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Microsoft Compiled HTML Help (CHM)[a] is a Microsoft proprietary online help format, consisting of a collection of HTML pages, an index and other navigation tools. The files are compressed and deployed in a binary format with the extension .CHM. The format was intended to succeed Microsoft WinHelp.
Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported by many document viewers.
File format
Help is delivered as a binary file with the .chm extension. It contains a set of HTML files, a hyperlinked table of contents, and an index file. The file format has been reverse-engineered and documentation of it is freely available.[8][9]
The file starts with bytes "ITSF" (in ASCII), for "Info-Tech Storage Format", which is the internal name given by Microsoft to the generic storage file format used for CHM files.[10]
CHM files support the following features:
- Data compression (using LZX)
- Built-in search engine
- Ability to merge multiple .chm help files
- Extended character support, although it does not fully support Unicode.[11]
The Microsoft Reader's .lit file format is a modification of the HTML Help CHM format. CHM files are sometimes used for e-books.[12]
Viewers
In addition to Microsoft Windows, the following apps support CHM:
| Name |
Operating system |
Website |
|
|---|
| Okular |
Linux, Unix-like, Windows |
okular.kde.org
invent.kde.org/graphics/okular |
Uses hh.exe on Windows |
| Calibre |
Windows, macOS, Linux |
calibre-ebook.com
calibre on GitHub |
|
| Sumatra PDF |
Windows |
www.sumatrapdfreader.org
sumatrapdf on GitHub |
|
| GnoCHM |
Linux, BSD |
GnoCHM on SourceForge |
|
| CHM View |
None/Uncompiled |
chmviewkit on GitHub |
|
| kchmviewer |
Windows, Linux |
ulduzsoft.com/kchmviewer/ |
|
| KCHM |
Linux, BSD, Solaris |
KCHM on SourceForge |
|
| CHMPane |
Windows, macOS, Linux |
CHMPane on SourceForge |
|
| CHMate Neue |
iOS, iPadOS, visionOS |
CHMate Neue on iTunes |
|
| iChm |
iOS, macOS |
iChm on iTunes |
Discontinued |
| ChmPlus |
iOS, iPadOS, macOS, visionOS |
ChmPlus on iTunes |
|
| Chmox |
macOS |
chmox.sourceforge.net
Chmox on SourceForge |
|
| Clearview |
macOS |
Clearview on iTunes |
|
| DisplayCHM |
Linux |
linux-apps.com/p/998057 |
|
| KOReader |
Android, Linux |
koreader.rocks
Koreader on GitHub |
|
| 7-zip |
Windows (GUI), MacOS (command-line), Linux (command-line) |
www.7-zip.org
Ip7z on GitHub |
7-zip can show the contents of .chm files as though they are a compressed archive, but does not render the HTML as a help-viewer. |
Other utilities
The official viewer in Microsoft Windows (hh.exe) can decompile a CHM file. So can Microsoft HTML Help Workshop and 7-Zip. Calibre and arCHMage can convert CHM into another format.
Footnotes
- ↑ An initialism of "Compiled Help Module".[3]
References
- ↑ Techtonik, Anatoly (11 April 2006). "application/vnd.ms-htmlhelp". Retrieved 7 March 2012.
- ↑ "Microsoft HTML Help 1.4". Windows Dev Center. Microsoft. Retrieved 10 January 2017.
- ↑ "System Requirements". Microsoft Sync Framework Documentation. Microsoft Download Center. 2024-07-15.
Note: On computers with security update 896358 installed, downloaded Compiled Help Module (CHM) files cannot be opened by default.
- ↑ "Microsoft Security Bulletin MS02-055 - Critical". docs.microsoft.com. Retrieved 2020-10-29.
- ↑ "Full Disclosure: HtmlHelp - .CHM File Heap Overflow". seclists.org. Retrieved 2020-10-29.
- ↑ "[SOLVED] Windows 10 CHM Help Files showing up blank. - Spiceworks". community.spiceworks.com. February 2017. Retrieved 2020-10-29.
- ↑ "Microsoft HTML Help Downloads". Microsoft. Retrieved 6 March 2012.
- ↑ Wise, Paul; Wing, Jed (2005). "Unofficial (Preliminary) HTML Help Specification". Retrieved 15 August 2012.
- ↑ Palade, Alexandru (2005). "Archive::Chm". MetaCPAN. Retrieved 22 July 2014.
- ↑ "Virus Bulletin :: Chamber of horrors". www.virusbulletin.com. Retrieved 2022-01-05.
- ↑ "INFO: Limited Unicode Support in HTML Help". Microsoft. 11 April 2001. Archived from the original on 13 March 2016. Retrieved 17 March 2012.
- ↑ Salomon, David; Motta, Giovanni; Bryant, David (CON) (2009). Handbook of Data Compression (5th, illustrated ed.). Springer. ISBN 978-1-84882-902-2.
- ↑ Wise, Paul; Wing, Jed (2005). "Unofficial (Preliminary) HTML Help Specification INI formats". Retrieved 1 February 2018.
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