tar.specs

TAR(5)			  FreeBSD File Formats Manual			TAR(5)

NAME
     tar -- format of tape archive files

DESCRIPTION
     The tar 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.

   General Format
     A tar 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.

     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 ``blocks'' are always a multiple
     of the record size.  The most common block size--and the maximum sup-
     ported by historic implementations--is 10240 bytes or 20 records.	(Note:
     the terms ``block'' and ``record'' here are not entirely standard; this
     document follows the convention established by John Gilmore in document-
     ing pdtar.)

   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
     Version 7 AT&T UNIX, which is one of the earliest widely-used versions of
     the tar program.

     The header record for an old-style tar archive consists of the following:

	   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];
	   };
     All unused bytes in the header record are filled with nulls.

     name    Pathname, stored as a null-terminated string.  Early tar imple-
	     mentations only stored regular files (including hardlinks to
	     those files).  One common early convention used a trailing "/"
	     character to indicate a directory name, allowing directory per-
	     missions and owner information to be archived and restored.

     mode    File mode, stored as an octal number in ASCII.

     uid, gid
	     User id and group id of owner, as octal numbers in ASCII.

     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.

     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.

     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 inter-
	     operability problems when transferring archives between systems.
	     Modern robust readers compute the checksum both ways and accept
	     the header if either computation matches.

     linkflag, 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 linkflag is
	     set to an ASCII `1' and the linkname field holds the first name
	     under which this file appears.  (Note that regular files have a
	     null value in the linkflag field.)

     Early tar implementations varied in how they terminated these fields.
     The tar command in Version 7 AT&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 prac-
     tice until the IEEE Std 1003.1-1988 (``POSIX.1'') standard was released.
     For best portability, modern implementations should fill the numeric
     fields with leading zeros.

   Pre-POSIX Archives
     An early draft of IEEE Std 1003.1-1988 (``POSIX.1'') served as the basis
     for John Gilmore's 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:
     o	     The magic value is ``ustar '' (note the following space).	The
	     version field contains a space character followed by a null.
     o	     The numeric fields are generally filled with leading spaces (not
	     leading zeros as recommended in the final standard).
     o	     The prefix field is often not used, limiting pathnames to the 100
	     characters of old-style archives.

   POSIX ustar Archives
     IEEE Std 1003.1-1988 (``POSIX.1'') defined a standard tar file format to
     be read and written by compliant implementations of tar(1).  This format
     is often called the ``ustar'' format, after the magic value used in the
     header.  (The name is an acronym for ``Unix Standard TAR''.)  It extends
     the historic format with new fields:

	   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];
	   };

     typeflag
	     Type of entry.  POSIX extended the earlier linkflag field with
	     several new type values:
	     ``0''   Regular file.  NULL should be treated as a synonym, for
		     compatibility purposes.
	     ``1''   Hard link.
	     ``2''   Symbolic link.
	     ``3''   Character device node.
	     ``4''   Block device node.
	     ``5''   Directory.
	     ``6''   FIFO node.
	     ``7''   Reserved.
	     Other   A POSIX-compliant implementation must treat any unrecog-
		     nized typeflag value as a regular file.  In particular,
		     writers should ensure that all entries have a valid file-
		     name 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.
	     It is worth noting that the size field, in particular, has dif-
	     ferent 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.

     magic   Contains the magic value ``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.

     version
	     Version.  This should be ``00'' (two copies of the ASCII digit
	     zero) for POSIX standard archives.

     uname, 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.

     devmajor, devminor
	     Major and minor numbers for character device or block device
	     entry.

     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 / character with the first portion going here.  If the prefix
	     field is not empty, the reader will prepend the prefix value and
	     a / character to the regular name field to obtain the full path-
	     name.

     Note that all unused bytes must be set to NULL.

     Field termination is specified slightly differently by POSIX than by pre-
     vious implementations.  The magic, uname, and gname fields must have a
     trailing NULL.  The pathname, linkname, and prefix fields must have a
     trailing NULL unless they fill the entire field.  (In particular, it is
     possible to store a 256-character pathname if it happens to have a / 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 NULL
     characters.

     Currently, most tar implementations comply with the ustar format, occa-
     sionally extending it by adding new fields to the blank area at the end
     of the header record.

   Pax Interchange Format
     There are many attributes that cannot be portably stored in a POSIX ustar
     archive.  IEEE Std 1003.1-2001 (``POSIX.1'') defined a ``pax interchange
     format'' that uses two new types of entries to hold text-formatted meta-
     data that applies to following entries.  Note that a pax interchange for-
     mat archive is a ustar archive in every respect.  The new data is stored
     in ustar-compatible archive entries that use the ``x'' or ``g'' typeflag.
     In particular, older implementations that do not fully support these
     extensions will extract the metadata into regular files, where the meta-
     data can be examined as necessary.

     An entry in a pax interchange format archive consists of one or two stan-
     dard 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 indi-
     cates 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:
	   25 ctime=1084839148.1212\n
     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 deci-
     mal, not octal.  A description of some common keys follows:

     atime, ctime, mtime
	     File access, inode change, and modification times.  These fields
	     can be negative or include a decimal point and a fractional
	     value.

     uname, uid, gname, 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.

     linkpath
	     The full path of the linked-to file.  Note that this is encoded
	     in UTF8 and can thus include non-ASCII characters.

     path    The full pathname of the entry.  Note that this is encoded in
	     UTF8 and can thus include non-ASCII characters.

     realtime.*, security.*
	     These keys are reserved and may be used for future standardiza-
	     tion.

     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.

     SCHILY.*
	     Vendor-specific attributes used by Joerg Schilling's star imple-
	     mentation.

     SCHILY.acl.access, SCHILY.acl.default
	     Stores the access and default 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 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).

     SCHILY.devminor, SCHILY.devmajor
	     The full minor and major numbers for device nodes.

     SCHILY.dev, SCHILY.ino, 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 SCHILY.* extensions can store all of the data
	     from struct stat.

     LIBARCHIVE.xattr.namespace.key
	     Libarchive stores POSIX.1e-style extended attributes using keys
	     of this form.  The key value is URL-encoded: All non-ASCII char-
	     acters and the two special characters ``='' and ``%'' are encoded
	     as ``%'' 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

     VENDOR.*
	     XXX document other vendor-specific extensions XXX

     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 arbitrar-
     ily 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.

     In addition to the x entry described above, the pax interchange format
     also supports a g entry.  The g entry is identical in format, but speci-
     fies attributes that serve as defaults for all subsequent archive
     entries.  The g entry is not widely used.

     Besides the new x and 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.

   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 mod-
     ify following entries (similar in principle to the x entry described
     above, but each GNU special entry is single-purpose, unlike the general-
     purpose x entry).	As a result, GNU tar archives are not POSIX compati-
     ble, although more lenient POSIX-compliant readers can successfully
     extract most GNU tar archives.

	   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];
	   };

     typeflag
	     GNU tar uses the following special entry types, in addition to
	     those defined by POSIX:

	     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.

	     D	     This indicates a directory entry.	Unlike the POSIX-stan-
		     dard "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 ar-
		     chive was made.

		     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.

	     K	     The data for this entry is a long linkname for the fol-
		     lowing regular entry.

	     L	     The data for this entry is a long pathname for the fol-
		     lowing regular entry.

	     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 vol-
		     ume.  The "M" typeflag indicates that this entry contin-
		     ues 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 size field spec-
		     ifies the size of this entry.  The offset field at bytes
		     369-380 specifies the offset where this file fragment
		     begins.  The realsize field specifies the total size of
		     the file (which must equal size plus 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.

	     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
		     ``Rename %s to %s\n'' or ``Symlink %s to %s\n''; in
		     either case, both filenames are escaped using K&R C syn-
		     tax.

	     S	     This is a ``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 nec-
		     essary with ``extra'' header extensions (an older format
		     that is no longer used), or ``sparse'' extensions.

	     V	     The name field should be interpreted as a tape/volume
		     header name.  This entry should generally be ignored on
		     extraction.

     magic   The magic field holds the five characters ``ustar'' followed by a
	     space.  Note that POSIX ustar archives have a trailing null.

     version
	     The version field holds a space character followed by a null.
	     Note that POSIX ustar archives use two copies of the ASCII digit
	     ``0''.

     atime, ctime
	     The time the file was last accessed and the time of last change
	     of file information, stored in octal as with mtime.

     longnames
	     This field is apparently no longer used.

     Sparse offset / 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.

     isextended
	     If this is set to non-zero, the header will be followed by addi-
	     tional ``sparse header'' records.	Each such record contains
	     information about as many as 21 additional sparse blocks as shown
	     here:

		   struct gnu_sparse_header {
			   struct {
				   char offset[12];
				   char numbytes[12];
			   } sparse[21];
			   char    isextended[1];
			   char    padding[7];
		   };

     realsize
	     A binary representation of the file's complete size, with a much
	     larger range than the POSIX file size.  In particular, with 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 realsize field will indicate the total size of the
	     file.

   Solaris Tar
     XXX More Details Needed XXX

     Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an
     ``extended'' format that is fundamentally similar to pax interchange for-
     mat, with the following differences:
     o	     Extended attributes are stored in an entry whose type is X, not
	     x, as used by pax interchange format.  The detailed format of
	     this entry appears to be the same as detailed above for the x
	     entry.
     o	     An additional 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.

   Other Extensions
     One common extension, utilized by GNU tar, star, and other newer tar
     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.

     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.

SEE ALSO
     ar(1), pax(1), tar(1)

STANDARDS
     The tar utility is no longer a part of POSIX or the Single Unix Standard.
     It last appeared in Version 2 of the Single UNIX Specification
     (``SUSv2'').  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 (``POSIX.1'').

HISTORY
     A tar command appeared in Seventh Edition Unix, which was released in
     January, 1979.  It replaced the tp program from Fourth Edition Unix which
     in turn replaced the tap program from First Edition Unix.	John Gilmore's
     pdtar public-domain implementation (circa 1987) was highly influential
     and formed the basis of GNU tar.  Joerg Shilling's star archiver is
     another open-source (GPL) archiver (originally developed circa 1985)
     which features complete support for pax interchange format.

FreeBSD 6.0			 May 20, 2004			   FreeBSD 6.0