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Microsoft's HTML Help (.chm) format


This is documentation on the .chm format used by Microsoft HTML Help. This format has been reverse engineered in the past, but as far as I know this is the first freely available documentation on it. One Usenet message indicates that these .chm files are actually IStorage files documented in the Microsoft Platform SDK. However, I have not been able to locate such documentation.


The word "section" is badly overloaded in this document. Sorry about that.

All numbers are in hexadecimal unless otherwise indicated in the text. Except in tabular listings, this will be indicated by $ or 0x as appropriate. All values within the file are Intel byte order (little endian) unless indicated otherwise.

The overall format of a .chm file

The .chm file begins with a short ($38 byte) initial header. This is followed by the header section table and the offset to the content. Collectively, this is the "header".

The header is followed by the header sections. There are two header sections. One header section is the file directory, the other contains the file length and some unknown data. Immediately following the header sections is the content.

The header starts with the initial header, which has the following format

0000: char[4]  'ITSF'
0004: DWORD    3 (Version number)
0008: DWORD    Total header length, including header section table and
               following data.
000C: DWORD    1 (unknown)
0010: DWORD    a timestamp.
               Considered as a big-endian DWORD, it appears to contain
               seconds (MSB) and fractional seconds (second byte).
	       The third and fourth bytes may contain even more fractional
               bits.  The 4 least significant bits in the last byte are
0014: DWORD    Windows Language ID.  The two I've seen
               $0409 = LANG_ENGLISH/SUBLANG_ENGLISH_US
               $0407 = LANG_GERMAN/SUBLANG_GERMAN
0018: GUID     {7C01FD10-7BAA-11D0-9E0C-00A0-C922-E6EC}
0028: GUID     {7C01FD11-7BAA-11D0-9E0C-00A0-C922-E6EC}

Note: a GUID is $10 bytes, arranged as 1 DWORD, 2 WORDs, and 8 BYTEs.

It is followed by the header section table, which is 2 entries, where each entry is $10 bytes long and has this format:

0000: QWORD    Offset of section from beginning of file
0008: QWORD    Length of section

Following the header section table is 8 bytes of additional header data. In Version 2 files, this data is not there and the content section starts immediately after the directory.

0000: QWORD    Offset within file of content section 0

The Header Sections

Header Section 0

This section contains the total size of the file, and not much else

0000: DWORD    $01FE (unknown)
0004: DWORD    0 (unknown)
0008: QWORD    File Size
0010: DWORD    0 (unknown)
0014: DWORD    0 (unknown)

Header Section 1: The Directory Listing

The central part of the .chm file: A directory of the files and information it contains.

Directory header

The directory starts with a header; its format is as follows:

0000: char[4]  'ITSP'
0004: DWORD    Version number 1
0008: DWORD    Length of the directory header
000C: DWORD    $0a (unknown)
0010: DWORD    $1000    Directory chunk size
0014: DWORD    "Density" of quickref section, usually 2.
0018: DWORD    Depth of the index tree
               1 there is no index, 2 if there is one level of PMGI
001C: DWORD    Chunk number of root index chunk, -1 if there is none
               (though at least one file has 0 despite there being no
	       index chunk, probably a bug.) 
0020: DWORD    Chunk number of first PMGL (listing) chunk
0024: DWORD    Chunk number of last PMGL (listing) chunk
0028: DWORD    -1 (unknown)
002C: DWORD    Number of directory chunks (total)
0030: DWORD    Windows language ID
0034: GUID     {5D02926A-212E-11D0-9DF9-00A0C922E6EC}
0044: DWORD    $54 (This is the length again)
0048: DWORD    -1 (unknown)
004C: DWORD    -1 (unknown)
0050: DWORD    -1 (unknown)

The Listing Chunks

The header is directly followed by the directory chunks. There are two types of directory chunks -- index chunks, and listing chunks. The index chunk will be omitted if there is only one listing chunk. A listing chunk has the following format:

0000: char[4]  'PMGL'
0004: DWORD    Length of free space and/or quickref area at end of
               directory chunk 
0008: DWORD    Always 0. 
000C: DWORD    Chunk number of previous listing chunk when reading
               directory in sequence (-1 if this is the first listing chunk)
0010: DWORD    Chunk number of next listing chunk when reading
               directory in sequence (-1 if this is the last listing chunk)
0014: Directory listing entries (to quickref area)  Sorted by
      filename; the sort is case-insensitive.

The quickref area is written backwards from the end of the chunk. One quickref entry exists for every n entries in the file, where n is calculated as 1 + (1 << quickref density). So for density = 2, n = 5.

Chunklen-0002: WORD     Number of entries in the chunk
Chunklen-0004: WORD     Offset of entry n from entry 0
Chunklen-0008: WORD     Offset of entry 2n from entry 0
Chunklen-000C: WORD     Offset of entry 3n from entry 0

The format of a directory listing entry is as follows

      ENCINT: length of name
      BYTEs: name  (UTF-8 encoded)
      ENCINT: content section
      ENCINT: offset
      ENCINT: length

The offset is from the beginning of the content section the file is in, after the section has been decompressed (if appropriate). The length also refers to length of the file in the section after decompression.

There are two kinds of file represented in the directory: user data and format related files. The files which are format-related have names which begin with '::', the user data files have names which begin with "/".

The Index Chunk

An index chunk has the following format

0000: char[4]  'PMGI'
0004: DWORD    Length of quickref/free area at end of directory chunk
0008: Directory index entries (to quickref/free area)

The quickref area in an PMGI is the same as in an PMGL

The format of a directory index entry is as follows

      ENCINT: length of name
      BYTEs: name  (UTF-8 encoded)
      ENCINT: directory listing chunk which starts with name

When higher-level indexes exist (when the depth of the index tree is 3 or higher), presumably the upper-level indexes will contain the numbers of lower-level index chunks rather than listing chunks

Encoded Integers

An ENCINT is a variable-length integer. The high bit of each byte indicates "continued to the next byte". Bytes are stored most significant to least significant. So, for example, $EA $15 is (((0xEA&0x7F)<<7)|0x15) = 0x3515.

The Content

In Version 3, the content typically immediately follows the header sections, and is at the location indicated by the DWORD following the header section table. In Version 2, the content immediately follows the header. All content section 0 locations in the directory are relative to that point. The other content sections are stored WITHIN content section 0.

The Namelist file

There exists in content section 0 and in the directory a file called "::DataSpace/NameList". This file contains the names of all the content sections. The format is as follows:

0000: WORD     Length of file, in words
0002: WORD     Number of entries in file

Each entry:
0000: WORD     Length of name in words, excluding terminating null
0002: WORD     Double-byte characters
xxxx: WORD     0

Yes, the names have a length word AND are null terminated; sort of a belt-and-suspenders approach. The coding system is likely UTF-16 (little endian).

The section names seen so far are

  • Uncompressed
  • MSCompressed

"Uncompressed" is self-explanatory. The section "MSCompressed" is compressed with Microsoft's LZX algorithm.

The Section Data

For each section other than 0, there exists a file called '::DataSpace/Storage/<Section Name>/Content'. This file contains the compressed data for the section. So, conceptually, getting a file from a nonzero section is a multi-step process. First you must get the content file from section 0. Then you decompress (if appropriate) the section. Then you get the desired file from your decompressed section.

Other section format-related files

There are several other files associated with the sections

  • ::DataSpace/Storage/<SectionName>/ControlData

    This file contains $20 bytes of information on the compression. The information is partially known:

    0000: DWORD    Number of DWORDs following 'LZXC', must be 6 if version is 2
    0004: ASCII    'LZXC'  Compression type identifier
    0008: DWORD    Version (Must be <=2)
    000C: DWORD    The LZX reset interval
    0010: DWORD    The window size
    0014: DWORD    The cache size
    0018: DWORD    0 (unknown)

    Reset interval, window size, and cache size are in bytes if version is 1, $8000-byte blocks if version is 2.

  • ::DataSpace/Storage/<SectionName>/SpanInfo

    This file contains a quadword containing the uncompressed length of the section.

  • ::DataSpace/Storage/<SectionName>/Transform/List

    It appears this file was intended to contain a list of GUIDs belonging to methods of decompressing (or otherwise transforming) the section. However, it actually contains only half of the string representation of a GUID, apparently because it was sized for characters but contains wide characters.

Appendix: The Compression

The compressed sections are compressed using LZX, a compression method Microsoft also uses for its cabinet files. To ensure this, check the second DWORD of compression info in the ControlData file for the section — it should be 'LZXC'. To decompress, first read the file "::DataSpace/Storage/<SectionName>/Transform/{7FC28940-9D31-11D0-9B27-00A0C91E9C7C}/InstanceData/ResetTable". This reset table has the following format

0000: DWORD    2     unknown (possibly a version number)
0004: DWORD    Number of entries in reset table
0008: DWORD    8     Size of table entry (bytes)
000C: DWORD    $28   Length of table header (area before table entries)
0010: QWORD    Uncompressed Length
0018: QWORD    Compressed Length
0020: QWORD    0x8000 block size for locations below
0028: QWORD    0 (zeroth entry of table)
0030: QWORD    location in compressed data of 1st block boundary in
               uncompressed data

Repeat to end of file

Now you can finally obtain the section (from its Content file). The window size for the LZX compression is 16 (decimal) on all the files seen so far. This is specified by the DWORD at $10 in the ControlData file (but note that DWORD gives the window size in 0x8000-byte blocks, not the LZX code for the window size)

The rule that the input bit-stream is to be re-aligned to a 16-bit boundary after $8000 output characters have been processed IS in effect, despite this LZX not being part of a CAB file. The reset table tells you when this was done, though there is no need for that during decompression; you can just keep track of the number of output characters. Furthermore, while this does not appear to be documented in the LZX format, the uncompressed stream is padded to an $8000 byte boundary.

There is one change from LZX as defined by Microsoft: After each LZX reset interval (defined in the ControlData file, but in practice equal to the window size) of compressed data is processed, the LZX state is fully reset, as if an entirely new file was being encoded. This allows semi-random access to the compressed data; you can start reading on any reset interval boundary using the reset interval size and the reset table.

Earlier versions of this document stated that the reset interval only reset the Huffman tables and required outputting the 1-bit header again. This was erroneous. The Lempel Ziv state is reset as well. In practice, a decoder works just as well with the incorrect assumption, but encoding a file with match positions which refer to a time before the most recent LZX reset causes crashes on decoding.


The following people in (no particular order) have submitted information which has helped correct and close the gaps in this document.

  • Peter Ferrie (peter_ferrie at Web Site
  • Pabs (pabs at who also runs the CHM Spec page.

And others I have not been able to reach.

Copyright 2001-2003 Matthew T. Russotto

You may freely copy and distribute unmodified copies of this file, or copies where the only modification is a change in line endings, padding after the html end tag, coding system, or any combination thereof. The original is in ASCII with Unix line endings.


HTML Help (CHM) Tools and Information

HTML Help format
An incomplete description of Microsoft's .CHM format.

ITOL/ITLS format
A description of Microsoft's ITOL/ITLS format, which is used by HTML Help 2.0 among other things.

CHM Tools package
A set of tools for working with the CHM files, consisting of a C language library 'chmlib' and a program called 'chmdump' which dumps out the files in a CHM file.

Not everything in the document is implemented here, but it is a start, and an LZX decompression engine (from Stuart Caie's "cabextract", suitably modified) is included. License is the GPL, following "cabextract".

I also have a C++ library for reading CHM and ITOL/ITLS formats, including the ability to use arbitrary transforms in the latter.

LZX compression package (lzxcomp)
An LGPLed LZX compression engine, suitable for creating compressed CHM files. Or for use in a CAB-making utility or for any other purpose LZX is useful for.

Documentation for the lzxcomp library included and on-line.


Changed May 3 2100 EST: fixed a really dumb bug introduced last minute.
Also allowed LZ compressor to look into the match buffer, for a significant compression improvement.


  • hhm (GPL2): hhm (HTML Help Maker) is a program that makes ITS files and in the future it will also make Compiled HTML Help (CHM) files. Both types of files are a kind of compressed archive format used on Win98, Win2K and other Microsoft operating systems to store documentation.
  • chmdeco (GPL2): chmdeco (CHM decompiler) is a program that converts the internal files of CHM files back into the hhp, hhc, hhk etc used to compile the documentation.
  • chmspec (GPL2): chmspec (CHM specification) is an effort to document Microsoft's Compiled HTML Help files (CHMs), mainly the internal files, since the archive format is documented already.
  • istorage (BSD): This is just a simple Windows proggie to extract files from those pesky MS compound file objects accessible via OLE's StgOpenStorage fuction and the IStorage interface exposed by that function. These compound file objects are created by word, excel, & probably other MS progs. Also Macromedia Flash source files (*.fla - there are some of these available from, which is an interesting site) are these compound files. These compound file objects can be thought of as the equivalent of tar files, but of course MS went & invented some new format without even considering .zip, .lha, .tar.gz, .cab, blah blah blah. One weird thing about them is that the IStreams inside can & for word & excel do have freaky chars in their names, often as the first char such as in word 2000 docs there are streams named "SummaryInformation" (that is an 0x05 - there are also ones with 0x01). MSDEV .opt files are also these compound files. I updated this in April 2002 to extract Compiled HTML Help (chm) files (also known as InfoTech Storage (ITS) files) too. This feature uses the same IStorage interface, but uses an ITStorage (CLSID=5D02926A-212E-11D0-9DF9-00A0C922E6EC) object (from itss.dll) got from CoCreateInstance to open the file. I found out how to do this from these two code samples: &
  • istorage-make (unicode version) (BSD): This is just a simple proggie to create those pesky MS compound file and InfoTech Storage (ITS) files too. It uses the same interfaces as istorage (an extractor for these files).
  • indychat (public domain): this is the code behind, the old page was really crappy, so I wrote the new php version and patched and installed the new chat software.
  • pinballcheat (GPL2): This is a little proggy to help you change the High score table for Microsoft's 3D Pinball game - specifically SpaceCadet but you (or I on request) can easily recompile it for other tables.
  • clap (public domain): clap is a Windows program that monitors the Win32 clipboard and appends each piece of data copied to an internal clipboard, which it then sends back to the Win32 clipboard. The effect is that it seems that each copy operation appends data to the clipboard. Currently only works with the CF_TEXT clipboard format.
  • emxwrapper (public domain): & are files from the emx runtime on OS/2. I have created a ncurses/panel wrapper for these files, which means that programs written specifically for these files can be compiled for ncurses/panel on platforms that support these two libraries, such as GNU/Linux, various UNIXes & GNU/Cygwin. It was written on GNU/Cygwin using ncurses & panel so there may be incompatibility issues. There are likely lots of bugs so if it doesn't work try to fix it & send me a diff -u or just complain & I'll try to fix it. It is by no means complete since the program I developed it for (VBinDiff 1.7, which is available at the Hobbes OS/2 Archive) doesn't use all the winmgr.h & kbdscan.h functions & constants.
  • bezier (public domain): Get my bezier include file that I was using in a space animation. Use the functions HermiteLP, BezierLP, BezierLPTension to load points into the geometry matrix. Use p(u) to get the point, pd(u) to get the derivative, pdd(u) to get the second derivative & pddd(u) to get the third derivative. Other functions return curvature, the principle normal and binormal vectors, torsion (doesn't work quite right yet), and some as yet unfinished macros for orienting objects along the spline & doing banking & stuff. I think the math is right - email me if not
  • df3maker (public domain): a simple proggie to make df3 files for povray. Ported to C++ from the Visual Basic version by Mark James Lewin. His page is down, so it is available here and at
  • xchat scripts (public domain): Some python/etc scripts for xchat that I find useful.
  • ~/bin scripts (public domain): Some scripts I have written for my ~/bin folder that I find useful.
  • zwiki to twiki (public domain): a script to extract some data from zwiki and prepare it for importing to twiki.



I have contributed various patches that are not listed here to various projects.


I've some developed patches for the following packages that are still pending.

mailman cgiirc et al.

not included

  • scite
    • A half complete 133t
    • A patch to make the window title be a reversed path (eg patches.txt\ideas\pabs\:H). This is useful when you have enough windows open for your taskbar to only show part of the filename, but Neil refused to put it into the official distribution. I have diffs against 1.46 and 1.49.
  • imc_aggregator
  • povray
    • particles (same licence as upstream - will relicence if they change licences): This was based on a project by James Neill - see his projects page. Chris Huff had integrated the ParticlePatch into his MegaPOV+ (with new particle system simulations) and had windows & mac compiles of it at his homepage. Mark Gordon had a Linux compile of MegaPOV+ at his homepage. Also the glow patch in MegaPOV (&MP+) is better but has no refraction.
    • some abandoned patches:
      • I was working on a custom version (Win32 version) that keeps the scene in memory between renders. I attempted to developing the controls used to interface with the scene memory. I have now given up. If you want the sources please contact me.
      • The ObjectCameraPatch progressed slowly - few obj types done-torus, cone/cyl, bicubic, sphere, then I waited for a bit as there was not much interest in the ObjectCamPatch at After that I lost interest in it and moved onto other things.


  • frhed: I developed a whole heap of patches for frhed, all of which are included in v1.1 (the latest version ATM). You can still view the old patch page if you wish.
  • chmlib: A coupla simple fixes.
  • wine: Removed one FIXME that frhed was showing.
  • nsis: Fixed build system on Linux



I'm not a DD, or even in the NM queue yet. I have filed a few ITPs, found a sponsor for some debian packages, experienced the melting of NEW and the "flood" of FTBFS/etc bugreports. I plan to ITP some more packages, a couple of fonts, some of my software and some orphaned packages at some point, and enter the NM queue after a while of having sponsored packages in debian. I'm thinking that I will get more involved in QA work during that period and start sponsoring new maintainers once I become a DD.

I also maintain debian packages of some of the software I have written, which can be found on (this means you must build it yourself).


I maintain a crappy build of xchm for windows that can be found in the xchm downloads page. Fixing its weird crashyness is at the top of the todo list.

I also maintain win32 packages of the some software I have written. These packages will be uploaded to their respective download pages.


CHM lib0.37


CHMLIB is a library for dealing with Microsoft ITSS/CHM format files. Right now, it is a very simple library, but sufficient for dealing with all of the .chm files I've come across. Due to the fairly well-designed indexing built into this particular file format, even a small library is able to gain reasonably good performance indexing into ITSS archives.

Version 0.37 is primarily a security release. On October 25th, a security vulnerability was located by Sven Tantau. This release is primarily to fix this, as well as a broken which didn't properly install the library for people who did:

    ./configure; make; make install

If you did this, and were unable to subsequently build the example programs, this release should fix it for you. 0.37.2 includes yet another small patch to the The change in 0.37.2 will be mainly of importance to packagers who use:

    make install DESTDIR=/path/to/sandbox

as DESTDIR had been inadvertently omitted from one of the actions in the "make install" target.

In the continuing saga, 0.37.3 contains yet one more minor patch to make DESTDIR work properly. The symlinks were being created pointing to $(DESTDIR)$(libdir) When DESTDIR was set to a temporary build location for packaging, this meant that the symlinks were broken. Thanks to Mark Rosenstand for pointing this out and supplying a patch.

Once more with feeling! 0.37.4 contains yet another fix to the, from Thomas Klausner. 'make install' was not using libtool to install the shared library, which is a portability issue. (For anyone who has had difficulty with 'make install' on non-Linux platforms, this may be the cause.) Furthermore, exec_prefix was not being set, so the library itself was being installed in /lib, regardless of the chosen installation prefix.

Note: UTF-8 support is fairly minimal at present. By this, I mean that I return the filename verbatim. Filename comparisons are done using strcasecmp, which is clearly not correct for UTF-8. I'm very interested in hearing from anyone who has dealt with internationalized filenames before, and can tell me the "right" way to deal with them. (Hopefully in a portable way.)

I've set up a sourceforge project to host this library, but I haven't really had time to move the project over. Maybe someday...

To do:

  • an index layer which sits on top of the basic chmlib functionality and understands the full-text index and possibly other indexing features. (topic structure?)
  • More functionality for querying the contents of an archive
  • Add write support (maybe?)

Right now this library supports enumerating the contents of the archive, and reading files from the archive.

This code is now being distributed under the LGPL. It incorporates LZX decompression code from the cabextract project. Thanks to Stuart Caie for authorizing the relicensing of this code in the context of chmlib.

Thanks to Stan Tobias for bugfixes and Andrew Hodgetts for bugfixes and portability fixes!

For those interested in the CHM format, a good resource is Pabs' HtmlHelp Maker, which is a free software project for creating HTML Help files. More importantly, the author maintains a reverse engineered spec for HTML Help files, including the structure of the internal files, which maintain the "topic" structure of the help file, the full-text index, and other useful things. At the time of writing, the spec was not available for download; however, the author has plans to publish it on his site when it is more complete, and an offer to mail out the current version to anyone who expresses interest.

Another "free software" tool which fulfills approximately the same niche as this library may be downloaded from Matthew T. Russotto's CHM site. If, for some reason, my library does not meet your needs, try out the chmtools from this site. Apparently, this site also offers LZX compression code.

Download version 0.37.4:

Download version 0.37:

Download version 0.36:

Download version 0.35:

Download version 0.33:

Download version 0.32:

Download version 0.31:

Download version 0.3:

Download version 0.2:

Download version 0.1:

Applications which use chmlib:

Language bindings for chmlib:

re: MS的chm文件格式及库和原代码
re: MS的chm文件格式及库和原代码
re: MS的chm文件格式及库和原代码