freebsd-dev/lib/libarchive/tar.5

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.Dd October 1, 2003
.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---and the maximum supported by historical
implementations---is 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 a variant that is compatible with
most historic
.Nm
implementations.
.Pp
The header record for an old-style
.Nm
archive consists of the following:
.Bd -literal -offset indent
struct tarfile_header_old {
  char name[100];
  char mode[8];
  char uid[8];
  char gid[8];
  char size[12];
  char mtime[12];
  char checksum[8];
};
.Ed
The remaining bytes in the header record are filled with nulls.
.Bl -tag -width indent
.It Va name
Pathname, stored as a null-terminated string.
Some very early implementations only supported regular files.
However, a common early convention added
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 number in ASCII.
.It Va size
Size of file, as octal number in ASCII.
.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 for many years, Sun tar used signed arithmetic
for the checksum field, which can cause interoperability problems
when transferring archives between systems.
This error was propagated to other implementations that used Sun
tar as a reference.
Modern robust readers compute the checksum both ways and accept the
header if either computation matches.
.El
.Pp
Early implementations of
.Nm
varied in how they terminated these fields.
Early BSD documentation specified the following: 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.
For best portability, writers of
.Nm
archives should fill the numeric fields with leading zeros.
.Ss Early Extensions
Very early
.Nm
implementations only supported regular files.
Two early extensions added support for directories, hard links, and
symbolic links.
.Pp
Early
.Nm
archives indicated directories by adding a trailing
.Pa /
to the name.
The size field was often used to indicate the total size of all files
in the directory.
This was intended to facilitate extraction on systems that pre-allocated
directory storage; most modern readers should simply ignore the
size field for directories.
.Pp
To support hard links and symbolic links,
.Va linktype
and
.Va linkname
fields were added:
.Bd -literal -offset indent
struct tarfile_entry_common {
  char name[100];
  char mode[8];
  char uid[8];
  char gid[8];
  char size[12];
  char mtime[12];
  char checksum[8];
  char linktype[1];
  char linkname[100];
};
.Ed
.Pp
The
.Va linktype
field indicates the type of entry.
For backwards compatibility, a NULL
character here indicates a regular file or directory.
An ASCII "1" here indicates a hard link entry, ASCII "2" indicates
a symbolic link.
The
.Va linkname
field holds the name of the file linked to.
.Ss POSIX Standard Archives
POSIX 1003.1 defines a standard
.Nm
file format that is read and written
by POSIX-compliant implementations
of
.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 format above
with new fields:
.Bd -literal -offset indent
struct tarfile_entry_posix {
  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];
};
.Ed
.Bl -tag -width indent
.It Va typeflag
Type of entry.  POSIX adopted the BSD
.Va linktype
field and extended it 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 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.
(Note that GNU tar archives uses a trailing space rather than a trailing
NULL here and are therefore not POSIX standard archives.)
.It Va version
Version.  This should be
.Dq 00
(two copies of the ASCII digit zero) for POSIX standard archives.
(Note that GNU tar archives fill this with a space and a null.)
.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.
.Ss Pax Interchange Format
There are many attributes that cannot be portably stored in a
POSIX ustar archive.
POSIX 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 entries, each with its own header and data.
The first optional entry stores the extended attributes
for the second 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.
Keys are always encoded in portable 7-bit ASCII.
Keys in all lowercase are reserved for future standardization.
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.
.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 by SUSv3 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
.Nm
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 XXXX draft 17.
In particular, each user or group access specification can include a fourth
field with the integer 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.ino
The inode number for the entry.
.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.
.Ss GNU Tar Archives
The GNU tar program added new features by starting with an early draft
of POSIX and using three different extension mechanisms:  They added
new fields to the empty space in the header (some of which was later
used by POSIX for conflicting purposes);
they allowed the header to
be continued over multiple records;
and they 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 tarfile_entry_gnu {
  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];
};
.Ed
.Bl -tag -width indent
.It Va typeflag
GNU tar uses the following special entry types.
.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 gaurantee 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 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 archive 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 size, with a much larger range
than the POSIX file size.
.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 any official standard.
It last appeared in SUSv2.
It has been supplanted in subsequent standards by
.Xr pax 1 .
The ustar format is defined in
.St -p1003.1
as 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 Sixth Edition Unix.
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.