;">User name, group name, and
numeric UID and GID values. The user name and group name
stored here are encoded in UTF8 and can thus include
non-ASCII characters. The UID and GID fields can be of
arbitrary length.</p>

<p style="margin-top: 1em"><b>linkpath</b></p>

<p style="margin-left:17%;">The full path of the linked-to
file. Note that this is encoded in UTF8 and can thus include
non-ASCII characters.</p>

<p style="margin-top: 1em"><b>path</b></p>

<p style="margin-left:17%; margin-top: 1em">The full
pathname of the entry. Note that this is encoded in UTF8 and
can thus include non-ASCII characters.</p>

<p style="margin-top: 1em"><b>realtime.*</b>,
<b>security.*</b></p>

<p style="margin-left:17%;">These keys are reserved and may
be used for future standardization.</p>

<p style="margin-top: 1em"><b>size</b></p>

<p style="margin-left:17%; margin-top: 1em">The size of the
file. Note that there is no length limit on this field,
allowing conforming archives to store files much larger than
the historic 8GB limit.</p>

<p style="margin-top: 1em"><b>SCHILY.*</b></p>

<p style="margin-left:17%;">Vendor-specific attributes used
by Joerg Schilling&rsquo;s <b>star</b> implementation.</p>

<p style="margin-top: 1em"><b>SCHILY.acl.access</b>,
<b>SCHILY.acl.default</b>, <b>SCHILY.acl.ace</b></p>

<p style="margin-left:17%;">Stores the access, default and
NFSv4 ACLs as textual strings in a format that is an
extension of the format specified by POSIX.1e draft 17. In
particular, each user or group access specification can
include an additional colon-separated field with the numeric
UID or GID. This allows ACLs to be restored on systems that
may not have complete user or group information available
(such as when NIS/YP or LDAP services are temporarily
unavailable).</p>

<p style="margin-top: 1em"><b>SCHILY.devminor</b>,
<b>SCHILY.devmajor</b></p>

<p style="margin-left:17%;">The full minor and major
numbers for device nodes.</p>

<p style="margin-top: 1em"><b>SCHILY.fflags</b></p>

<p style="margin-left:17%;">The file flags.</p>

<p style="margin-top: 1em"><b>SCHILY.realsize</b></p>

<p style="margin-left:17%;">The full size of the file on
disk. XXX explain? XXX</p>

<p style="margin-top: 1em"><b>SCHILY.dev</b>,
<b>SCHILY.ino</b>, <b>SCHILY.nlinks</b></p>

<p style="margin-left:17%;">The device number, inode
number, and link count for the entry. In particular, note
that a pax interchange format archive using Joerg
Schilling&rsquo;s <b>SCHILY.*</b> extensions can store all
of the data from <i>struct stat</i>.</p>

<p style="margin-top: 1em"><b>LIBARCHIVE.*</b></p>

<p style="margin-left:17%;">Vendor-specific attributes used
by the <b>libarchive</b> library and programs that use
it.</p>


<p style="margin-top: 1em"><b>LIBARCHIVE.creationtime</b></p>

<p style="margin-left:17%;">The time when the file was
created. (This should not be confused with the POSIX
&ldquo;ctime&rdquo; attribute, which refers to the time when
the file metadata was last changed.)</p>


<p style="margin-top: 1em"><b>LIBARCHIVE.xattr</b>.<i>namespace</i>.<i>key</i></p>

<p style="margin-left:17%;">Libarchive stores
POSIX.1e-style extended attributes using keys of this form.
The <i>key</i> value is URL-encoded: All non-ASCII
characters and the two special characters &ldquo;=&rdquo;
and &ldquo;%&rdquo; are encoded as &ldquo;%&rdquo; followed
by two uppercase hexadecimal digits. The value of this key
is the extended attribute value encoded in base 64. XXX
Detail the base-64 format here XXX</p>

<p style="margin-top: 1em"><b>VENDOR.*</b></p>

<p style="margin-left:17%;">XXX document other
vendor-specific extensions XXX</p>

<p style="margin-left:6%; margin-top: 1em">Any values
stored in an extended attribute override the corresponding
values in the regular tar header. Note that compliant
readers should ignore the regular fields when they are
overridden. This is important, as existing archivers are
known to store non-compliant values in the standard header
fields in this situation. There are no limits on length for
any of these fields. In particular, numeric fields can be
arbitrarily large. All text fields are encoded in UTF8.
Compliant writers should store only portable 7-bit ASCII
characters in the standard ustar header and use extended
attributes whenever a text value contains non-ASCII
characters.</p>

<p style="margin-left:6%; margin-top: 1em">In addition to
the <b>x</b> entry described above, the pax interchange
format also supports a <b>g</b> entry. The <b>g</b> entry is
identical in format, but specifies attributes that serve as
defaults for all subsequent archive entries. The <b>g</b>
entry is not widely used.</p>

<p style="margin-left:6%; margin-top: 1em">Besides the new
<b>x</b> and <b>g</b> entries, the pax interchange format
has a few other minor variations from the earlier ustar
format. The most troubling one is that hardlinks are
permitted to have data following them. This allows readers
to restore any hardlink to a file without having to rewind
the archive to find an earlier entry. However, it creates
complications for robust readers, as it is no longer clear
whether or not they should ignore the size field for
hardlink entries.</p>

<p style="margin-left:6%; margin-top: 1em"><b>GNU Tar
Archives</b> <br>
The GNU tar program started with a pre-POSIX format similar
to that described earlier and has extended it using several
different mechanisms: It added new fields to the empty space
in the header (some of which was later used by POSIX for
conflicting purposes); it allowed the header to be continued
over multiple records; and it defined new entries that
modify following entries (similar in principle to the
<b>x</b> entry described above, but each GNU special entry
is single-purpose, unlike the general-purpose <b>x</b>
entry). As a result, GNU tar archives are not POSIX
compatible, although more lenient POSIX-compliant readers
can successfully extract most GNU tar archives.</p>

<p style="margin-left:14%; margin-top: 1em">struct
header_gnu_tar {</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char name[100];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char mode[8];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char uid[8];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char gid[8];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char size[12];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char mtime[12];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char checksum[8];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char typeflag[1];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char linkname[100];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char magic[6];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char version[2];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char uname[32];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char gname[32];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char devmajor[8];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char devminor[8];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char atime[12];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char ctime[12];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char offset[12];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char longnames[4];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char unused[1];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>struct {</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">
</td>
<td width="10%">


<p>char offset[12];</p></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">
</td>
<td width="10%">


<p>char numbytes[12];</p></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>} sparse[4];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char isextended[1];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char realsize[12];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
<tr valign="top" align="left">
<td width="24%"></td>
<td width="11%">


<p>char pad[17];</p></td>
<td width="10%"></td>
<td width="55%">
</td></tr>
</table>

<p style="margin-left:14%;">};</p>

<p style="margin-top: 1em"><i>typeflag</i></p>

<p style="margin-left:17%;">GNU tar uses the following
special entry types, in addition to those defined by
POSIX:</p>

<p style="margin-top: 1em">7</p>

<p style="margin-left:27%; margin-top: 1em">GNU tar treats
type &quot;7&quot; records identically to type &quot;0&quot;
records, except on one obscure RTOS where they are used to
indicate the pre-allocation of a contiguous file on
disk.</p>

<p style="margin-top: 1em">D</p>

<p style="margin-left:27%; margin-top: 1em">This indicates
a directory entry. Unlike the POSIX-standard &quot;5&quot;
typeflag, the header is followed by data records listing the
names of files in this directory. Each name is preceded by
an ASCII &quot;Y&quot; if the file is stored in this archive
or &quot;N&quot; if the file is not stored in this archive.
Each name is terminated with a null, and an extra null marks
the end of the name list. The purpose of this entry is to
support incremental backups; a program restoring from such
an archive may wish to delete files on disk that did not
exist in the directory when the archive was made.</p>

<p style="margin-left:27%; margin-top: 1em">Note that the
&quot;D&quot; typeflag specifically violates POSIX, which
requires that unrecognized typeflags be restored as normal
files. In this case, restoring the &quot;D&quot; entry as a
file could interfere with subsequent creation of the
like-named directory.</p>

<p style="margin-top: 1em">K</p>

<p style="margin-left:27%; margin-top: 1em">The data for
this entry is a long linkname for the following regular
entry.</p>

<p style="margin-top: 1em">L</p>

<p style="margin-left:27%; margin-top: 1em">The data for
this entry is a long pathname for the following regular
entry.</p>

<p style="margin-top: 1em">M</p>

<p style="margin-left:27%; margin-top: 1em">This is a
continuation of the last file on the previous volume. GNU
multi-volume archives guarantee that each volume begins with
a valid entry header. To ensure this, a file may be split,
with part stored at the end of one volume, and part stored
at the beginning of the next volume. The &quot;M&quot;
typeflag indicates that this entry continues an existing
file. Such entries can only occur as the first or second
entry in an archive (the latter only if the first entry is a
volume label). The <i>size</i> field specifies the size of
this entry. The <i>offset</i> field at bytes 369-380
specifies the offset where this file fragment begins. The
<i>realsize</i> field specifies the total size of the file
(which must equal <i>size</i> plus <i>offset</i>). When
extracting, GNU tar checks that the header file name is the
one it is expecting, that the header offset is in the
correct sequence, and that the sum of offset and size is
equal to realsize.</p>

<p style="margin-top: 1em">N</p>

<p style="margin-left:27%; margin-top: 1em">Type
&quot;N&quot; records are no longer generated by GNU tar.
They contained a list of files to be renamed or symlinked
after extraction; this was originally used to support long
names. The contents of this record are a text description of
the operations to be done, in the form &ldquo;Rename %s to
%s\n&rdquo; or &ldquo;Symlink %s to %s\n&rdquo;; in either
case, both filenames are escaped using K&amp;R C syntax. Due
to security concerns, &quot;N&quot; records are now
generally ignored when reading archives.</p>

<p style="margin-top: 1em">S</p>

<p style="margin-left:27%; margin-top: 1em">This is a
&ldquo;sparse&rdquo; regular file. Sparse files are stored
as a series of fragments. The header contains a list of
fragment offset/length pairs. If more than four such entries
are required, the header is extended as necessary with
&ldquo;extra&rdquo; header extensions (an older format that
is no longer used), or &ldquo;sparse&rdquo; extensions.</p>

<p style="margin-top: 1em">V</p>

<p style="margin-left:27%; margin-top: 1em">The <i>name</i>
field should be interpreted as a tape/volume header name.
This entry should generally be ignored on extraction.</p>

<p style="margin-top: 1em"><i>magic</i></p>

<p style="margin-left:17%; margin-top: 1em">The magic field
holds the five characters &ldquo;ustar&rdquo; followed by a
space. Note that POSIX ustar archives have a trailing
null.</p>

<p style="margin-top: 1em"><i>version</i></p>

<p style="margin-left:17%;">The version field holds a space
character followed by a null. Note that POSIX ustar archives
use two copies of the ASCII digit &ldquo;0&rdquo;.</p>

<p style="margin-top: 1em"><i>atime</i>, <i>ctime</i></p>

<p style="margin-left:17%;">The time the file was last
accessed and the time of last change of file information,
stored in octal as with <i>mtime</i>.</p>

<p style="margin-top: 1em"><i>longnames</i></p>

<p style="margin-left:17%;">This field is apparently no
longer used.</p>

<p style="margin-top: 1em">Sparse <i>offset /
numbytes</i></p>

<p style="margin-left:17%;">Each such structure specifies a
single fragment of a sparse file. The two fields store
values as octal numbers. The fragments are each padded to a
multiple of 512 bytes in the archive. On extraction, the
list of fragments is collected from the header (including
any extension headers), and the data is then read and
written to the file at appropriate offsets.</p>

<p style="margin-top: 1em"><i>isextended</i></p>

<p style="margin-left:17%;">If this is set to non-zero, the
header will be followed by additional &ldquo;sparse
header&rdquo; records. Each such record contains information
about as many as 21 additional sparse blocks as shown
here:</p>

<p style="margin-left:24%; margin-top: 1em">struct
gnu_sparse_header {</p>

<table width="100%" border="0" rules="none" frame="void"
       cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="35%"></td>
<td width="10%">


<p>struct {</p></td>
<td width="10%"></td>
<td width="45%">
</td></tr>
<tr valign="top" align="left">
<td width="35%"></td>
<td width="10%">
</td>
<td width="10%">


<p>char offset[12];</p></td>
<td width="45%">
</td></tr>
<tr valign="top" align="left">
<td width="35%"></td>
<td width="10%">
</td>
<td width="10%">


<p>char numbytes[12];</p></td>
<td width="45%">
</td></tr>
<tr valign="top" align="left">
<td width="35%"></td>
<td width="10%">


<p>} sparse[21];</p></td>
<td width="10%"></td>
<td width="45%">
</td></tr>
<tr valign="top" align="left">
<td width="35%"></td>
<td width="10%">


<p>char isextended[1];</p></td>
<td width="10%"></td>
<td width="45%">
</td></tr>
<tr valign="top" align="left">
<td width="35%"></td>
<td width="10%">


<p>char padding[7];</p></td>
<td width="10%"></td>
<td width="45%">
</td></tr>
</table>

<p style="margin-left:24%;">};</p>

<p style="margin-top: 1em"><i>realsize</i></p>

<p style="margin-left:17%;">A binary representation of the
file&rsquo;s complete size, with a much larger range than
the POSIX file size. In particular, with <b>M</b> type
files, the current entry is only a portion of the file. In
that case, the POSIX size field will indicate the size of
this entry; the <i>realsize</i> field will indicate the
total size of the file.</p>

<p style="margin-left:6%; margin-top: 1em"><b>GNU tar pax
archives</b> <br>
GNU tar 1.14 (XXX check this XXX) and later will write pax
interchange format archives when you specify the
<b>--posix</b> flag. This format follows the pax interchange
format closely, using some <b>SCHILY</b> tags and
introducing new keywords to store sparse file information.
There have been three iterations of the sparse file support,
referred to as &ldquo;0.0&rdquo;, &ldquo;0.1&rdquo;, and
&ldquo;1.0&rdquo;.</p>

<p style="margin-top: 1em"><b>GNU.sparse.numblocks</b>,
<b>GNU.sparse.offset</b>, <b>GNU.sparse.numbytes</b>,
<b>GNU.sparse.size</b></p>

<p style="margin-left:17%;">The &ldquo;0.0&rdquo; format
used an initial <b>GNU.sparse.numblocks</b> attribute to
indicate the number of blocks in the file, a pair of
<b>GNU.sparse.offset</b> and <b>GNU.sparse.numbytes</b> to
indicate the offset and size of each block, and a single
<b>GNU.sparse.size</b> to indicate the full size of the
file. This is not the same as the size in the tar header
because the latter value does not include the size of any
holes. This format required that the order of attributes be
preserved and relied on readers accepting multiple
appearances of the same attribute names, which is not
officially permitted by the standards.</p>

<p style="margin-top: 1em"><b>GNU.sparse.map</b></p>

<p style="margin-left:17%;">The &ldquo;0.1&rdquo; format
used a single attribute that stored a comma-separated list
of decimal numbers. Each pair of numbers indicated the
offset and size, respectively, of a block of data. This does
not work well if the archive is extracted by an archiver
that does not recognize this extension, since many pax
implementations simply discard unrecognized attributes.</p>

<p style="margin-top: 1em"><b>GNU.sparse.major</b>,
<b>GNU.sparse.minor</b>, <b>GNU.sparse.name</b>,
<b>GNU.sparse.realsize</b></p>

<p style="margin-left:17%;">The &ldquo;1.0&rdquo; format
stores the sparse block map in one or more 512-byte blocks
prepended to the file data in the entry body. The pax
attributes indicate the existence of this map (via the
<b>GNU.sparse.major</b> and <b>GNU.sparse.minor</b> fields)
and the full size of the file. The <b>GNU.sparse.name</b>
holds the true name of the file. To avoid confusion, the
name stored in the regular tar header is a modified name so
that extraction errors will be apparent to users.</p>

<p style="margin-left:6%; margin-top: 1em"><b>Solaris
Tar</b> <br>
XXX More Details Needed XXX</p>

<p style="margin-left:6%; margin-top: 1em">Solaris tar
(beginning with SunOS XXX 5.7 ?? XXX) supports an
&ldquo;extended&rdquo; format that is fundamentally similar
to pax interchange format, with the following
differences:</p>

<p><b>&bull;</b></p>

<p style="margin-left:17%;">Extended attributes are stored
in an entry whose type is <b>X</b>, not <b>x</b>, as used by
pax interchange format. The detailed format of this entry
appears to be the same as detailed above for the <b>x</b>
entry.</p>

<p><b>&bull;</b></p>

<p style="margin-left:17%;">An additional <b>A</b> header
is used to store an ACL for the following regular entry. The
body of this entry contains a seven-digit octal number
followed by a zero byte, followed by the textual ACL
description. The octal value is the number of ACL entries
plus a constant that indicates the ACL type: 01000000 for
POSIX.1e ACLs and 03000000 for NFSv4 ACLs.</p>

<p style="margin-left:6%; margin-top: 1em"><b>AIX Tar</b>
<br>
XXX More details needed XXX</p>

<p style="margin-left:6%; margin-top: 1em">AIX Tar uses a
ustar-formatted header with the type <b>A</b> for storing
coded ACL information. Unlike the Solaris format, AIX tar
writes this header after the regular file body to which it
applies. The pathname in this header is either <b>NFS4</b>
or <b>AIXC</b> to indicate the type of ACL stored. The
actual ACL is stored in platform-specific binary format.</p>

<p style="margin-left:6%; margin-top: 1em"><b>Mac OS X
Tar</b> <br>
The tar distributed with Apple&rsquo;s Mac OS X stores most
regular files as two separate files in the tar archive. The
two files have the same name except that the first one has
&ldquo;._&rdquo; prepended to the last path element. This
special file stores an AppleDouble-encoded binary blob with
additional metadata about the second file, including ACL,
extended attributes, and resources. To recreate the original
file on disk, each separate file can be extracted and the
Mac OS X <b>copyfile</b>() function can be used to unpack
the separate metadata file and apply it to th regular file.
Conversely, the same function provides a &ldquo;pack&rdquo;
option to encode the extended metadata from a file into a
separate file whose contents can then be put into a tar
archive.</p>

<p style="margin-left:6%; margin-top: 1em">Note that the
Apple extended attributes interact badly with long
filenames. Since each file is stored with the full name, a
separate set of extensions needs to be included in the
archive for each one, doubling the overhead required for
files with long names.</p>

<p style="margin-left:6%; margin-top: 1em"><b>Summary of
tar type codes</b> <br>
The following list is a condensed summary of the type codes
used in tar header records generated by different tar
implementations. More details about specific implementations
can be found above:</p>

<p>NUL</p>

<p style="margin-left:13%; margin-top: 1em">Early tar
programs stored a zero byte for regular files.</p>

<p><b>0</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX standard
type code for a regular file.</p>

<p><b>1</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX standard
type code for a hard link description.</p>

<p><b>2</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX standard
type code for a symbolic link description.</p>

<p><b>3</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX standard
type code for a character device node.</p>

<p><b>4</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX standard
type code for a block device node.</p>

<p><b>5</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX standard
type code for a directory.</p>

<p><b>6</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX standard
type code for a FIFO.</p>

<p><b>7</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX
reserved.</p>

<p><b>7</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar used
for pre-allocated files on some systems.</p>

<p><b>A</b></p>

<p style="margin-left:13%; margin-top: 1em">Solaris tar ACL
description stored prior to a regular file header.</p>

<p><b>A</b></p>

<p style="margin-left:13%; margin-top: 1em">AIX tar ACL
description stored after the file body.</p>

<p><b>D</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar
directory dump.</p>

<p><b>K</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar long
linkname for the following header.</p>

<p><b>L</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar long
pathname for the following header.</p>

<p><b>M</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar
multivolume marker, indicating the file is a continuation of
a file from the previous volume.</p>

<p><b>N</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar long
filename support. Deprecated.</p>

<p><b>S</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar sparse
regular file.</p>

<p><b>V</b></p>

<p style="margin-left:13%; margin-top: 1em">GNU tar
tape/volume header name.</p>

<p><b>X</b></p>

<p style="margin-left:13%; margin-top: 1em">Solaris tar
general-purpose extension header.</p>

<p><b>g</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX pax
interchange format global extensions.</p>

<p><b>x</b></p>

<p style="margin-left:13%; margin-top: 1em">POSIX pax
interchange format per-file extensions.</p>

<p style="margin-top: 1em"><b>SEE ALSO</b></p>

<p style="margin-left:6%;">ar(1), pax(1), tar(1)</p>

<p style="margin-top: 1em"><b>STANDARDS</b></p>

<p style="margin-left:6%;">The <b>tar</b> utility is no
longer a part of POSIX or the Single Unix Standard. It last
appeared in Version&nbsp;2 of the Single UNIX Specification
(&ldquo;SUSv2&rdquo;). It has been supplanted in subsequent
standards by pax(1). The ustar format is currently part of
the specification for the pax(1) utility. The pax
interchange file format is new with IEEE Std 1003.1-2001
(&ldquo;POSIX.1&rdquo;).</p>

<p style="margin-top: 1em"><b>HISTORY</b></p>

<p style="margin-left:6%;">A <b>tar</b> command appeared in
Seventh Edition Unix, which was released in January, 1979.
It replaced the <b>tp</b> program from Fourth Edition Unix
which in turn replaced the <b>tap</b> program from First
Edition Unix. John Gilmore&rsquo;s <b>pdtar</b>
public-domain implementation (circa 1987) was highly
influential and formed the basis of <b>GNU tar</b> (circa
1988). Joerg Shilling&rsquo;s <b>star</b> archiver is
another open-source (CDDL) archiver (originally developed
circa 1985) which features complete support for pax
interchange format.</p>

<p style="margin-left:6%; margin-top: 1em">This
documentation was written as part of the <b>libarchive</b>
and <b>bsdtar</b> project by Tim Kientzle
&lt;[email protected]&gt;.</p>

<p style="margin-left:6%; margin-top: 1em">BSD
December&nbsp;27, 2016 BSD</p>
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