Editing 7-Zip (section)

==Archive formats==

===7z===
{{Main|7z}}

By default, 7-Zip creates 7z-format archives with a <code>.7z</code> [[file extension]]. Each archive can contain multiple directories and files. As a ''container'' format, security or size reduction are achieved by looking for similarities throughout the data using a stacked combination of filters. These can consist of pre-processors, [[Data compression|compression algorithms]], and [[encryption]] filters.

The core 7z compression uses a variety of [[algorithm]]s, the most common of which are [[bzip2]], [[Prediction by partial matching|PPMd]], [[Lempel–Ziv–Markov chain algorithm#LZMA2_format|LZMA2]], and [[Lempel–Ziv–Markov chain algorithm|LZMA]]. Developed by Pavlov, LZMA is a relatively new system, making its debut as part of the 7z format. LZMA uses an LZ-based sliding dictionary of up to 3840 MB in size, backed by a [[Range encoding|range coder]].<ref>{{cite web|last=Diaz|first=Antonio Diaz |title=Lzip|url=http://lzip.nongnu.org/lzip.html |website=LZip.NonGNU.org |access-date=29 July 2010| archive-url= https://web.archive.org/web/20100729135045/http://lzip.nongnu.org/lzip.html| archive-date= 29 July 2010 | url-status= live}}</ref>

The native 7z [[file format]] is open and [[Modular programming|modular]]. [[File name]]s are stored as [[UTF-16|Unicode]].<ref>{{cite web|url=https://www.7-zip.org/7z.html|title=7z Format|last=Pavlov|first=Igor|website=7-zip.org|access-date=16 April 2017}}</ref>

In 2011, [[TopTenReviews]] found that the 7z compression was at least 17% better than [[Zip (file format)|ZIP]],<ref>{{cite web|url=http://file-compression-software-review.toptenreviews.com/7-zip-review.html|title=7-Zip 2011 – TopTenREVIEWS|archive-url=https://web.archive.org/web/20121025200631/http://file-compression-software-review.toptenreviews.com/7-zip-review.html|archive-date=25 October 2012}}</ref> and 7-Zip's own site has since 2002 reported that while compression ratio results are very dependent upon the data used for the tests, "Usually, 7-Zip compresses to 7z format 30–70% better than to zip format, and 7-Zip compresses to zip format 2–10% better than most other zip-compatible programs."<ref>{{cite web|url=http://www.7-zip.org/ |title=7-Zip (home page) |archive-url=https://web.archive.org/web/20020121054121/http://www.7-zip.org/ |archive-date=21 January 2002 |url-status=live |website=7-zip.org |author=Igor Pavlov }} Statement on compression performance very similar in January 2002 and April 2017.</ref>

The 7z file format specification is distributed with the program's source code, in the "doc" sub-directory.

===Others===
7-Zip supports a number of other compression and non-compression archive formats (both for packing and unpacking), including [[ZIP (file format)|ZIP]], [[gzip]], [[bzip2]], [[XZ Utils|xz]], [[tar (file format)|tar]], and [[Windows Imaging Format|WIM]]. The utility also supports unpacking [[Apple Partition Map|APM]], [[Ar (Unix)|ar]], [[ARJ]], [[Microsoft Compiled HTML Help|chm]], [[cpio]], [[deb (file format)|deb]], [[Flash Video|FLV]], [[JAR (file format)|JAR]], [[LHA (file format)|LHA/LZH]], [[Lempel–Ziv–Markov chain algorithm|LZMA]], MSLZ, [[Office Open XML]], [[onepkg]], [[RAR (file format)|RAR]], [[RPM Package Manager|RPM]], [[smzip]], [[SWF]], [[Xar (archiver)|XAR]], and [[compress|Z]] archives and [[cramfs]], [[Apple Disk Image|DMG]], [[File Allocation Table|FAT]], [[Hierarchical File System (Apple)|HFS]], [[ISO image|ISO]], [[Master boot record|MBR]], [[NTFS]], [[SquashFS]], [[Universal Disk Format|UDF]], and [[Virtual Hard Disk|VHD]] [[disk image]]s. 7-Zip supports the ZIPX format for unpacking only. It has had this support since at least version 9.20, which was released in late 2010. 7-Zip added support for RAR5 in 2015.<ref>[https://www.7-zip.org/history.txt 7-zip.org/history.txt]</ref>

7-Zip can open some [[Windows Installer|MSI]] files, allowing access to the meta-files within along with the main contents. Some Microsoft CAB ([[LZX]] compression) and [[Nullsoft Scriptable Install System|NSIS]] (LZMA) installer formats can be opened. Similarly, some Microsoft executable programs (.[[EXE]]s) that are self-extracting archives or otherwise contain archived content (e.g., some setup files) may be opened as archives.

When compressing ZIP or gzip files, 7-Zip uses its own [[DEFLATE]] encoder, which may achieve higher compression, but at lower speed, than the more common [[zlib]] DEFLATE implementation. The 7-Zip deflate encoder implementation is available separately as part of the AdvanceCOMP suite of tools.

The decompression engine for [[RAR (file format)|RAR]] archives was developed using freely available source code of the unRAR program, which has a licensing restriction against creation of a RAR compressor. 7-Zip v15.06 and later support extraction of files in the RAR5 format.<ref>{{cite web|last1=Pavlov|first1=Igor|title=7-Zip 15.06 beta release notes|url=http://sourceforge.net/p/sevenzip/discussion/45797/thread/c611394e/#1783}}</ref> Some [[backup]] systems use formats supported by archiving programs such as 7-Zip; e.g., some [[Android (operating system)|Android]] backups are in <code>tar</code> format, and can be extracted by archivers such as 7-Zip.<ref>{{cite web|url=https://ajqi.com/how-to-extract-files-from-a-nandroid-backup/|title=How to extract files from a Nandroid Backup|author=Vijay|date=11 May 2013|website=AJQI|access-date=16 April 2017|archive-date=31 March 2017|archive-url=https://web.archive.org/web/20170331134328/http://ajqi.com/how-to-extract-files-from-a-nandroid-backup/|url-status=dead}}</ref>

Some forks add more formats.