Cabinet (file format)
Cabinet (file format)
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Cabinet (file format)

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Cabinet (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 four bytes (also called their magic number) MSCF. Cabinet files were known originally as Diamond files.

A CAB archive can contain up to 65,535 folders (distinct from standard operating system directories), each of which can contain up to 65,535 files for a maximum of 4,294,836,225. Internally, each folder is treated as a single compressed block, which provides more efficient compression than individually compressing each file.

Every entry in a folder has to be a file. 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 folders and files:

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

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.

Microsoft Windows supports creating CAB archive files using the makecab command-line utility. It supports extracting the contents of a CAB archive files using File Explorer, Setup API, and using the command-line commands expand.exe, extract.exe and extrac32.exe.

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