Cabinet (file format)

Cabinet
Filename extension .cab
Internet media type application/vnd.ms-cab-compressed
Uniform Type Identifier (UTI) public.archive.cab
UTI conformation public.data
public.archive
Magic number MSCF
Developed by Microsoft
Type of format Archive file format

Cabinet (or CAB) is an archive-file format for Microsoft Windows that supports lossless data compression and embedded digital certificates used for maintaining archive integrity. Cabinet files have .cab filename extensions and are recognized by their first 4 bytes MSCF. Cabinet files were known originally as Diamond files.

The CAB file format may employ the following compression algorithms:

A CAB archive can reserve empty spaces in the archive as well as for each file in the archive, for some application-specific uses like digital signatures or arbitrary data. A variety of Microsoft installation technologies use the CAB format - these include Windows Installer, Setup API, Device Installer and AdvPack (used by Internet Explorer to install ActiveX components). CAB files are also often associated with self-extracting programs like IExpress where the executable program extracts the associated CAB file. CAB files are also sometimes embedded into other files. For example, MSI and MSU files (the latter are CAB files with just another filename extension) usually include one or more embedded CAB files.

File structure

A CAB archive can contain up to 65535 CAB-folders, (not to be confused with file system folders) each can contain up to 65535 files. Internally, each CAB-folder is treated as a single compressed block, which provides more efficient compression than individually compressing each file.

Every entry in a CAB-folder has to be a file.[1] Due to this structure, it is not possible to store empty folders in CAB archives.

The following shows an example a CAB file structure, demonstrating the relationship between CAB-folders and files:

CAB file
1st CAB-folder
Path: Records/Student_01.tsv
Path: Records/Photos/Student_01.jpg
2nd CAB-folder
Path: Records/Student_02.tsv
Path: Records/Photos/Student_02.jpg

How paths should be handled is not specified in the CAB file format, leaving it to the software implementation.

  • Some affix file paths to filenames only, as if all files in a CAB archive are in a single folder. IExpress works this way, as does Microsoft Windows Explorer, which can open CAB archives as a folder.
  • Some can store the paths, and upon extraction, create folders as necessary. CABARC.EXE and EXTRACT.EXE (tools from Microsoft Cabinet SDK[2]) as well as lcab[3] and cabextract[4] (third-party open-source tools) work this way.
  • EXPAND.EXE, only since version 6 (which is included from Windows Vista to above) can extract files to their paths. The previous versions don't do it.[5]

Software

Software that can extract the contents of a CAB archive file are numerous, including Microsoft Windows itself (through File Explorer, Setup API, expand.exe or extract.exe) as well as well-known software such as WinZip, WinRAR or 7-Zip. However, fewer programs can create CAB archives. For a full list, see Comparison of file archivers § archive formats.

The Windows program makecab.exe is used to create CAB archives:

  • Compress a single file into a CAB archive
makecab.exe <input_file>[ <output_file>]
  • Read the diamond directive file (with .ddf filename extension) and create a CAB archive containing multiple files in a flat or hierarchical structure like a file system.
makecab.exe /F <diamond_directive_file>[ <output_file>]

The .cab filename extension is also used by other installer programs (e.g. InstallShield) for their own proprietary archiving formats. InstallShield uses zlib for compression (see Deflate), but their headers are not the same as for Microsoft CAB files so they are incompatible and cannot be manipulated or edited with the programs that are made for standard Cabinet format. Specialized third-party utilities, such as Unshield,[6] can extract this specific proprietary format.

Microsoft Publisher has a "Pack and Go" feature that bundles a publisher document, together with all external links, into a CAB file with a .PUZ extension. These files are meant to be activated with a companion .EXE file which is distributed along with the .PUZ file. These files may be opened with any CAB file extraction program.

See also

References

  1. ↑ https://msdn.microsoft.com/en-us/library/bb267310.aspx Microsoft Cabinet Format
  2. ↑ "Microsoft Cabinet Software Development Kit". Support. Microsoft. Retrieved 27 March 2013.
  3. ↑ "lcab". Freecode. Dice. Retrieved 27 March 2013.
  4. ↑ "cabextract". Freecode. Dice. Retrieved 27 March 2013.
  5. ↑ "[Undocumented] [Bugs] Expand.exe (more about it) (Page 1) / Windows CMD Shell / SS64 Forum". ss64.org. Retrieved 21 April 2018.
  6. ↑ "twogood/unshield". GitHub. Retrieved 21 April 2018.


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