.dmg. Sit

Developer(s)Aladdin Systems, Smith Micro Software
Stable release
Operating systemmacOS, Windows
TypeFile Compressor
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StuffIt is a family of computer software utilities for archiving and compressing files. Originally produced for the Macintosh, versions for Microsoft Windows, Linux (x86), and Sun Solaris were later created.[1][2] The proprietary compression format used by the StuffIt utilities is also termed StuffIt.

  • 3Notable features
  • 4Issues


Filename extension.sit
Internet media typeapplication/x-stuffit
Type codeSIT!, SITD, SIT2, SIT5 (depending on file version)
Uniform Type Identifier (UTI)com.stuffit.archive.sit
UTI conformationpublic.date
Developed byRaymond Lau (creator), currently Smith Micro
Initial release1987; 32 years ago
Container forfiles, including resource forks

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StuffIt was originally developed in the summer of 1987 by Raymond Lau,[3] who was then a student at Stuyvesant High School in New York City. It combined the fork-combining capabilities of utilities such as MacBinary with newer compression algorithms similar to those used in ZIP. Compared to existing utilities on the Mac, notably PackIt, StuffIt offered 'one step' operation and higher compression ratios. By the fall of 1987 StuffIt had largely replaced PackIt in the Mac world, with many software sites even going so far as to convert existing PackIt archives to save more space.[citation needed]

StuffIt soon became very popular and Aladdin Systems was formed to market it (the last shareware release by Lau was version 1.5.1). They split the product line in two, offering StuffIt Classic in shareware and StuffIt Deluxe as a commercial package. Deluxe added a variety of additional functions, including additional compression methods and integration into the Mac Finder to allow files to be compressed from a 'Magic Menu' without opening StuffIt itself.[1]

StuffIt was upgraded several times, and Lau removed himself from direct development as major upgrades to the 'internal machinery' were rare. Because new features and techniques appeared regularly on the Macintosh platform, the shareware utility Compact Pro emerged as a competitor to StuffIt in the early 1990s.[citation needed]

A major competitive upgrade followed, accompanied by the release of the freeware StuffIt Expander, to make the format more universally readable, as well as the shareware StuffIt Lite which made it easier to produce. Prior to this anyone attempting to use the format needed to buy StuffIt, making Compact Pro more attractive. This move was a success, and Compact Pro subsequently fell out of use.

Several other Mac compression utilities appeared and disappeared during the 1990s, but none became a real threat to StuffIt's dominance. The only ones to see any widespread use were special-purpose 'disk expanders' like DiskDoubler and SuperDisk!, which served a different niche. Apparently as a side-effect, StuffIt once again saw few upgrades. The file format changed in a number of major revisions, leading to incompatible updates. PC-based formats long surpassed StuffIt in terms of compression, notably newer systems like RAR and 7z. These had little impact on the Mac market, as most of these never appeared in an easy-to-use program on the Mac.[citation needed]

With the introduction of Mac OS X, newer Mac software lost their forks and no longer needed anything except the built-in Unix utilities like gzip and tar. Numerous programs 'wrapping' these utilities were distributed, and since these files could be opened on any machine, they were considerably more practical than StuffIt in an era when most data is cross-platform. With the release of OS X Public Beta, Aladdin Systems released StuffIt 6.0 which runs under OS X, with Expander no longer relying on the StuffIt Engine™.[citation needed]

Stuffit X
Filename extension.sitx
Internet media typeapplication/x-stuffitx
Type codeSITX
Uniform Type Identifier (UTI)com.stuffit.archive.sitx
UTI conformationpublic.date
Initial release2002; 17 years ago
Type of formatarchive file format
Container forfiles, including resource forks

Although it was late to market, Aladdin Systems introduced the completely new StuffIt X format in September 2002 with StuffIt Deluxe 7.0 for Macintosh. It was designed to be extendable, support more compression methods, support long file names, and support Unix and Windows file attributes. StuffIt X improves over the original StuffIt format and its descendants by adding multiple compression algorithms such as PPM, and BWT to LZW-type compression. It also added a 'block mode' option and several encryption options. In January 2005, JPEG compression was added as a StuffIt X compression option (see the related 'SIF Format' below).

From the mid-1990s until the 2005 acquisition by Smith Micro Software, coinciding with the release of Mac OS X v10.4 'Tiger,' StuffIt Expander came bundled with the Macintosh operating system.

Although Mac files generally did not use filename extensions, one of StuffIt's primary uses was to allow Mac files to be stored on non-Mac systems where extensions were required.So, StuffIt-compressed files save the resource forks of the Macintosh files inside them, and typically have the extension .sit. Newer (non-backwards compatible) Stuffit X-compressed files carry the file extension .sitx. Encrypted StuffIt archives created with the now-discontinued Private File utility will have .pf extensions. StuffIt-compressed ShrinkWrapdisk images will carry .img or .image extensions. However, a Classic Mac OS version of StuffIt is needed to mount the images or convert them to a newer format readable in macOS.

Smith Micro Software offers free downloads of StuffIt Expander for Mac and Windows, which expands (uncompresses) files compressed using the StuffIt and StuffIt X format, as well as many other compressed, encoded, encrypted and segmented formats. The shareware application DropStuff permits the compressing of files into the StuffIt X format.

The StuffIt and StuffIt X formats remain, unlike some other file compression formats, proprietary, and Smith Micro Software charge license fees for its use in other programs. Given this, few alternative programs support the format.

There was also an 'self-expanding' variant of StuffIt files with a .sea extension that runs as an executable. A utility called unsea exists to turn such an executable into a vanilla sit file.[4]

StuffIt Image Format (SIF)[edit]

Early in 2005, a new JPEG compression system was released that regularly obtained compression in the order of 25%[5] (meaning a compressed file size 75% of the original file size) without any further loss of image quality and with the ability to rebuild the original file, not just the original image. (ZIP-like programs typically achieve JPEG compression rates in the order of 1 to 3%. Programs that optimise JPEGs without regard for the original file, only the original image, obtain compression rates from 3 to 10% (depending on the efficiency of the original JPEG). Programs that use the rarely implemented arithmetic coding option available to the JPEG standard typically achieve rates around 12%.)

The new technique was implemented as a StuffIt X format option in their StuffIt Deluxe product. They have also proposed a new image format known as SIF, which simply consists of a single JPEG file compressed using this new technique.

Pending filing of their patent, they retain knowledge of the details of this algorithm as a trade secret.

Notable features[edit]

Duplicate Folding[edit]

Duplicate Folding™ is a feature which saves even more space by only keeping one copy of a duplicate file in an archive.[6][7][8][9]


Backwards compatibility[edit]

Changes to the Stuffit compression software, claimed by the developer to be upgrades, frequently render previous versions of Stuffit unable to decompress newer archives without first downloading or purchasing the new version. This incompatibility can be inconvenient for work flows where timely execution is of importance. Though users are able to create archives in a legacy format, this functionality is not clearly exposed.[10]

StuffIt Wireless[edit]

On July 5, 2005, Smith Micro Software announced their acquisition and intention to expand the new JPEG recompression technique to wireless platforms and other file formats. The initial press release and preliminary information saw the first use of the title “StuffIt Wireless.”


macOS includes Archive Utility which is compatible with the open formats ZIP, gzip, and bzip2. In versions since 10.3 (Panther), it now preserves resource forks in the ZIP format, so Stuffit is no longer a requirement for Mac file compression. ZIP is also a de facto standard, making it more widely accepted for archives and sharing.

While StuffIt used to be a standard way of packaging Mac software for download, macOS native compressed disk images (DMG) have largely replaced this practice.

StuffIt might still be used in situations where its specific features are required (archive editing/browsing, better compression, JPEG compression, encryption, old packages). An open source alternative might be The Unarchiver, even if it doesn't support newer StuffIt file formats.[11] Some 3rd-party software, such as the Macintosh Finder replacement Path Finder, use the Stuffit SDK to gain all the features of Stuffit.

See also[edit]


  1. ^Official vendor information about StuffIt for Linux and SolarisArchived 2008-11-16 at the Wayback Machine
  2. ^Stuffit Archives, explains how to open StuffIt and StuffIt X archives on Linux
  3. ^'Raymond Lau: StuffIt'. www.raylau.com.
  4. ^'Archivers of Syllable'. www.angelfire.com.
  5. ^Gilchrist, Jeff. 'ACT JPEG Compression Test'. www.compression.ca.
  6. ^'Archived copy'. Archived from the original on 2011-08-31. Retrieved 2011-08-15.CS1 maint: archived copy as title (link)
  7. ^'StuffIt Deluxe 2009 Keeps Evolving After 20 Years'. TidBITS. 17 September 2008.
  8. ^'Review: StuffIt Deluxe 2010'. 30 January 2010.
  9. ^'StuffIt - The Premier Compression Solution Celebrates Its 20th Anniversary with the Launch of StuffIt Deluxe 2009 for the Mac'. www.businesswire.com. 17 September 2008.
  10. ^'Macworld - News, tips, and reviews from the Apple experts'. Macworld.
  11. ^'The Unarchiver'. StuffIt: No encryption; StuffIt X: Partial

External links[edit]

Retrieved from 'https://en.wikipedia.org/w/index.php?title=StuffIt&oldid=917703845'
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Apple Disk Image
The icon represents an internal hard drive within a generic file icon.
Filename extension.dmg, .smi, .img
Internet media typeapplication/x-apple-diskimage
Uniform Type Identifier (UTI)com.apple.disk-image
Developed byApple Inc.
Type of formatDisk image

Apple Disk Image is a disk image format commonly used by the macOS operating system. When opened, an Apple Disk Image is mounted as a volume within the Macintosh Finder.

An Apple Disk Image can be structured according to one of several proprietary disk image formats, including the Universal Disk Image Format (UDIF) and the New Disk Image Format (NDIF). An Apple disk image file's name usually has '.dmg' as its extension.

  • 3File format


Apple Disk Image files are published with a MIME type of application/x-apple-diskimage.

Different file systems can be contained inside these disk images, and there is also support for creating hybrid optical media images that contain multiple file systems.[1] Some of the file systems supported include Hierarchical File System (HFS), HFS Plus, File Allocation Table (FAT), ISO9660 and Universal Disk Format (UDF).[1][2]

Apple Disk Images can be created using utilities bundled with Mac OS X, specifically Disk Copy in Mac OS X v10.2 and earlier and Disk Utility in Mac OS X v10.3 and later. These utilities can also use Apple disk image files as images for burning CDs and DVDs. Disk image files may also be managed via the command line interface using the hdiutil utility.[3]

In Mac OS X v10.2.3, Apple introduced Compressed Disk Images[4] and Internet-Enabled Disk Images for use with the Apple utility Disk Copy, which was later integrated into Disk Utility in 10.3. The Disk Copy application had the ability to display a multi-lingual software license agreement before mounting a disk image. The image will not be mounted unless the user indicates agreement with the license.[5]

An Apple Disk Image allows secure password protection as well as file compression, and hence serves both security and file distribution functions; such a disk image is most commonly used to distribute software over the Internet.

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Apple originally created its disk image formats because the resource fork used by Mac applications could not easily be transferred over mixed networks such as those that make up the Internet. Even as the use of resource forks declined with Mac OS X, disk images remained the standard software distribution format. Disk images allow the distributor to control the Finder's presentation of the window, which is commonly used to instruct the user to copy the application to the correct folder.

A previous version of the format, intended only for floppy disk images, is usually referred to as 'Disk Copy 4.2' format, after the version of the Disk Copy utility that was used to handle these images.[1] A similar format that supported compression of floppy disk images is called DART.[1][6]

New Disk Image Format (NDIF) was the previous default disk image format in Mac OS 9,[1] and disk images with this format generally have a .img (not to be confused with raw .img disk image files) or .smi file extension. Files with the .smi extension are actually applications that mount an embedded disk image, thus a 'Self Mounting Image', and are intended only for Mac OS 9 and earlier.[7][2]

Universal Disk Image Format (UDIF) is the native disk image format for Mac OS X. Disk images in this format typically have a .dmg extension.[1]

File format[edit]

Apple has not released any documentation on the format, but attempts to reverse engineer parts of the format have been successful. The encrypted layer was reverse engineered in an implementation called VileFault[8] (a spoonerism of FileVault).

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Apple disk image files are essentially raw disk images (i.e. contain block data) with some added metadata, optionally with one or two layers applied that provide compression and encryption. In hdiutil these layers are called CUDIFEncoding and CEncryptedEncoding.[1]

UDIF supports ADC (an old proprietary compression format by Apple), zlib, bzip2 (as of Mac OS X v10.4), and LZFSE (as of Mac OS X v10.11)[9] compression internally.

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The trailer can be described using the following C structure.[10] All values are big-endian (PowerPC byte ordering)

Here is an explanation:

Position(in Hex)Length (in bytes)Description
0004Magic bytes ('koly').
0044File version (current is 4)
0084The length of this header, in bytes. Should be 512.
0188Data fork offset (usually 0, beginning of file)
0208Size of data fork (usually up to the XMLOffset, below)
0288Resource fork offset, if any
0308Resource fork length, if any
0384Segment number. Usually 1, may be 0
03C4Segment count. Usually 1, may be 0
04016128-bit GUID identifier of segment
0504Data fork checksum type
0544Data fork checksum size
058128Data fork checksum
0D88Offset of XML property list in DMG, from beginning
0E08Length of XML property list
0E8120Reserved bytes
1604Master checksum type
1644Master checksum size
168128Master checksum
1E84Unknown, commonly 1
1EC8Size of DMG when expanded, in sectors
1F412Reserved bytes (zeroes)


There are few options available to extract files or mount the proprietary Apple Disk Image format. Some cross-platform conversion utilities are:

  • dmg2img was originally written in Perl; however, the Perl version is no longer maintained, and the project was rewritten in C. Currently, without additional tools, the resulting images may be mounted only under Mac OS X and under Linux (provided hfsplus support has been enabled). UDIF ADC-compressed images have been supported since version 1.5.[11]
  • DMGEXtractor is written in Java with GUI, and it supports more advanced features of dmg including AES-128 encrypted images but not UDCO images.[12]
  • 7-Zip, including the free cross-platform port of its command-line interface, p7zip.

In Windows, most dmg images can be opened using several other programs such as UltraISO and IsoBuster. MacDrive can also mount simple dmg files as drives under windows, but not sparse disk or encrypted dmgs.[13] A free Apple DMG Disk Image Viewer also exists.[14]

In Linux and possibly other Unix flavors, most .dmg files can be burned to CD/DVD using any CD-burner program (using cdrecord directly or a front-end such as K3B or Brasero) or directly mounted to a mountpoint (e.g. mount -o loop,ro -t hfsplus imagefile.dmg /mnt/mountpoint).[15][16] darling-dmg is a FUSE module enabling easy DMG file mounting on Linux.[17]

See also[edit]


  1. ^ abcdefg'hdiutil(1) Mac OS X Manual Page'. Archived from the original on 2016-05-14. Retrieved 2016-05-14.
  2. ^ ab'Mac OS X: Using Disk Copy disk image files'. Archived from the original on 2013-12-19. Retrieved 2013-04-25.
  3. ^hdiutil(1) – Darwin and macOS General Commands Manual
  4. ^'Re: Some apps refuse to launch in 10.2.8! (OT, but very important)'. Archived from the original on 2014-01-17.
  5. ^'Guides'. Apple. Archived from the original on 2009-03-06. Retrieved 2010-10-27.
  6. ^'DART 1.5.3: Version Change History'. Archived from the original on 2013-12-19. Retrieved 2013-04-25.
  7. ^'Software Downloads: Formats and Common Error Messages'. Archived from the original on 2010-12-24. Retrieved 2009-05-06.
  8. ^'VileFault'. 2006-12-29. Archived from the original on 2007-01-09. Retrieved 2010-10-27.
  9. ^Michael Tsai (2015-10-07). 'LZFSE Disk Images in El Capitan'. Archived from the original on 2017-04-09. Retrieved 2017-04-09.
  10. ^'Demystifying the DMG File Format'. Archived from the original on 2013-03-17.
  11. ^'dmg2img'. Archived from the original on 29 November 2010. Retrieved 2010-10-27.
  12. ^'DMGExtractor'. Archived from the original on 2011-01-02. Retrieved 2011-01-03.
  13. ^MacDrive Features / Boot Camp / System Requirements /. 'MacDrive Home page'. Mediafour. Archived from the original on 4 October 2010. Retrieved 2010-10-27.
  14. ^Olivia Dehaviland (2015-03-03). 'Apple DMG Disk Image Viewer'. DataForensics.org. Archived from the original on 2015-04-02. Retrieved 2015-03-10.
  15. ^'How To Convert DMG To ISO in Windows, Linux & Mac'. Archived from the original on 2010-03-07.
  16. ^'Convert DMG To ISO using PowerISO'. Archived from the original on 2009-05-02. Retrieved 2009-02-15.
  17. ^'darling-dmg'. darling-dmg. Retrieved 29 March 2015.

External links[edit]

  • Apple Developer Connection A Quick Look at PackageMaker and Installer
  • O'Reilly Mac DevCenter Tip 16-5. Create a Disk Image from a Directory in the Terminal
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Apple_Disk_Image&oldid=917998972'