RAR

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RAR stands for Roshal ARchive. It is a proprietary^[2] archive file format that supports data compression, error recovery, and file spanning. It was developed by a Russian software engineer, Eugene Roshal (the first letter of his surname contributing to the name of the archive format), and is currently licensed by win.rar GmbH.^[3]

The filename extension used by RAR is .rar for the data volume set and .rev for the recovery volume set. In previous versions, if a RAR-archive was split into many smaller files (a "multi-volume archive"), the smaller files used the extensions .rar, .r00, .r01, .r02 etc.

Version 1 and 2 archive files were often used in conjunction with a parchive file archiver to create parity files for error recovery when using less-than-perfect file transmission and storage media such as newsgroups, satellite transmission, and optical discs. Version 3 has eliminated the need for third party post-processing.

Compression algorithm

The RAR compression utility is proprietary, with a closed algorithm. RAR is owned by Alexander L. Roshal,^[2] the elder brother of Eugene Roshal. Version 3 of RAR is based on Lempel-Ziv (LZSS) and prediction by partial matching (PPM) compression, specifically the PPMd implementation of PPMII by Dmitry Shkarin.^[4]

Versions

Several versions of the RAR format have been noted by third party developers:

• RAR1.3 (original, does not have "Rar!" signature)
• RAR1.5
• RAR2
• RAR3 (current) - implemented by developers of Rarlab WinRAR version 2.9 and released in WinRAR version 3.00. Many changes including:
  • File extensions changed from {volume name}.rar, {volume name}.r00, {volume name}.r01, etc. to {volume name}.part001.rar, {volume name}.part002.rar, etc.
  • Encryption algorithm is changed to cipher block chaining (CBC) mode over AES (Advanced Encryption Standard) with 128 bit key length.
  • Encryption of both file data and file headers.
  • Improved compression algorithm using 4MB dictionary size, Dmitry Shkarin's PPMII algorithm for file data, and selective preprocessing algorithms based on platform and source file type.
  • Optional creation of "recovery volumes" (.rev files) with redundancy data which can be used to reconstruct missing files in a volume set.
  • Support for archive files larger than 9 GB.
  • Support for Unicode names.

Software

RAR files may be created only with commercial software WinRAR, RAR, and software that has permission from the licensor Alexander Roshal (Eugene's brother).^[2] RAR for Pocket PC is the only freeware for creating RAR files.^{[citation needed]}

Third-party programs that can only read (unpack) RAR files include: WinZip (Windows), 7-Zip (multiplatform), Bandizip (Windows, Mac OS X), IZArc (Windows), PeaZip (Windows, Linux), Zipeg (Windows, Mac OS X), ALZip (Windows) and PowerArchiver (Windows), along with the free version of unrar (which is available for Linux and FreeBSD). Mac OS X readers include StuffIt Expander, The Unarchiver, and Zipeg. Stuffit Expander is also available for Mac OS Classic with RAR support for this platform.

WinRAR

WinRAR is commercial software (or 40-day trial)^[2] available for Microsoft Windows. It is the only graphical tool that can write modern RAR files (RAR version 3). WinRAR's command line equivalent, RAR, is also commercial software (or 40-day trial), available for Windows, Linux, Mac OS X, MS-DOS, OS/2, and FreeBSD. Additionally, the unrar tool from the same distributor can be used on the aforementioned platforms to extract RAR files but not to write them.

Roshal created the RAR file format and developed programs for packing and unpacking RAR files, originally for DOS in 1993, which were later ported to other platforms. The main Windows version of the archiver, known as WinRAR, is distributed as trialware, requiring payment after 40 days (although it can still be used after this period, albeit with nags^{[citation needed]}); shareware versions of this program are also available for GNU/Linux, Mac OS X, DOS, OS/2, and FreeBSD, though they are all called simply "RAR".

RARLAB distributes the source code and binaries for a freeware command-line "unrar" program,^[5] although it is not under a free software license. This program can only decompress/extract, not create RAR files.

Other software

• There is a free software decompression library called "unrarlib", licensed under the GPL, based on an old version of unrar with permission from the author Eugene Roshal, but it can only decompress archives created by RAR versions up to 2.x. Archives created by RAR 2.9 and later (which are most RAR archives found today) use a different format which is not supported by the free library.

• The paid versions of Stuffit Expander decode RAR files.
• The open source software archiver 7-Zip decompresses newer RAR files using a closed-source, free-of-charge plug-in under the "unRAR license";^[6] the license makes the plug-in source-available but not free software.
• The Unarchiver is another open source unarchiver which can decompress newer RAR files.
• The free software Linux archivers File Roller, Ark, and Xarchiver can use the free UnRAR program to decompress RAR files, if they are found in the system path.

UnRAR.dll

UnRAR.dll is a free of charge dynamic-link library (DLL) that can be downloaded from the official WinRAR site.

Comparison to other compression algorithms

RAR is typically slower than the "zip" command for creating ZIP files, but it has a moderately better rate of compression.^[7]

7z's LZMA algorithm has a higher compression ratio than RAR, except for "multimedia" files like .wav and .bmp files, for which RAR uses specialized routines that outperform LZMA.^[8] Other free compression software, such as FreeArc, usually outperform WinRAR.^[9]

RAR can create "recovery volumes" (.rev files), which can provide redundancy for a main archive.

RAR file limitations

The minimum size of a RAR file is 20 bytes. The maximum size of a RAR file is 9,223,372,036,854,775,807 (2⁶³-1) bytes, which is 8 exabytes.

See also

• Comparison of archive formats
• List of archive formats
• Comparison of file archivers

Further reading

• RAR Archiver Frequently Asked Questions by Eugene Roshal
• RAR File format by WinRAR
• LAMPdocs: quick guide with annotation of the advantages of RAR in comparison to other file archivers

External links

• Official RAR site

References