Microsoft Compressed HTML Help

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Microsoft Compressed HTML Help is a proprietary format for online help files, developed by Microsoft and first released in 1997 as a successor to the Microsoft WinHelp format. It was first introduced with the release of Windows 98, and is still supported and distributed through Windows XP platforms.

HTML Help files are made with help authoring tools. Microsoft ships the HTML Help Workshop with supported versions of Microsoft Windows and makes the tool available for free download. There are also a lot of third-party help authoring tools available.

The Microsoft Reader .LIT file format is basically a modification of the HTML Help CHM format.

In 2002, Microsoft announced some security risks associated with the .CHM format, as well as some security bulletins and patches [1]. They have since announced their intentions not to develop the .CHM format further, and will be moving to a new generation of Windows Help deemed Microsoft Assistance Markup Language in the Windows Vista operating system.

1 History
2 File Format
3 Advantages
4 Applications
5 Extracting to HTML
6 External links

[edit] History

February 1996 - Microsoft announced plans to stop development of WinHelp and start development on HTML Help.
August 1997 - HTML Help 1.0 (HH 1.0) was released with Internet Explorer 4.
February 1998 - HTML Help 1.1a shipped with Windows 98.
January 2000 - HTML Help 1.3 shipped with Windows 2000.
July 2000 - HTML Help 1.32 released with Internet Explorer 5.5 and Windows ME.
October 2001 - HTML Help 1.33 released with Internet Explorer 6 and Windows XP.
March 2001 - At the WritersUA (formerly WinWriters) conference, Microsoft announced plans for a new help platform Help 2, also HTML based.
January 2003 - Microsoft decided not to release Microsoft Help 2 as a general Help platform.

[edit] File Format

A CHM help file has a ".chm" extension and is often referred to as a "chum" file.^{[citation needed]} It has a set of web pages written in a subset of HTML and a hyperlinked table of contents. CHM format is optimized for reading, as files are heavily indexed. All files are compressed together with LZX compression. Most CHM browsers have the capability to display a table of contents outside of the body text of the help file.

The file starts with bytes "ITSF" (in ASCII), for "Info-Tech Storage Format". The format has been partially reverse engineered and there are container and internal files specifications available.

There are some open source tools which can read and explore these files (see for example xCHM, KchmViewer, GnoCHM or Chmox for OS X), but they lack various features of the Microsoft Windows tools.

[edit] Advantages

File size smaller than plain HTML
Range of formatting options that HTML gives for text presentation
Ability to search the full text
Ability to assemble several CHM files into one file with common TOC, index and search (see MSDN)

[edit] Applications

This format was originally intended only for encoding help files, but other uses have since been found. It is very handy for packing saved HTML pages in one compact and browsable archive and for creating compact ebooks. Some people use it to keep personal notes, because it can organize them in an ordered hierarchical table and allows quick text searching. There is a Firefox extension capable of viewing CHM files [2].

[edit] Extracting to HTML

On Windows, a CHM file can be extracted to plain HTML with the command:

hh.exe -decompile extracted filename.chm

This will decompress all files embedded in filename.chm to folder extracted.

or by using HTML Help Workshop.

On Unix systems which use apt as a packaging tool, a CHM file is extracted to plain HTML with (first command is for a Debian based OS)

 $ sudo apt-get install libchm-bin
 $ extract_chmLib tero.chm tero/

Another useful set of tools for CHM files in non-Windows environments is the CHM Tools Package. It's available as source code, and includes a program, chmdump, which extracts the HTML from a CHM file into a separate directory.

It's also available for Mac OS X via MacPorts.

If DarwinPorts is installed on your system, you can type: