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25 .\" $FreeBSD: src/lib/libarchive/tar.5,v 1.9 2004/08/07 17:24:50 kientzle Exp $
32 .Nd format of tape archive files
36 archive format collects any number of files, directories, and other
37 filesystem objects (symbolic links, device nodes, etc.) into a single
39 The format was originally designed to be used with
40 tape drives that operate with fixed-size blocks, but is widely used as
41 a general packaging mechanism.
45 archive consists of a series of 512-byte records.
46 Each filesystem object requires a header record which stores basic metadata
47 (pathname, owner, permissions, etc.) and zero or more records containing any
49 The end of the archive is indicated by two records consisting
50 entirely of zero bytes.
52 For compatibility with tape drives that use fixed block sizes,
53 programs that read or write tar files always read or write a fixed
54 number of records with each I/O operation.
57 are always a multiple of the record size.
58 The most common block size\(emand the maximum supported by historic
59 implementations\(emis 10240 bytes or 20 records.
64 here are not entirely standard; this document follows the
65 convention established by John Gilmore in documenting
67 .Ss Old-Style Archive Format
68 The original tar archive format has been extended many times to
69 include additional information that various implementors found
71 This section describes the variant implemented by the tar command
74 which is one of the earliest widely-used versions of the tar program.
76 The header record for an old-style
78 archive consists of the following:
79 .Bd -literal -offset indent
80 struct header_old_tar {
93 All unused bytes in the header record are filled with nulls.
94 .Bl -tag -width indent
96 Pathname, stored as a null-terminated string.
97 Early tar implementations only stored regular files (including
98 hardlinks to those files).
99 One common early convention used a trailing "/" character to indicate
100 a directory name, allowing directory permissions and owner information
101 to be archived and restored.
103 File mode, stored as an octal number in ASCII.
105 User id and group id of owner, as octal numbers in ASCII.
107 Size of file, as octal number in ASCII.
108 For regular files only, this indicates the amount of data
109 that follows the header.
110 In particular, this field was ignored by early tar implementations
111 when extracting hardlinks.
112 Modern writers should always store a zero length for hardlink entries.
114 Modification time of file, as an octal number in ASCII.
115 This indicates the number of seconds since the start of the epoch,
116 00:00:00 UTC January 1, 1970.
117 Note that negative values should be avoided
118 here, as they are handled inconsistently.
120 Header checksum, stored as an octal number in ASCII.
121 To compute the checksum, set the checksum field to all spaces,
122 then sum all bytes in the header using unsigned arithmetic.
123 This field should be stored as six octal digits followed by a null and a space
125 Note that many early implementations of tar used signed arithmetic
126 for the checksum field, which can cause interoperability problems
127 when transferring archives between systems.
128 Modern robust readers compute the checksum both ways and accept the
129 header if either computation matches.
130 .It Va linkflag , Va linkname
131 In order to preserve hardlinks and conserve tape, a file
132 with multiple links is only written to the archive the first
133 time it is encountered.
134 The next time it is encountered, the
140 field holds the first name under which this file appears.
141 (Note that regular files have a null value in the
146 Early tar implementations varied in how they terminated these fields.
149 used the following conventions (this is also documented in early BSD manpages):
150 the pathname must be null-terminated;
151 the mode, uid, and gid fields must end in a space and a null byte;
152 the size and mtime fields must end in a space;
153 the checksum is terminated by a null and a space.
154 Early implementations filled the numeric fields with leading spaces.
155 This seems to have been common practice until the
157 standard was released.
158 For best portability, modern implementations should fill the numeric
159 fields with leading zeros.
160 .Ss Pre-POSIX Archives
163 served as the basis for John Gilmore's
165 program and many system implementations from the late 1980s
167 These archives generally follow the POSIX ustar
168 format described below with the following variations:
169 .Bl -bullet -compact -width indent
173 (note the following space).
174 The version field contains a space character followed by a null.
176 The numeric fields are generally filled with leading spaces
177 (not leading zeros as recommended in the final standard).
179 The prefix field is often not used, limiting pathnames to
180 the 100 characters of old-style archives.
182 .Ss POSIX ustar Archives
184 defined a standard tar file format to be read and written
185 by compliant implementations of
189 This format is often called the
191 format, after the magic value used
193 (The name is an acronym for
194 .Dq Unix Standard TAR. )
195 It extends the historic format with new fields:
196 .Bd -literal -offset indent
197 struct header_posix_ustar {
217 .Bl -tag -width indent
220 POSIX extended the earlier
222 field with several new type values:
223 .Bl -tag -width indent -compact
226 NULL should be treated as a synonym, for compatibility purposes.
232 Character device node.
242 A POSIX-compliant implementation must treat any unrecognized typeflag value
244 In particular, writers should ensure that all entries
245 have a valid filename so that they can be restored by readers that do not
246 support the corresponding extension.
247 Uppercase letters "A" through "Z" are reserved for custom extensions.
248 Note that sockets and whiteout entries are not archivable.
250 It is worth noting that the
252 field, in particular, has different meanings depending on the type.
253 For regular files, of course, it indicates the amount of data
254 following the header.
255 For directories, it may be used to indicate the total size of all
256 files in the directory, for use by operating systems that pre-allocate
258 For all other types, it should be set to zero by writers and ignored
261 Contains the magic value
263 followed by a NULL byte to indicate that this is a POSIX standard archive.
264 Full compliance requires the uname and gname fields be properly set.
269 (two copies of the ASCII digit zero) for POSIX standard archives.
270 .It Va uname , Va gname
271 User and group names, as null-terminated ASCII strings.
272 These should be used in preference to the uid/gid values
273 when they are set and the corresponding names exist on
275 .It Va devmajor , Va devminor
276 Major and minor numbers for character device or block device entry.
278 First part of pathname.
279 If the pathname is too long to fit in the 100 bytes provided by the standard
280 format, it can be split at any
282 character with the first portion going here.
283 If the prefix field is not empty, the reader will prepend
284 the prefix value and a
286 character to the regular name field to obtain the full pathname.
289 Note that all unused bytes must be set to
292 Field termination is specified slightly differently by POSIX
293 than by previous implementations.
299 fields must have a trailing
306 fields must have a trailing
308 unless they fill the entire field.
309 (In particular, it is possible to store a 256-character pathname if it
312 as the 156th character.)
313 POSIX requires numeric fields to be zero-padded in the front, and allows
314 them to be terminated with either space or
318 Currently, most tar implementations comply with the ustar
319 format, occasionally extending it by adding new fields to the
320 blank area at the end of the header record.
321 .Ss Pax Interchange Format
322 There are many attributes that cannot be portably stored in a
326 .Dq pax interchange format
327 that uses two new types of entries to hold text-formatted
328 metadata that applies to following entries.
329 Note that a pax interchange format archive is a ustar archive in every
331 The new data is stored in ustar-compatible archive entries that use the
336 In particular, older implementations that do not fully support these
337 extensions will extract the metadata into regular files, where the
338 metadata can be examined as necessary.
340 An entry in a pax interchange format archive consists of one or
341 two standard ustar entries, each with its own header and data.
342 The first optional entry stores the extended attributes
343 for the following entry.
344 This optional first entry has an "x" typeflag and a size field that
345 indicates the total size of the extended attributes.
346 The extended attributes themselves are stored as a series of text-format
347 lines encoded in the portable UTF-8 encoding.
348 Each line consists of a decimal number, a space, a key string, an equals
349 sign, a value string, and a new line.
350 The decimal number indicates the length of the entire line, including the
351 initial length field and the trailing newline.
352 An example of such a field is:
353 .Dl 25 ctime=1084839148.1212\en
354 Keys in all lowercase are standard keys.
355 Vendors can add their own keys by prefixing them with an all uppercase
356 vendor name and a period.
357 Note that, unlike the historic header, numeric values are stored using
359 A description of some common keys follows:
360 .Bl -tag -width indent
361 .It Cm atime , Cm ctime , Cm mtime
362 File access, inode change, and modification times.
363 These fields can be negative or include a decimal point and a fractional value.
364 .It Cm uname , Cm uid , Cm gname , Cm gid
365 User name, group name, and numeric UID and GID values.
366 The user name and group name stored here are encoded in UTF8
367 and can thus include non-ASCII characters.
368 The UID and GID fields can be of arbitrary length.
370 The full path of the linked-to file.
371 Note that this is encoded in UTF8 and can thus include non-ASCII characters.
373 The full pathname of the entry.
374 Note that this is encoded in UTF8 and can thus include non-ASCII characters.
375 .It Cm realtime.* , Cm security.*
376 These keys are reserved and may be used for future standardization.
378 The size of the file.
379 Note that there is no length limit on this field, allowing conforming
380 archives to store files much larger than the historic 8GB limit.
382 Vendor-specific attributes used by Joerg Schilling's
385 .It Cm SCHILY.acl.access , Cm SCHILY.acl.default
386 Stores the access and default ACLs as textual strings in a format
387 that's an extension of the format specified by POSIX.1e draft 17.
388 In particular, each user or group access specification can include a fourth
389 colon-separated field with the numeric UID or GID.
390 This allows ACLs to be restored on systems that may not have complete
391 user or group information available (such as when NIS/YP or LDAP services
392 are temporarily unavailable).
393 .It Cm SCHILY.devminor , Cm SCHILY.devmajor
394 The full minor and major numbers for device nodes.
395 .It Cm SCHILY.dev, Cm SCHILY.ino , Cm SCHILY.nlinks
396 The device number, inode number, and link count for the entry.
397 In particular, note that a pax interchange format archive using Joerg
400 extensions can store all of the data from
403 XXX document other vendor-specific extensions XXX
406 Any values stored in an extended attribute override the corresponding
407 values in the regular tar header.
408 Note that compliant readers should ignore the regular fields when they
410 This is important, as existing archivers are known to store non-compliant
411 values in the standard header fields in this situation.
412 There are no limits on length for any of these fields.
413 In particular, numeric fields can be arbitrarily large.
414 All text fields are encoded in UTF8.
415 Compliant writers should store only portable 7-bit ASCII characters in
416 the standard ustar header and use extended
417 attributes whenever a text value contains non-ASCII characters.
421 entry described above, the pax interchange format
427 entry is identical in format, but specifies attributes that serve as
428 defaults for all subsequent archive entries.
431 entry is not widely used.
437 entries, the pax interchange format has a few other minor variations
438 from the earlier ustar format.
439 The most troubling one is that hardlinks are permitted to have
441 This allows readers to restore any hardlink to a file without
442 having to rewind the archive to find an earlier entry.
443 However, it creates complications for robust readers, as it is no longer
444 clear whether or not they should ignore the size field for hardlink entries.
446 The GNU tar program started with a pre-POSIX format similar to that
447 described earlier and has extended it using several different mechanisms:
448 It added new fields to the empty space in the header (some of which was later
449 used by POSIX for conflicting purposes);
450 it allowed the header to be continued over multiple records;
451 and it defined new entries that modify following entries
452 (similar in principle to the
454 entry described above, but each GNU special entry is single-purpose,
455 unlike the general-purpose
458 As a result, GNU tar archives are not POSIX compatible, although
459 more lenient POSIX-compliant readers can successfully extract most
461 .Bd -literal -offset indent
462 struct header_gnu_tar {
492 .Bl -tag -width indent
494 GNU tar uses the following special entry types, in addition to
495 those defined by POSIX:
496 .Bl -tag -width indent
498 GNU tar treats type "7" records identically to type "0" records,
499 except on one obscure RTOS where they are used to indicate the
500 pre-allocation of a contiguous file on disk.
502 This indicates a directory entry.
503 Unlike the POSIX-standard "5"
504 typeflag, the header is followed by data records listing the names
505 of files in this directory.
506 Each name is preceded by an ASCII "Y"
507 if the file is stored in this archive or "N" if the file is not
508 stored in this archive.
509 Each name is terminated with a null, and
510 an extra null marks the end of the name list.
512 entry is to support incremental backups; a program restoring from
513 such an archive may wish to delete files on disk that did not exist
514 in the directory when the archive was made.
516 Note that the "D" typeflag specifically violates POSIX, which requires
517 that unrecognized typeflags be restored as normal files.
518 In this case, restoring the "D" entry as a file could interfere
519 with subsequent creation of the like-named directory.
521 The data for this entry is a long linkname for the following regular entry.
523 The data for this entry is a long pathname for the following regular entry.
525 This is a continuation of the last file on the previous volume.
526 GNU multi-volume archives guarantee that each volume begins with a valid
528 To ensure this, a file may be split, with part stored at the end of one volume,
529 and part stored at the beginning of the next volume.
530 The "M" typeflag indicates that this entry continues an existing file.
531 Such entries can only occur as the first or second entry
532 in an archive (the latter only if the first entry is a volume label).
535 field specifies the size of this entry.
538 field at bytes 369-380 specifies the offset where this file fragment
542 field specifies the total size of the file (which must equal
546 When extracting, GNU tar checks that the header file name is the one it is
547 expecting, that the header offset is in the correct sequence, and that
548 the sum of offset and size is equal to realsize.
549 FreeBSD's version of GNU tar does not handle the corner case of an
550 archive's being continued in the middle of a long name or other
553 Type "N" records are no longer generated by GNU tar.
555 list of files to be renamed or symlinked after extraction; this was
556 originally used to support long names.
557 The contents of this record
558 are a text description of the operations to be done, in the form
559 .Dq Rename %s to %s\en
561 .Dq Symlink %s to %s\en ;
563 filenames are escaped using K&R C syntax.
568 Sparse files are stored as a series of fragments.
569 The header contains a list of fragment offset/length pairs.
570 If more than four such entries are required, the header is
571 extended as necessary with
573 header extensions (an older format that's no longer used), or
579 field should be interpreted as a tape/volume header name.
580 This entry should generally be ignored on extraction.
583 The magic field holds the five characters
586 Note that POSIX ustar archives have a trailing null.
588 The version field holds a space character followed by a null.
589 Note that POSIX ustar archives use two copies of the ASCII digit
591 .It Va atime , Va ctime
592 The time the file was last accessed and the time of
593 last change of file information, stored in octal as with
596 This field is apparently no longer used.
597 .It Sparse Va offset / Va numbytes
598 Each such structure specifies a single fragment of a sparse
600 The two fields store values as octal numbers.
601 The fragments are each padded to a multiple of 512 bytes
603 On extraction, the list of fragments is collected from the
604 header (including any extension headers), and the data
605 is then read and written to the file at appropriate offsets.
607 If this is set to non-zero, the header will be followed by additional
610 Each such record contains information about as many as 21 additional
611 sparse blocks as shown here:
612 .Bd -literal -offset indent
613 struct gnu_sparse_header {
623 A binary representation of the file's complete size, with a much larger range
624 than the POSIX file size.
627 type files, the current entry is only a portion of the file.
628 In that case, the POSIX size field will indicate the size of this
631 field will indicate the total size of the file.
634 XXX More Details Needed XXX
636 Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an
638 format that is fundamentally similar to pax interchange format,
639 with the following differences:
640 .Bl -bullet -compact -width indent
642 Extended attributes are stored in an entry whose type is
646 as used by pax interchange format.
647 The detailed format of this entry appears to be the same
648 as detailed above for the
654 entry is used to store an ACL for the following regular entry.
655 The body of this entry contains a seven-digit octal number
656 (whose value is 01000000 plus the number of ACL entries)
657 followed by a zero byte, followed by the
658 textual ACL description.
661 One common extension, utilized by GNU tar, star, and other newer
663 implementations, permits binary numbers in the standard numeric
665 This is flagged by setting the high bit of the first character.
666 This permits 95-bit values for the length and time fields
667 and 63-bit values for the uid, gid, and device numbers.
668 GNU tar supports this extension for the
669 length, mtime, ctime, and atime fields.
670 Joerg Schilling's star program supports this extension for
672 Note that this extension is largely obsoleted by the extended attribute
673 record provided by the pax interchange format.
675 Another early GNU extension allowed base-64 values rather
677 This extension was short-lived and such archives are almost never seen.
678 However, there is still code in GNU tar to support them; this code is
679 responsible for a very cryptic warning message that is sometimes seen when
680 GNU tar encounters a damaged archive.
688 utility is no longer a part of POSIX or the Single Unix Standard.
691 It has been supplanted in subsequent standards by
693 The ustar format is currently part of the specification for the
696 The pax interchange file format is new with
701 command appeared in Seventh Edition Unix, which was released in January, 1979.
704 program from Fourth Edition Unix which in turn replaced the
706 program from First Edition Unix.
709 public-domain implementation (circa 1987) was highly influential
710 and formed the basis of GNU tar.
713 archiver is another open-source (GPL) archiver (originally developed
714 circa 1985) which features complete support for pax interchange