Microsoft Compiled HTML Help

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Microsoft Compiled HTML Help
Filename extension
.chm
Internet media type
application/vnd.ms-htmlhelp[1]
Developed byMicrosoft
Initial release1997
Latest release
1.4[2]
Extended to.lit
Microsoft HTML Help Executable
Other nameshh.exe
Help system

Microsoft Compiled HTML Help (CHM) is a

Microsoft WinHelp
.

Although the format was designed by Microsoft, it has been successfully reverse-engineered and is now supported by many document viewers.

History

CHM was introduced as the successor to

Windows NT family, the CHM file support is introduced in Windows NT 4.0[3][4] and is still supported in Windows 11.[5]

Month Year Description
February 1996 Microsoft announces plans to stop development of WinHelp and start development on HTML Help.
August 1997 HTML Help 1.0 (HH 1.0) is released with Internet Explorer 4.
February 1998 HTML Help 1.1a ships with Windows 98.
January 2000 HTML Help 1.3 ships with Windows 2000.
July HTML Help 1.32 releases with Internet Explorer 5.5 and Windows Me.
October 2001 HTML Help 1.33 releases with Internet Explorer 6 and Windows XP.
March At the WritersUA (formerly WinWriters) conference, Microsoft announces plans for a new help platform, Help 2, which is also HTML based.
January 2003 Microsoft decides not to release Microsoft Help 2 as a general Help platform.

Microsoft has announced that they do not intend to add any new features to HTML Help.[6]

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.[7][8]

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.[9]

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.[10]

The Microsoft Reader's .lit file format is a modification of the HTML Help CHM format. CHM files are sometimes used for e-books.[11]

Viewers

In addition to Microsoft Windows, the following apps support CHM:

Name Operating system Website
Okular Windows, Linux, Unix-like 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

Creators

Microsoft's HTML Help Workshop generates CHM files by instructions stored in a HTML Help project file, which bears a .HHP

file name extension and is a specialized form of INI file.[12]

command-line
compiler called chmcmd.

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.

See also

The Wikibook Windows Programming has a page on the topic of: Compiled HTML Help

References

  1. ^ Techtonik, Anatoly (11 April 2006). "application/vnd.ms-htmlhelp". Retrieved 7 March 2012.
  2. ^ "Microsoft HTML Help 1.4". Windows Dev Center. Microsoft. Retrieved 10 January 2017.
  3. ^ "Microsoft Security Bulletin MS02-055 - Critical". docs.microsoft.com. Retrieved 2020-10-29.
  4. ^ "Full Disclosure: HtmlHelp - .CHM File Heap Overflow". seclists.org. Retrieved 2020-10-29.
  5. ^ "[SOLVED] Windows 10 CHM Help Files showing up blank. - Spiceworks". community.spiceworks.com. Retrieved 2020-10-29.
  6. ^ "Microsoft HTML Help Downloads". Microsoft. Retrieved 6 March 2012.
  7. ^ Wise, Paul; Wing, Jed (2005). "Unofficial (Preliminary) HTML Help Specification". Retrieved 15 August 2012.
  8. ^ Palade, Alexandru (2005). "Archive::Chm". Retrieved 22 July 2014.
  9. ^ "Virus Bulletin :: Chamber of horrors". www.virusbulletin.com. Retrieved 2022-01-05.
  10. ^ "INFO: Limited Unicode Support in HTML Help". Microsoft. 11 April 2001. Archived from the original on 13 March 2016. Retrieved 17 March 2012.
  11. ISBN 978-1-84882-902-2
    .
  12. ^ Wise, Paul; Wing, Jed (2005). "Unofficial (Preliminary) HTML Help Specification INI formats". Retrieved 1 February 2018.
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