freebsd-skq/lib/libarchive/tar.5

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.Dd May 20, 2004
.Dt TAR 5
.Os
.Sh NAME
.Nm tar
.Nd format of tape archive files
.Sh DESCRIPTION
The
.Nm
archive format collects any number of files, directories, and other
filesystem 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.
.Ss General Format
A
.Nm
archive consists of a series of 512-byte records.
Each filesystem 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.
.Pp
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
.Dq blocks
are always a multiple of the record size.
The most common block size\(emand the maximum supported by historic
implementations\(emis 10240 bytes or 20 records.
(Note: the terms
.Dq block
and
.Dq record
here are not entirely standard; this document follows the
convention established by John Gilmore in documenting
.Nm pdtar . )
.Ss Old-Style Archive Format
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
.At v7 ,
which is one of the earliest widely-used versions of the tar program.
.Pp
The header record for an old-style
.Nm
archive consists of the following:
.Bd -literal -offset indent
struct header_old_tar {
	char name[100];
	char mode[8];
	char uid[8];
	char gid[8];
	char size[12];
	char mtime[12];
	char checksum[8];
	char linkflag[1];
	char linkname[100];
	char pad[255];
};
.Ed
All unused bytes in the header record are filled with nulls.
.Bl -tag -width indent
.It Va name
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 "/" character to indicate
a directory name, allowing directory permissions and owner information
to be archived and restored.
.It Va mode
File mode, stored as an octal number in ASCII.
.It Va uid , Va gid
User id and group id of owner, as octal numbers in ASCII.
.It Va size
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.
.It Va mtime
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.
.It Va checksum
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.
.It Va linkflag , Va linkname
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
.Va linkflag
is set to an ASCII
.Sq 1
and the
.Va linkname
field holds the first name under which this file appears.
(Note that regular files have a null value in the
.Va linkflag
field.)
.El
.Pp
Early tar implementations varied in how they terminated these fields.
The tar command in
.At v7
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
.St -p1003.1
standard was released.
For best portability, modern implementations should fill the numeric
fields with leading zeros.
.Ss Pre-POSIX Archives
An early draft of
.St -p1003.1-88
served as the basis for John Gilmore's
.Nm pdtar
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:
.Bl -bullet -compact -width indent
.It
The magic value is
.Dq ustar\ \&
(note the following space).
The version field contains a space character followed by a null.
.It
The numeric fields are generally filled with leading spaces
(not leading zeros as recommended in the final standard).
.It
The prefix field is often not used, limiting pathnames to
the 100 characters of old-style archives.
.El
.Ss POSIX ustar Archives
.St -p1003.1-88
defined a standard tar file format to be read and written
by compliant implementations of
.Xr tar 1
and
.Xr pax 1 .
This format is often called the
.Dq ustar
format, after the magic value used
in the header.
(The name is an acronym for
.Dq Unix Standard TAR. )
It extends the historic format with new fields:
.Bd -literal -offset indent
struct header_posix_ustar {
	char name[100];
	char mode[8];
	char uid[8];
	char gid[8];
	char size[12];
	char mtime[12];
	char checksum[8];
	char typeflag[1];
	char linkname[100];
	char magic[6];
	char version[2];
	char uname[32];
	char gname[32];
	char devmajor[8];
	char devminor[8];
	char prefix[155];
	char pad[12];
};
.Ed
.Bl -tag -width indent
.It Va typeflag
Type of entry.
POSIX extended the earlier
.Va linkflag
field with several new type values:
.Bl -tag -width indent -compact
.It Dq 0
Regular file.
NULL should be treated as a synonym, for compatibility purposes.
.It Dq 1
Hard link.
.It Dq 2
Symbolic link.
.It Dq 3
Character device node.
.It Dq 4
Block device node.
.It Dq 5
Directory.
.It Dq 6
FIFO node.
.It Dq 7
Reserved.
.It Other
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 "A" through "Z" are reserved for custom extensions.
Note that sockets and whiteout entries are not archivable.
.El
It is worth noting that the
.Va size
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.
.It Va magic
Contains the magic value
.Dq ustar
followed by a NULL byte to indicate that this is a POSIX standard archive.
Full compliance requires the uname and gname fields be properly set.
.It Va version
Version.
This should be
.Dq 00
(two copies of the ASCII digit zero) for POSIX standard archives.
.It Va uname , Va gname
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.
.It Va devmajor , Va devminor
Major and minor numbers for character device or block device entry.
.It Va prefix
First part of pathname.
If the pathname is too long to fit in the 100 bytes provided by the standard
format, it can be split at any
.Pa /
character with the first portion going here.
If the prefix field is not empty, the reader will prepend
the prefix value and a
.Pa /
character to the regular name field to obtain the full pathname.
.El
.Pp
Note that all unused bytes must be set to
.Dv NULL .
.Pp
Field termination is specified slightly differently by POSIX
than by previous implementations.
The
.Va magic ,
.Va uname ,
and
.Va gname
fields must have a trailing
.Dv NULL .
The
.Va pathname ,
.Va linkname ,
and
.Va prefix
fields must have a trailing
.Dv NULL
unless they fill the entire field.
(In particular, it is possible to store a 256-character pathname if it
happens to have a
.Pa /
as the 156th character.)
POSIX requires numeric fields to be zero-padded in the front, and allows
them to be terminated with either space or
.Dv NULL
characters.
.Pp
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.
.Ss Pax Interchange Format
There are many attributes that cannot be portably stored in a
POSIX ustar archive.
.St -p1003.1-2001
defined a
.Dq pax interchange format
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
.Dq x
or
.Dq g
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.
.Pp
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 "x" 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:
.Dl 25 ctime=1084839148.1212\en
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:
.Bl -tag -width indent
.It Cm atime , Cm ctime , Cm mtime
File access, inode change, and modification times.
These fields can be negative or include a decimal point and a fractional value.
.It Cm uname , Cm uid , Cm gname , Cm gid
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.
.It Cm linkpath
The full path of the linked-to file.
Note that this is encoded in UTF8 and can thus include non-ASCII characters.
.It Cm path
The full pathname of the entry.
Note that this is encoded in UTF8 and can thus include non-ASCII characters.
.It Cm realtime.* , Cm security.*
These keys are reserved and may be used for future standardization.
.It Cm size
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.
.It Cm SCHILY.*
Vendor-specific attributes used by Joerg Schilling's
.Nm star
implementation.
.It Cm SCHILY.acl.access , Cm SCHILY.acl.default
Stores the access and default ACLs as textual strings in a format
that's an extension of the format specified by POSIX.1e draft 17.
In particular, each user or group access specification can include a fourth
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).
.It Cm SCHILY.devminor , Cm SCHILY.devmajor
The full minor and major numbers for device nodes.
.It Cm SCHILY.dev, Cm SCHILY.ino , Cm SCHILY.nlinks
The device number, inode number, and link count for the entry.
In particular, note that a pax interchange format archive using Joerg
Schilling's
Cm SCHILY.*
extensions can store all of the data from
.Va struct stat .
.It Cm VENDOR.*
XXX document other vendor-specific extensions XXX
.El
.Pp
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.
.Pp
In addition to the
.Cm x
entry described above, the pax interchange format
also supports a
.Cm g
entry.
The
.Cm g
entry is identical in format, but specifies attributes that serve as
defaults for all subsequent archive entries.
The
.Cm g
entry is not widely used.
.Pp
Besides the new
.Cm x
and
.Cm g
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.
.Ss GNU Tar Archives
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
.Cm x
entry described above, but each GNU special entry is single-purpose,
unlike the general-purpose
.Cm x
entry).
As a result, GNU tar archives are not POSIX compatible, although
more lenient POSIX-compliant readers can successfully extract most
GNU tar archives.
.Bd -literal -offset indent
struct header_gnu_tar {
	char name[100];
	char mode[8];
	char uid[8];
	char gid[8];
	char size[12];
	char mtime[12];
	char checksum[8];
	char typeflag[1];
	char linkname[100];
	char magic[6];
	char version[2];
	char uname[32];
	char gname[32];
	char devmajor[8];
	char devminor[8];
	char atime[12];
	char ctime[12];
	char offset[12];
	char longnames[4];
	char unused[1];
	struct {
		char offset[12];
		char numbytes[12];
	} sparse[4];
	char isextended[1];
	char realsize[12];
	char pad[17];
};
.Ed
.Bl -tag -width indent
.It Va typeflag
GNU tar uses the following special entry types, in addition to
those defined by POSIX:
.Bl -tag -width indent
.It "7"
GNU tar treats type "7" records identically to type "0" records,
except on one obscure RTOS where they are used to indicate the
pre-allocation of a contiguous file on disk.
.It "D"
This indicates a directory entry.
Unlike the POSIX-standard "5"
typeflag, the header is followed by data records listing the names
of files in this directory.
Each name is preceded by an ASCII "Y"
if the file is stored in this archive or "N" 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.
.Pp
Note that the "D" typeflag specifically violates POSIX, which requires
that unrecognized typeflags be restored as normal files.
In this case, restoring the "D" entry as a file could interfere
with subsequent creation of the like-named directory.
.It "K"
The data for this entry is a long linkname for the following regular entry.
.It "L"
The data for this entry is a long pathname for the following regular entry.
.It "M"
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 "M" 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
.Va size
field specifies the size of this entry.
The
.Va offset
field at bytes 369-380 specifies the offset where this file fragment
begins.
The
.Va realsize
field specifies the total size of the file (which must equal
.Va size
plus
.Va offset ) .
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.
FreeBSD's version of GNU tar does not handle the corner case of an
archive's being continued in the middle of a long name or other
extension header.
.It "N"
Type "N" 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
.Dq Rename %s to %s\en
or
.Dq Symlink %s to %s\en ;
in either case, both
filenames are escaped using K&R C syntax.
.It "S"
This is a
.Dq sparse
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
.Dq extra
header extensions (an older format that's no longer used), or
.Dq sparse
extensions.
.It "V"
The
.Va name
field should be interpreted as a tape/volume header name.
This entry should generally be ignored on extraction.
.El
.It Va magic
The magic field holds the five characters
.Dq ustar
followed by a space.
Note that POSIX ustar archives have a trailing null.
.It Va version
The version field holds a space character followed by a null.
Note that POSIX ustar archives use two copies of the ASCII digit
.Dq 0 .
.It Va atime , Va ctime
The time the file was last accessed and the time of
last change of file information, stored in octal as with
.Va mtime.
.It Va longnames
This field is apparently no longer used.
.It Sparse Va offset / Va numbytes
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.
.It Va isextended
If this is set to non-zero, the header will be followed by
additional
.Dq sparse header
records.
Each such record contains XXX more details needed XXX
.It Va realsize
A binary representation of the file's complete size, with a much larger range
than the POSIX file size.
In particular, with
.Cm M
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
.Va realsize
field will indicate the total size of the file.
.El
.Ss Solaris Tar
XXX More Details Needed XXX
.Pp
Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an
.Dq extended
format that is fundamentally similar to pax interchange format,
with the following differences:
.Bl -bullet -compact -width indent
.It
Extended attributes are stored in an entry whose type is
.Cm X ,
not
.Cm x ,
as used by pax interchange format.
The detailed format of this entry appears to be the same
as detailed above for the
.Cm x
entry.
.It
An additional
.Cm A
entry is used to store an ACL for the following regular entry.
The body of this entry contains a seven-digit octal number
(whose value is 01000000 plus the number of ACL entries)
followed by a zero byte, followed by the
textual ACL description.
.El
.Ss Other Extensions
One common extension, utilized by GNU tar, star, and other newer
.Nm
implementations, permits binary numbers in the standard numeric
fields.
This is flagged by setting the high bit of the first character.
This permits 95-bit values for the length and time fields
and 63-bit values for the uid, gid, and device numbers.
GNU tar supports this extension for the
length, mtime, ctime, and atime fields.
Joerg Schilling's star program supports this extension for
all numeric fields.
Note that this extension is largely obsoleted by the extended attribute
record provided by the pax interchange format.
.Pp
Another early GNU extension allowed base-64 values rather
than octal.
This extension was short-lived and such archives are almost never seen.
However, there is still code in GNU tar to support them; this code is
responsible for a very cryptic warning message that is sometimes seen when
GNU tar encounters a damaged archive.
.Sh SEE ALSO
.Xr ar 1 ,
.Xr pax 1 ,
.Xr tar 1
.Sh STANDARDS
The
.Nm tar
utility is no longer a part of POSIX or the Single Unix Standard.
It last appeared in
.St -susv2 .
It has been supplanted in subsequent standards by
.Xr pax 1 .
The ustar format is currently part of the specification for the
.Xr pax 1
utility.
The pax interchange file format is new with
.St -p1003.1-2001 .
.Sh HISTORY
A
.Nm tar
command appeared in Seventh Edition Unix, which was released in January, 1979.
John Gilmore's
.Nm pdtar
public-domain implementation (circa 1987) was highly influential
and formed the basis of GNU tar.
Joerg Shilling's
.Nm star
archiver is another open-source (GPL) archiver (originally developed
circa 1985) which features complete support for pax interchange
format.