“Xar (archiver)”的意思、由来-开放百科全书

词条 Xar (archiver)

释义

File structure
Header Table of contents
References
External links

{{lowercase}}{{Infobox file format
| name = XAR
| icon =
| logo =
| screenshot =
| caption =
|_noextcode = on
| extensions = {{code|.xar}}, {{code|.pkg}}, {{code|.xip}}^[1]
| mime = application/x-xar^[1]
| type code =
| uniform type = com.apple.xar-archive
| magic = xar!
| owner =
| released =
| latest release version = 1.6.1
| latest release date = {{Start date and age|2012|09|17|df=yes}}
| genre = archive file format
| container for =
| contained by =
| extended from = XML, gzip
| extended to =
| standard =
| free =
| url = {{URL|https://mackyle.github.io/xar/}} {{URL|https://github.com/mackyle/xar}}
}}XAR (short for eXtensible ARchive format) is an open source file archiver and the archiver’s file format. It was created within the OpenDarwin project and is used in macOS X 10.5 and up for software installation routines, as well as browser extensions in Safari 5.0 and up. Xar replaced the use of gzipped pax files.^[2]

One development branch of RPM, RPM5, uses xar.^[3]

File structure

The file consists of three sections, the header, the table of contents and the heap. All fields are stored in big endian order.^[4]

Header

Offset#	Size (in bytes)	Purpose
0	4	File signature used to identify the file format as Xar. This should always equal `xar!`
4	2	Header size
6	2	Version of Xar format to use. Currently there is only version 1.
8	8	Length of the TOC compressed data.
16	8	Length of the TOC uncompressed data.
24	4	Checksum algorithm:{{plainlist}} 0 = none 1 = SHA1 2 = MD5 3 = SHA-256^[5] 4 = SHA-512 {{endplainlist}}
28	0, 4 , 36	sometimes padding bytes or checksum algorithm name

Xar is different from cpio, tar or ar in that it stores the TOC (table of contents) in the beginning of the file, making appending to an archive more complicated, but making it unnecessary to scan through the archive to extract an individual contained file. The table of contents is stored as a zlib compressed, UTF-8 encoded, XML document.^[6]^[4] Each file that is stored in the Xar is independently compressed/encoded. This gives the ability to have the file(s) encoded using gzip while having another file in the same archive encoded using a different method such as bzip2. On some systems the archive member can also be encoded by xz or lzma compression method.^[5]

Example Table of contents020staff20joe5010755directorycom.foobar205010775directoryContents428201005

a5f6f1461213a904f831d4ef6f214638342842ed

21d21a0c90378248ce0dfb6f345376d1b00d65fc

205010664fileInfo.plist205010775directoryResources14868448274432

efe5c97921de7ccc5aebc158d158e9d4280d6814

45c8be42d1d9afdb57ddd5e9311453010ec46161

205010664filefoobar176351531645056

3c761ffcc81ee6e232e4f4a1c4a81654c26c4e52

0ea31f8ef0e5987a1838a64ab5c26ebf3ee4bc37

205010664filedocSet.skidx205010775directoryDocuments3579032951209242

5242cd71585c34e722932f324706f8c00e1ae0c5

c0e013e53d829511835e2b429abb5198731e9a3e

205010664filefoobar.html

References

1. ^¹{{cite web|url=https://reposcope.com/mimetype/application/x-xar|language=en|title=application/x-xar|accessdate=2019-03-19}}
2. ^Nabble – xar in Mac OS X {{webarchive |url=https://web.archive.org/web/20070207045420/http://www.nabble.com/xar-in-Mac-OS-X-t2081148.html |date=February 7, 2007 }}
3. ^RPM 5 package manager
4. ^¹https://github.com/mackyle/xar/wiki/xarformat
5. ^¹{{cite web|url=https://www.unix.com/man-page/mojave/1/xar|title=xar - eXtensible ARchiver mojave man page|accessdate=2019-03-26|last1=Braun|first1=Rob|last2=Fuller|first2=Landon|last3=Leimbach|first3=David|last4=Van Vechten|first4=Kevin|date=2015-06-04}}
6. ^https://code.google.com/archive/p/xar/wikis/xarformat.wiki

External links

Xar homepage
[https://opensource.apple.com/source/xar/ Apple xar homepage]

1 : Archive formats

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