1.\" Copyright (c) 2003-2009 Tim Kientzle 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD: head/lib/libarchive/tar.5 201077 2009-12-28 01:50:23Z kientzle $ 26.\" 27.Dd December 27, 2009 28.Dt tar 5 29.Os 30.Sh NAME 31.Nm tar 32.Nd format of tape archive files 33.Sh DESCRIPTION 34The 35.Nm 36archive format collects any number of files, directories, and other 37file system objects (symbolic links, device nodes, etc.) into a single 38stream of bytes. 39The format was originally designed to be used with 40tape drives that operate with fixed-size blocks, but is widely used as 41a general packaging mechanism. 42.Ss General Format 43A 44.Nm 45archive consists of a series of 512-byte records. 46Each file system object requires a header record which stores basic metadata 47(pathname, owner, permissions, etc.) and zero or more records containing any 48file data. 49The end of the archive is indicated by two records consisting 50entirely of zero bytes. 51.Pp 52For compatibility with tape drives that use fixed block sizes, 53programs that read or write tar files always read or write a fixed 54number of records with each I/O operation. 55These 56.Dq blocks 57are always a multiple of the record size. 58The maximum block size supported by early 59implementations was 10240 bytes or 20 records. 60This is still the default for most implementations 61although block sizes of 1MiB (2048 records) or larger are 62commonly used with modern high-speed tape drives. 63(Note: the terms 64.Dq block 65and 66.Dq record 67here are not entirely standard; this document follows the 68convention established by John Gilmore in documenting 69.Nm pdtar . ) 70.Ss Old-Style Archive Format 71The original tar archive format has been extended many times to 72include additional information that various implementors found 73necessary. 74This section describes the variant implemented by the tar command 75included in 76.At v7 , 77which seems to be the earliest widely-used version of the tar program. 78.Pp 79The header record for an old-style 80.Nm 81archive consists of the following: 82.Bd -literal -offset indent 83struct header_old_tar { 84 char name[100]; 85 char mode[8]; 86 char uid[8]; 87 char gid[8]; 88 char size[12]; 89 char mtime[12]; 90 char checksum[8]; 91 char linkflag[1]; 92 char linkname[100]; 93 char pad[255]; 94}; 95.Ed 96All unused bytes in the header record are filled with nulls. 97.Bl -tag -width indent 98.It Va name 99Pathname, stored as a null-terminated string. 100Early tar implementations only stored regular files (including 101hardlinks to those files). 102One common early convention used a trailing "/" character to indicate 103a directory name, allowing directory permissions and owner information 104to be archived and restored. 105.It Va mode 106File mode, stored as an octal number in ASCII. 107.It Va uid , Va gid 108User id and group id of owner, as octal numbers in ASCII. 109.It Va size 110Size of file, as octal number in ASCII. 111For regular files only, this indicates the amount of data 112that follows the header. 113In particular, this field was ignored by early tar implementations 114when extracting hardlinks. 115Modern writers should always store a zero length for hardlink entries. 116.It Va mtime 117Modification time of file, as an octal number in ASCII. 118This indicates the number of seconds since the start of the epoch, 11900:00:00 UTC January 1, 1970. 120Note that negative values should be avoided 121here, as they are handled inconsistently. 122.It Va checksum 123Header checksum, stored as an octal number in ASCII. 124To compute the checksum, set the checksum field to all spaces, 125then sum all bytes in the header using unsigned arithmetic. 126This field should be stored as six octal digits followed by a null and a space 127character. 128Note that many early implementations of tar used signed arithmetic 129for the checksum field, which can cause interoperability problems 130when transferring archives between systems. 131Modern robust readers compute the checksum both ways and accept the 132header if either computation matches. 133.It Va linkflag , Va linkname 134In order to preserve hardlinks and conserve tape, a file 135with multiple links is only written to the archive the first 136time it is encountered. 137The next time it is encountered, the 138.Va linkflag 139is set to an ASCII 140.Sq 1 141and the 142.Va linkname 143field holds the first name under which this file appears. 144(Note that regular files have a null value in the 145.Va linkflag 146field.) 147.El 148.Pp 149Early tar implementations varied in how they terminated these fields. 150The tar command in 151.At v7 152used the following conventions (this is also documented in early BSD manpages): 153the pathname must be null-terminated; 154the mode, uid, and gid fields must end in a space and a null byte; 155the size and mtime fields must end in a space; 156the checksum is terminated by a null and a space. 157Early implementations filled the numeric fields with leading spaces. 158This seems to have been common practice until the 159.St -p1003.1-88 160standard was released. 161For best portability, modern implementations should fill the numeric 162fields with leading zeros. 163.Ss Pre-POSIX Archives 164An early draft of 165.St -p1003.1-88 166served as the basis for John Gilmore's 167.Nm pdtar 168program and many system implementations from the late 1980s 169and early 1990s. 170These archives generally follow the POSIX ustar 171format described below with the following variations: 172.Bl -bullet -compact -width indent 173.It 174The magic value is 175.Dq ustar\ \& 176(note the following space). 177The version field contains a space character followed by a null. 178.It 179The numeric fields are generally filled with leading spaces 180(not leading zeros as recommended in the final standard). 181.It 182The prefix field is often not used, limiting pathnames to 183the 100 characters of old-style archives. 184.El 185.Ss POSIX ustar Archives 186.St -p1003.1-88 187defined a standard tar file format to be read and written 188by compliant implementations of 189.Xr tar 1 . 190This format is often called the 191.Dq ustar 192format, after the magic value used 193in the header. 194(The name is an acronym for 195.Dq Unix Standard TAR . ) 196It extends the historic format with new fields: 197.Bd -literal -offset indent 198struct header_posix_ustar { 199 char name[100]; 200 char mode[8]; 201 char uid[8]; 202 char gid[8]; 203 char size[12]; 204 char mtime[12]; 205 char checksum[8]; 206 char typeflag[1]; 207 char linkname[100]; 208 char magic[6]; 209 char version[2]; 210 char uname[32]; 211 char gname[32]; 212 char devmajor[8]; 213 char devminor[8]; 214 char prefix[155]; 215 char pad[12]; 216}; 217.Ed 218.Bl -tag -width indent 219.It Va typeflag 220Type of entry. 221POSIX extended the earlier 222.Va linkflag 223field with several new type values: 224.Bl -tag -width indent -compact 225.It Dq 0 226Regular file. 227NUL should be treated as a synonym, for compatibility purposes. 228.It Dq 1 229Hard link. 230.It Dq 2 231Symbolic link. 232.It Dq 3 233Character device node. 234.It Dq 4 235Block device node. 236.It Dq 5 237Directory. 238.It Dq 6 239FIFO node. 240.It Dq 7 241Reserved. 242.It Other 243A POSIX-compliant implementation must treat any unrecognized typeflag value 244as a regular file. 245In particular, writers should ensure that all entries 246have a valid filename so that they can be restored by readers that do not 247support the corresponding extension. 248Uppercase letters "A" through "Z" are reserved for custom extensions. 249Note that sockets and whiteout entries are not archivable. 250.El 251It is worth noting that the 252.Va size 253field, in particular, has different meanings depending on the type. 254For regular files, of course, it indicates the amount of data 255following the header. 256For directories, it may be used to indicate the total size of all 257files in the directory, for use by operating systems that pre-allocate 258directory space. 259For all other types, it should be set to zero by writers and ignored 260by readers. 261.It Va magic 262Contains the magic value 263.Dq ustar 264followed by a NUL byte to indicate that this is a POSIX standard archive. 265Full compliance requires the uname and gname fields be properly set. 266.It Va version 267Version. 268This should be 269.Dq 00 270(two copies of the ASCII digit zero) for POSIX standard archives. 271.It Va uname , Va gname 272User and group names, as null-terminated ASCII strings. 273These should be used in preference to the uid/gid values 274when they are set and the corresponding names exist on 275the system. 276.It Va devmajor , Va devminor 277Major and minor numbers for character device or block device entry. 278.It Va name , Va prefix 279If the pathname is too long to fit in the 100 bytes provided by the standard 280format, it can be split at any 281.Pa / 282character with the first portion going into the prefix field. 283If the prefix field is not empty, the reader will prepend 284the prefix value and a 285.Pa / 286character to the regular name field to obtain the full pathname. 287The standard does not require a trailing 288.Pa / 289character on directory names, though most implementations still 290include this for compatibility reasons. 291.El 292.Pp 293Note that all unused bytes must be set to 294.Dv NUL . 295.Pp 296Field termination is specified slightly differently by POSIX 297than by previous implementations. 298The 299.Va magic , 300.Va uname , 301and 302.Va gname 303fields must have a trailing 304.Dv NUL . 305The 306.Va pathname , 307.Va linkname , 308and 309.Va prefix 310fields must have a trailing 311.Dv NUL 312unless they fill the entire field. 313(In particular, it is possible to store a 256-character pathname if it 314happens to have a 315.Pa / 316as the 156th character.) 317POSIX requires numeric fields to be zero-padded in the front, and requires 318them to be terminated with either space or 319.Dv NUL 320characters. 321.Pp 322Currently, most tar implementations comply with the ustar 323format, occasionally extending it by adding new fields to the 324blank area at the end of the header record. 325.Ss Pax Interchange Format 326There are many attributes that cannot be portably stored in a 327POSIX ustar archive. 328.St -p1003.1-2001 329defined a 330.Dq pax interchange format 331that uses two new types of entries to hold text-formatted 332metadata that applies to following entries. 333Note that a pax interchange format archive is a ustar archive in every 334respect. 335The new data is stored in ustar-compatible archive entries that use the 336.Dq x 337or 338.Dq g 339typeflag. 340In particular, older implementations that do not fully support these 341extensions will extract the metadata into regular files, where the 342metadata can be examined as necessary. 343.Pp 344An entry in a pax interchange format archive consists of one or 345two standard ustar entries, each with its own header and data. 346The first optional entry stores the extended attributes 347for the following entry. 348This optional first entry has an "x" typeflag and a size field that 349indicates the total size of the extended attributes. 350The extended attributes themselves are stored as a series of text-format 351lines encoded in the portable UTF-8 encoding. 352Each line consists of a decimal number, a space, a key string, an equals 353sign, a value string, and a new line. 354The decimal number indicates the length of the entire line, including the 355initial length field and the trailing newline. 356An example of such a field is: 357.Dl 25 ctime=1084839148.1212\en 358Keys in all lowercase are standard keys. 359Vendors can add their own keys by prefixing them with an all uppercase 360vendor name and a period. 361Note that, unlike the historic header, numeric values are stored using 362decimal, not octal. 363A description of some common keys follows: 364.Bl -tag -width indent 365.It Cm atime , Cm ctime , Cm mtime 366File access, inode change, and modification times. 367These fields can be negative or include a decimal point and a fractional value. 368.It Cm uname , Cm uid , Cm gname , Cm gid 369User name, group name, and numeric UID and GID values. 370The user name and group name stored here are encoded in UTF8 371and can thus include non-ASCII characters. 372The UID and GID fields can be of arbitrary length. 373.It Cm linkpath 374The full path of the linked-to file. 375Note that this is encoded in UTF8 and can thus include non-ASCII characters. 376.It Cm path 377The full pathname of the entry. 378Note that this is encoded in UTF8 and can thus include non-ASCII characters. 379.It Cm realtime.* , Cm security.* 380These keys are reserved and may be used for future standardization. 381.It Cm size 382The size of the file. 383Note that there is no length limit on this field, allowing conforming 384archives to store files much larger than the historic 8GB limit. 385.It Cm SCHILY.* 386Vendor-specific attributes used by Joerg Schilling's 387.Nm star 388implementation. 389.It Cm SCHILY.acl.access , Cm SCHILY.acl.default 390Stores the access and default ACLs as textual strings in a format 391that is an extension of the format specified by POSIX.1e draft 17. 392In particular, each user or group access specification can include a fourth 393colon-separated field with the numeric UID or GID. 394This allows ACLs to be restored on systems that may not have complete 395user or group information available (such as when NIS/YP or LDAP services 396are temporarily unavailable). 397.It Cm SCHILY.devminor , Cm SCHILY.devmajor 398The full minor and major numbers for device nodes. 399.It Cm SCHILY.fflags 400The file flags. 401.It Cm SCHILY.realsize 402The full size of the file on disk. 403XXX explain? XXX 404.It Cm SCHILY.dev, Cm SCHILY.ino , Cm SCHILY.nlinks 405The device number, inode number, and link count for the entry. 406In particular, note that a pax interchange format archive using Joerg 407Schilling's 408.Cm SCHILY.* 409extensions can store all of the data from 410.Va struct stat . 411.It Cm LIBARCHIVE.xattr. Ns Ar namespace Ns . Ns Ar key 412Libarchive stores POSIX.1e-style extended attributes using 413keys of this form. 414The 415.Ar key 416value is URL-encoded: 417All non-ASCII characters and the two special characters 418.Dq = 419and 420.Dq % 421are encoded as 422.Dq % 423followed by two uppercase hexadecimal digits. 424The value of this key is the extended attribute value 425encoded in base 64. 426XXX Detail the base-64 format here XXX 427.It Cm VENDOR.* 428XXX document other vendor-specific extensions XXX 429.El 430.Pp 431Any values stored in an extended attribute override the corresponding 432values in the regular tar header. 433Note that compliant readers should ignore the regular fields when they 434are overridden. 435This is important, as existing archivers are known to store non-compliant 436values in the standard header fields in this situation. 437There are no limits on length for any of these fields. 438In particular, numeric fields can be arbitrarily large. 439All text fields are encoded in UTF8. 440Compliant writers should store only portable 7-bit ASCII characters in 441the standard ustar header and use extended 442attributes whenever a text value contains non-ASCII characters. 443.Pp 444In addition to the 445.Cm x 446entry described above, the pax interchange format 447also supports a 448.Cm g 449entry. 450The 451.Cm g 452entry is identical in format, but specifies attributes that serve as 453defaults for all subsequent archive entries. 454The 455.Cm g 456entry is not widely used. 457.Pp 458Besides the new 459.Cm x 460and 461.Cm g 462entries, the pax interchange format has a few other minor variations 463from the earlier ustar format. 464The most troubling one is that hardlinks are permitted to have 465data following them. 466This allows readers to restore any hardlink to a file without 467having to rewind the archive to find an earlier entry. 468However, it creates complications for robust readers, as it is no longer 469clear whether or not they should ignore the size field for hardlink entries. 470.Ss GNU Tar Archives 471The GNU tar program started with a pre-POSIX format similar to that 472described earlier and has extended it using several different mechanisms: 473It added new fields to the empty space in the header (some of which was later 474used by POSIX for conflicting purposes); 475it allowed the header to be continued over multiple records; 476and it defined new entries that modify following entries 477(similar in principle to the 478.Cm x 479entry described above, but each GNU special entry is single-purpose, 480unlike the general-purpose 481.Cm x 482entry). 483As a result, GNU tar archives are not POSIX compatible, although 484more lenient POSIX-compliant readers can successfully extract most 485GNU tar archives. 486.Bd -literal -offset indent 487struct header_gnu_tar { 488 char name[100]; 489 char mode[8]; 490 char uid[8]; 491 char gid[8]; 492 char size[12]; 493 char mtime[12]; 494 char checksum[8]; 495 char typeflag[1]; 496 char linkname[100]; 497 char magic[6]; 498 char version[2]; 499 char uname[32]; 500 char gname[32]; 501 char devmajor[8]; 502 char devminor[8]; 503 char atime[12]; 504 char ctime[12]; 505 char offset[12]; 506 char longnames[4]; 507 char unused[1]; 508 struct { 509 char offset[12]; 510 char numbytes[12]; 511 } sparse[4]; 512 char isextended[1]; 513 char realsize[12]; 514 char pad[17]; 515}; 516.Ed 517.Bl -tag -width indent 518.It Va typeflag 519GNU tar uses the following special entry types, in addition to 520those defined by POSIX: 521.Bl -tag -width indent 522.It "7" 523GNU tar treats type "7" records identically to type "0" records, 524except on one obscure RTOS where they are used to indicate the 525pre-allocation of a contiguous file on disk. 526.It "D" 527This indicates a directory entry. 528Unlike the POSIX-standard "5" 529typeflag, the header is followed by data records listing the names 530of files in this directory. 531Each name is preceded by an ASCII "Y" 532if the file is stored in this archive or "N" if the file is not 533stored in this archive. 534Each name is terminated with a null, and 535an extra null marks the end of the name list. 536The purpose of this 537entry is to support incremental backups; a program restoring from 538such an archive may wish to delete files on disk that did not exist 539in the directory when the archive was made. 540.Pp 541Note that the "D" typeflag specifically violates POSIX, which requires 542that unrecognized typeflags be restored as normal files. 543In this case, restoring the "D" entry as a file could interfere 544with subsequent creation of the like-named directory. 545.It "K" 546The data for this entry is a long linkname for the following regular entry. 547.It "L" 548The data for this entry is a long pathname for the following regular entry. 549.It "M" 550This is a continuation of the last file on the previous volume. 551GNU multi-volume archives guarantee that each volume begins with a valid 552entry header. 553To ensure this, a file may be split, with part stored at the end of one volume, 554and part stored at the beginning of the next volume. 555The "M" typeflag indicates that this entry continues an existing file. 556Such entries can only occur as the first or second entry 557in an archive (the latter only if the first entry is a volume label). 558The 559.Va size 560field specifies the size of this entry. 561The 562.Va offset 563field at bytes 369-380 specifies the offset where this file fragment 564begins. 565The 566.Va realsize 567field specifies the total size of the file (which must equal 568.Va size 569plus 570.Va offset ) . 571When extracting, GNU tar checks that the header file name is the one it is 572expecting, that the header offset is in the correct sequence, and that 573the sum of offset and size is equal to realsize. 574.It "N" 575Type "N" records are no longer generated by GNU tar. 576They contained a 577list of files to be renamed or symlinked after extraction; this was 578originally used to support long names. 579The contents of this record 580are a text description of the operations to be done, in the form 581.Dq Rename %s to %s\en 582or 583.Dq Symlink %s to %s\en ; 584in either case, both 585filenames are escaped using K&R C syntax. 586Due to security concerns, "N" records are now generally ignored 587when reading archives. 588.It "S" 589This is a 590.Dq sparse 591regular file. 592Sparse files are stored as a series of fragments. 593The header contains a list of fragment offset/length pairs. 594If more than four such entries are required, the header is 595extended as necessary with 596.Dq extra 597header extensions (an older format that is no longer used), or 598.Dq sparse 599extensions. 600.It "V" 601The 602.Va name 603field should be interpreted as a tape/volume header name. 604This entry should generally be ignored on extraction. 605.El 606.It Va magic 607The magic field holds the five characters 608.Dq ustar 609followed by a space. 610Note that POSIX ustar archives have a trailing null. 611.It Va version 612The version field holds a space character followed by a null. 613Note that POSIX ustar archives use two copies of the ASCII digit 614.Dq 0 . 615.It Va atime , Va ctime 616The time the file was last accessed and the time of 617last change of file information, stored in octal as with 618.Va mtime . 619.It Va longnames 620This field is apparently no longer used. 621.It Sparse Va offset / Va numbytes 622Each such structure specifies a single fragment of a sparse 623file. 624The two fields store values as octal numbers. 625The fragments are each padded to a multiple of 512 bytes 626in the archive. 627On extraction, the list of fragments is collected from the 628header (including any extension headers), and the data 629is then read and written to the file at appropriate offsets. 630.It Va isextended 631If this is set to non-zero, the header will be followed by additional 632.Dq sparse header 633records. 634Each such record contains information about as many as 21 additional 635sparse blocks as shown here: 636.Bd -literal -offset indent 637struct gnu_sparse_header { 638 struct { 639 char offset[12]; 640 char numbytes[12]; 641 } sparse[21]; 642 char isextended[1]; 643 char padding[7]; 644}; 645.Ed 646.It Va realsize 647A binary representation of the file's complete size, with a much larger range 648than the POSIX file size. 649In particular, with 650.Cm M 651type files, the current entry is only a portion of the file. 652In that case, the POSIX size field will indicate the size of this 653entry; the 654.Va realsize 655field will indicate the total size of the file. 656.El 657.Ss GNU tar pax archives 658GNU tar 1.14 (XXX check this XXX) and later will write 659pax interchange format archives when you specify the 660.Fl -posix 661flag. 662This format uses custom keywords to store sparse file information. 663There have been three iterations of this support, referred to 664as 665.Dq 0.0 , 666.Dq 0.1 , 667and 668.Dq 1.0 . 669.Bl -tag -width indent 670.It Cm GNU.sparse.numblocks , Cm GNU.sparse.offset , Cm GNU.sparse.numbytes , Cm GNU.sparse.size 671The 672.Dq 0.0 673format used an initial 674.Cm GNU.sparse.numblocks 675attribute to indicate the number of blocks in the file, a pair of 676.Cm GNU.sparse.offset 677and 678.Cm GNU.sparse.numbytes 679to indicate the offset and size of each block, 680and a single 681.Cm GNU.sparse.size 682to indicate the full size of the file. 683This is not the same as the size in the tar header because the 684latter value does not include the size of any holes. 685This format required that the order of attributes be preserved and 686relied on readers accepting multiple appearances of the same attribute 687names, which is not officially permitted by the standards. 688.It Cm GNU.sparse.map 689The 690.Dq 0.1 691format used a single attribute that stored a comma-separated 692list of decimal numbers. 693Each pair of numbers indicated the offset and size, respectively, 694of a block of data. 695This does not work well if the archive is extracted by an archiver 696that does not recognize this extension, since many pax implementations 697simply discard unrecognized attributes. 698.It Cm GNU.sparse.major , Cm GNU.sparse.minor , Cm GNU.sparse.name , Cm GNU.sparse.realsize 699The 700.Dq 1.0 701format stores the sparse block map in one or more 512-byte blocks 702prepended to the file data in the entry body. 703The pax attributes indicate the existence of this map 704(via the 705.Cm GNU.sparse.major 706and 707.Cm GNU.sparse.minor 708fields) 709and the full size of the file. 710The 711.Cm GNU.sparse.name 712holds the true name of the file. 713To avoid confusion, the name stored in the regular tar header 714is a modified name so that extraction errors will be apparent 715to users. 716.El 717.Ss Solaris Tar 718XXX More Details Needed XXX 719.Pp 720Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an 721.Dq extended 722format that is fundamentally similar to pax interchange format, 723with the following differences: 724.Bl -bullet -compact -width indent 725.It 726Extended attributes are stored in an entry whose type is 727.Cm X , 728not 729.Cm x , 730as used by pax interchange format. 731The detailed format of this entry appears to be the same 732as detailed above for the 733.Cm x 734entry. 735.It 736An additional 737.Cm A 738entry is used to store an ACL for the following regular entry. 739The body of this entry contains a seven-digit octal number 740followed by a zero byte, followed by the 741textual ACL description. 742The octal value is the number of ACL entries 743plus a constant that indicates the ACL type: 01000000 744for POSIX.1e ACLs and 03000000 for NFSv4 ACLs. 745.El 746.Ss AIX Tar 747XXX More details needed XXX 748.Ss Mac OS X Tar 749The tar distributed with Apple's Mac OS X stores most regular files 750as two separate entries in the tar archive. 751The two entries have the same name except that the first 752one has 753.Dq ._ 754added to the beginning of the name. 755This first entry stores the 756.Dq resource fork 757with additional attributes for the file. 758The Mac OS X 759.Fn CopyFile 760API is used to separate a file on disk into separate 761resource and data streams and to reassemble those separate 762streams when the file is restored to disk. 763.Ss Other Extensions 764One obvious extension to increase the size of files is to 765eliminate the terminating characters from the various 766numeric fields. 767For example, the standard only allows the size field to contain 76811 octal digits, reserving the twelfth byte for a trailing 769NUL character. 770Allowing 12 octal digits allows file sizes up to 64 GB. 771.Pp 772Another extension, utilized by GNU tar, star, and other newer 773.Nm 774implementations, permits binary numbers in the standard numeric fields. 775This is flagged by setting the high bit of the first byte. 776This permits 95-bit values for the length and time fields 777and 63-bit values for the uid, gid, and device numbers. 778GNU tar supports this extension for the 779length, mtime, ctime, and atime fields. 780Joerg Schilling's star program supports this extension for 781all numeric fields. 782Note that this extension is largely obsoleted by the extended attribute 783record provided by the pax interchange format. 784.Pp 785Another early GNU extension allowed base-64 values rather than octal. 786This extension was short-lived and is no longer supported by any 787implementation. 788.Sh SEE ALSO 789.Xr ar 1 , 790.Xr pax 1 , 791.Xr tar 1 792.Sh STANDARDS 793The 794.Nm tar 795utility is no longer a part of POSIX or the Single Unix Standard. 796It last appeared in 797.St -susv2 . 798It has been supplanted in subsequent standards by 799.Xr pax 1 . 800The ustar format is currently part of the specification for the 801.Xr pax 1 802utility. 803The pax interchange file format is new with 804.St -p1003.1-2001 . 805.Sh HISTORY 806A 807.Nm tar 808command appeared in Seventh Edition Unix, which was released in January, 1979. 809It replaced the 810.Nm tp 811program from Fourth Edition Unix which in turn replaced the 812.Nm tap 813program from First Edition Unix. 814John Gilmore's 815.Nm pdtar 816public-domain implementation (circa 1987) was highly influential 817and formed the basis of 818.Nm GNU tar 819(circa 1988). 820Joerg Shilling's 821.Nm star 822archiver is another open-source (GPL) archiver (originally developed 823circa 1985) which features complete support for pax interchange 824format. 825.Pp 826This documentation was written as part of the 827.Nm libarchive 828and 829.Nm bsdtar 830project by 831.An Tim Kientzle Aq kientzle@FreeBSD.org . 832