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File Date Author Commit
archlinux 2014-06-20 Eric Biggers Eric Biggers [5e2f33] Update version to v1.7.1-BETA
build-aux 2014-06-15 Eric Biggers Eric Biggers [19ce03] Re-visit SHA-1 code
debian 2014-06-13 Eric Biggers Eric Biggers [c0a642] Update Linux packaging
doc 2014-06-27 Eric Biggers Eric Biggers [c05c91] doc updates
examples 2014-06-13 Eric Biggers Eric Biggers [629719] Place programs in examples/ in public domain
include 7 days ago Eric Biggers Eric Biggers [cf8592] {de,}compressor_ops.h: Forward declare params h...
m4 2014-06-15 Eric Biggers Eric Biggers [19ce03] Re-visit SHA-1 code
programs 2014-06-25 Eric Biggers Eric Biggers [9f4d01] wimexport: Add --recompress option
rpm 2014-06-20 Eric Biggers Eric Biggers [5e2f33] Update version to v1.7.1-BETA
src 6 days ago Eric Biggers Eric Biggers [f957ca] win32_capture.c: Fix memory leak in error path
tests 2014-06-11 Eric Biggers Eric Biggers [157ec1] Don't exclude out-of-tree absolute symlinks in ...
AUTHORS unknown
COPYING 2014-06-13 Eric Biggers Eric Biggers [e66a12] Update library license: Add LGPLv3 exception
COPYING.GPLv3 2014-06-13 Eric Biggers Eric Biggers [e66a12] Update library license: Add LGPLv3 exception
COPYING.LGPLv3 2014-06-13 Eric Biggers Eric Biggers [e66a12] Update library license: Add LGPLv3 exception
INSTALL unknown 2014-06-20 Eric Biggers Eric Biggers [5e2f33] Update version to v1.7.1-BETA
NEWS 2014-06-25 Eric Biggers Eric Biggers [e61f93] wimlib: Allow custom error file
README 2014-06-20 Eric Biggers Eric Biggers [5e2f33] Update version to v1.7.1-BETA
README.WINDOWS 2014-05-26 Eric Biggers Eric Biggers [4dfc91] Remove references to libwim9
bootstrap unknown 2014-05-26 Eric Biggers Eric Biggers [260370] Use libdivsufsort-lite, not the full libdivsufsort 2014-06-20 Eric Biggers Eric Biggers [5e2f33] Update version to v1.7.1-BETA unknown
make-releases unknown
make-windows-release 2014-06-16 Eric Biggers Eric Biggers [ad6470] make-windows-release: Allow multiple extra args...
update-version 2014-06-20 Eric Biggers Eric Biggers [5e2f33] Update version to v1.7.1-BETA unknown

Read Me


This is wimlib version 1.7.1-BETA (June 2014).  wimlib is a C library for
creating, modifying, extracting, and mounting files in the Windows Imaging
Format (WIM files).  These files are normally created using the ImageX
(imagex.exe) or Dism (Dism.exe) utilities on Windows, but wimlib is distributed
with a free implementation of ImageX called "wimlib-imagex" for both UNIX-like
systems and Windows.


To install wimlib and wimlib-imagex on Windows you simply need to download and
extract the ZIP file containing the latest binaries from the SourceForge page
(, which you may have already done.

To install wimlib and wimlib-imagex on UNIX-like systems (with Linux being the
primary supported and tested platform), you must compile the source code, which
is also available at  Alternatively,
check if a package has been prepared for your Linux distribution.  Example files
for Debian and RPM packaging are in the debian/ and rpm/ directories.

                                    WIM FILES

A Windows Imaging (WIM) file is an archive designed primarily for archiving
Windows filesystems.  However, it can be used on other platforms as well, with
some limitations.  Like some other archive formats such as ZIP, files in WIM
archives may be compressed.  WIM files support multiple compression formats,
including LZX, XPRESS, and LZMS.  All these formats are supported by wimlib.

A WIM file consists of one or more "images".  Each image is an independent
top-level directory structure and is logically separate from all other images in
the WIM.  Each image has a name as well as a 1-based index in the WIM file.  To
save space, WIM archives automatically combine all duplicate files across all

A WIM file may be either stand-alone or split into multiple parts.  Split WIMs
are read-only and cannot be modified.

Since version 1.6.0, wimlib also supports ESD (.esd) files, except when
encrypted.  These are still WIM files but they use a newer version of the file

                             IMAGEX IMPLEMENTATION

wimlib itself is a C library, and it provides a documented public API (See: for other programs to use.  However, it is also
distributed with a command-line program called "wimlib-imagex" that uses this
library to implement an imaging tool similar to Microsoft's ImageX.
wimlib-imagex supports almost all the capabilities of Microsoft's ImageX as well
as additional capabilities.  wimlib-imagex works on both UNIX-like systems and
Windows, although some features differ between the platforms.

Run `wimlib-imagex' with no arguments to see an overview of the available
commands and their syntax.  For additional documentation:

  * If you have installed wimlib-imagex on a UNIX-like system, you will find
    further documentation in the man pages; run `man wimlib-imagex' to get

  * If you have downloaded the Windows binary distribution, you will find the
    documentation for wimlib-imagex in PDF format in the "doc" directory,
    ready for viewing with any PDF viewer.  Please note that although the PDF
    files are converted from UNIX-style "man pages", they do document
    Windows-specific behavior when appropriate.

                                COMPRESSION RATIO

wimlib (and wimlib-imagex) can create XPRESS, LZX, or LZMS compressed WIM files.
wimlib includes its own compression codecs and does not use the compression API
available on some versions of Windows.

I have gradually been improving the compression codecs in wimlib, and in some
cases they now outperform and outcompress the equivalent Microsoft
implementations.  Although results will vary depending on the data being
compressed, in the table below I present the results for a common use case:
compressing an x86 Windows PE image.  Each row displays the compression type,
the size of the resulting WIM file in bytes, and how many seconds it took to
create the file.  When applicable, the results with the equivalent Microsoft
implementation in WIMGAPI is included.

  | Compression type       ||  wimlib (v1.7.0)       |  WIMGAPI (Windows 8.1) |
  | None             [1]   ||  361,182,560 in 7.6s   |  361,183,674 in 11.9s  |
  | XPRESS           [2]   ||  138,992,007 in 11.5s  |  140,416,657 in 13.9s  |
  | LZX (quick)      [3]   ||  131,428,113 in 15.2s  |  N/A                   |
  | LZX (normal)     [4]   ||  126,807,853 in 58.9s  |  127,259,566 in 55.2s  |
  | LZX (slow)       [5]   ||  126,085,971 in 125s   |  N/A                   |
  | LZMS (non-solid) [6]   ||  122,082,982 in 59.8s  |  N/A                   |
  | LZMS (solid)     [7]   ||  92,171,126  in 260s   |  88,742,356 in 556s    |

   [1] '--compress=none' for wimlib-imagex;
       '/compress:none' for DISM.

   [2] '--compress=fast' or '--compress=XPRESS' for wimlib-imagex;
       '/compress:fast' for DISM.
       Compression chunk size is 32768 bytes, the default for XPRESS.

   [3] No compression option specified to wimlib-imagex;
       no known equivalent for DISM.
       Compression chunk size is 32768 bytes, the default for LZX.

   [4] '--compress=maximum' or '--compress=LZX' for wimlib-imagex;
       '/compress:maximum' for DISM.
       Compression chunk size is 32768 bytes, the default for LZX.

   [5] '--compress=maximum --compress-slow' for wimlib-imagex;
       no known equivalent for DISM.
       Compression chunk size is 32768 bytes, the default for LZX.

   [6] '--compress=recovery' or '--compress=LZMS' for wimlib-imagex;
       no known equivalent for DISM.
       Compression chunk size is 131072 bytes, the default for LZMS.

   [7] '--compress=recovery --solid' or '--compress=LZMS --solid' for
       wimlib-imagex.  Should be '/compress:recovery' for DISM, but it doesn't
       work; I called WIMGAPI directly instead.
       Compression chunk size in solid blocks is 67108864 bytes in both cases.

I did the timings on Windows 8.1 running in a virtual machine so that
side-by-side comparisons with the Microsoft implementation would be possible.
However, do note that wimlib may have even better performance on other operating
systems such as Linux.  I used the 64-bit builds of both programs.

The compression ratio provided by wimlib is also competitive with commonly used
archive formats.  Below are file sizes that result when the Canterbury corpus is
compressed with wimlib (v1.7.0), WIMGAPI (Windows 8), and some other

     | Format                         | Size (bytes) |
     | tar                            | 2,826,240    |
     | WIM (WIMGAPI, None)            | 2,814,278    |
     | WIM (wimlib, None)             | 2,813,856    |
     | WIM (WIMGAPI, XPRESS)          | 825,410      |
     | WIM (wimlib, XPRESS)           | 792,024      |
     | tar.gz (gzip, default)         | 738,796      |
     | ZIP (Info-ZIP, default)        | 735,334      |
     | tar.gz (gzip, -9)              | 733,971      |
     | ZIP (Info-ZIP, -9)             | 732,297      |
     | WIM (wimlib, LZX quick)        | 722,196      |
     | WIM (WIMGAPI, LZX)             | 651,766      |
     | WIM (wimlib, LZX normal)       | 649,204      |
     | WIM (wimlib, LZX slow)         | 639,618      |
     | WIM (wimlib, LZMS non-solid)   | 592,136      |
     | tar.bz2 (bzip, default)        | 565,008      |
     | tar.bz2 (bzip, -9)             | 565,008      |
     | WIM (wimlib, LZMS solid)       | 525,270      |
     | WIM (wimlib, LZMS solid, slow) | 521,700      |
     | WIM (WIMGAPI, LZMS solid)      | 521,232      |
     | tar.xz (xz, default)           | 486,916      |
     | tar.xz (xz, -9)                | 486,904      |
     | 7z  (7-zip, default)           | 484,700      |
     | 7z  (7-zip, -9)                | 483,239      |

Note: WIM does even better on directory trees containing duplicate files, which
the Canterbury corpus doesn't have.

                                  NTFS SUPPORT

WIM images may contain data, such as alternate data streams and
compression/encryption flags, that are best represented on the NTFS filesystem
used on Windows.  Also, WIM images may contain security descriptors which are
specific to Windows and cannot be represented on other operating systems.
wimlib handles this NTFS-specific or Windows-specific data in a
platform-dependent way:

  * In the Windows version of wimlib and wimlib-imagex, NTFS-specific and
    Windows-specific data are supported natively.

  * In the UNIX version of wimlib and wimlib-imagex, NTFS-specific and
    Windows-specific data are ordinarily ignored; however, there is also special
    support for capturing and extracting images directly to/from unmounted NTFS
    volumes.  This was made possible with the help of libntfs-3g from the
    NTFS-3g project.

For both platforms the code for NTFS capture and extraction is complete enough
that it is possible to apply an image from the "install.wim" contained in recent
Windows installation media (Vista, Windows 7, or Windows 8) directly to an NTFS
filesystem, and then boot Windows from it after preparing the Boot Configuration
Data.  In addition, a Windows installation can be captured (or backed up) into a
WIM file, and then re-applied later.

                                   WINDOWS PE

A major use for wimlib and wimlib-imagex is to create customized images of
Windows PE, the Windows Preinstallation Environment, on either UNIX-like systems
or Windows without having to rely on Microsoft's software and its restrictions
and limitations.

Windows PE is a lightweight version of Windows that can run entirely from memory
and can be used to install Windows from local media or a network drive or
perform maintenance.  It is the operating system that runs when you boot from
the Windows installation media.

You can find Windows PE on the installation DVD for Windows Vista, Windows 7, or
Windows 8, in the file `sources/boot.wim'.  Windows PE can also be found in the
Windows Automated Installation Kit (WAIK), which is free to download from
Microsoft, inside the `' file, which you can extract natively on
Windows, or on UNIX-like systems if you install either the `cabextract' or
`p7zip' programs.

In addition, Windows installations and recovery partitions frequently contain a
WIM containing an image of the Windows Recovery Environment, which is similar to
Windows PE.

A shell script `mkwinpeimg' is distributed with wimlib on UNIX-like systems to
ease the process of creating and customizing a bootable Windows PE image.


This section documents the dependencies of wimlib and the programs distributed
with it, when building for a UNIX-like system from source.  If you have
downloaded the Windows binary distribution of wimlib and wimlib-imagex then all
dependencies were already included and this section is irrelevant.

* libxml2 (required)
	This is a commonly used free library to read and write XML files.  You
	likely already have it installed as a dependency for some other program.
	For more information see

* libfuse (optional but highly recommended)
	Unless configured with --without-fuse, wimlib requires a non-ancient
	version of libfuse to be installed.  Most Linux distributions already
	include this, but make sure you have the libfuse package installed, and
	also libfuse-dev if your distribution distributes header files
	separately.  FUSE also requires a kernel module.  If the kernel module
	is available it will automatically be loaded if you try to mount a WIM
	file.  For more information see  FUSE is
	also available for FreeBSD.

* libntfs-3g (optional but highly recommended)
	Unless configured with --without-ntfs-3g, wimlib requires the library
	and headers for libntfs-3g version 2011-4-12 or later to be installed.
	Versions dated 2010-3-6 and earlier do not work because they are missing
	the header xattrs.h (and the file xattrs.c, which contains functions we
	need).  libntfs-3g version 2013-1-13 is compatible only with wimlib
	1.2.4 and later.

* OpenSSL / libcrypto (optional)
	wimlib can use the SHA1 message digest code from OpenSSL instead of
	compiling in yet another SHA1 implementation. (See LICENSE section.)

* cdrkit (optional)
* mtools (optional)
* syslinux (optional)
* cabextract (optional)
	The `mkwinpeimg' shell script will look for several other programs
	depending on what options are given to it.  Depending on your Linux
	distribution, you may already have these programs installed, or they may
	be in the software repository.  Making an ISO filesystem requires
	`mkisofs' from `cdrkit' (  Making a disk image
	requires `mtools' ( and `syslinux'
	(  Retrieving files from the Windows Automated
	Installation Kit requires `cabextract' (


This section documents the most important options that may be passed to the
"configure" script when building from source:

	If libntfs-3g is not available or is not version 2011-4-12 or later,
	wimlib can be built without it, in which case it will not be possible to
	apply or capture images directly to/from NTFS volumes.

	If libfuse or the FUSE kernel module is not available, wimlib can be
	compiled with --without-fuse.  This will remove the ability to mount and
	unmount WIM files.

	Build in functions for SHA1 rather than using external SHA1 functions
	from libcrypto (part of OpenSSL).  The default is to use libcrypto if it
	is found on the system.

	By default, data will be compressed using multiple threads when writing
	a WIM, unless only 1 processor is detected.  Specify this option to
	disable support for this.

	Use a very fast assembly language implementation of SHA1 from Intel.
	Only use this if the build target supports the SSSE3 instructions.

	Save some space by removing all error messages from the library.

	Remove assertions included by default.


wimlib has primarily been tested on Linux and Windows (primarily Windows 7, but
also Windows XP and Windows 8).

wimlib may work on FreeBSD and Mac OS X.  However, this is not well tested.  If
you do not have libntfs-3g 2011-4-12 or later available, you must configure
wimlib with --without-ntfs-3g.  On FreeBSD, before mounting a WIM you need to
load the POSIX message queue module (run `kldload mqueuefs').

The code has primarily been tested on x86 and x86_64 CPUs, but it's written to
be portable to other architectures and I've also tested it on ARM.  However,
although the code is written to correctly deal with endianness, it has not yet
actually been tested on a big-endian architecture.


The WIM file format is partially specified in a document that can be found in
the Microsoft Download Center.  However, this document really only provides an
overview of the format and is not a formal specification.

With regards to the supported compression formats:

- Microsoft has official documentation for XPRESS that is of reasonable quality.
- Microsoft has official documentation for LZX but it contains errors.
- There does not seem to be any official documentation for LZMS, so my comments
  and code in src/lzms-decompress.c may in fact be the best documentation
  available for this particular compression format.

The code in ntfs-3g_apply.c and ntfs-3g_capture.c uses the NTFS-3g library,
which is a library for reading and writing to NTFS filesystems (the filesystem
used by recent versions of Windows).  See for more information.

The LZX decompressor (lzx-decompress.c) was originally based on code from the
cabextract project (  The LZX compressor
(lzx-compress.c) was originally based on code written by Matthew Russotto
(  However I have since rewritten and made many
improvements to both the decompressor and compressor.

lz_hash.c contains LZ77 match-finding code that uses hash chains.  It is based
on code from zlib but I have since rewritten it.

lz_bt.c contains LZ77 match-finding code that uses binary trees.  It is based on
code from liblzma but I have since rewritten it.

A limited number of other free programs can handle some parts of the WIM
file format:

  * 7-zip is able to extract and create WIMs (as well as files in many
    other archive formats).  However, wimlib is designed specifically to handle
    WIM files and provides features previously only available in Microsoft's
    implementation, such as the ability to mount WIMs read-write as well as
    read-only, the ability to create compressed WIMs, and the correct handling
    of security descriptors and hard links.
  * ImagePyX ( is a Python program that
    provides similar capabilities to wimlib-imagex.  One thing to note, though,
    is that it does not support compression and decompression by itself, but
    instead relies on external native code, such as the codecs from wimlib.

If you are looking for an archive format that provides features similar to WIM
but was designed primarily for UNIX, you may want to consider SquashFS
(  However, you may find that wimlib works
surprisingly well on UNIX.  It will store hard links and symbolic links, and it
has optional support for storing UNIX owners, groups, modes, and special files
such as device nodes and FIFOs.  Actually, I use it to back up my own files on


See COPYING for information about the license.

wimlib is independently developed and does not contain any code, data, or files
copyrighted by Microsoft.  It is not known to be affected by any patents.

On UNIX-like systems, if you do not want wimlib to be dynamically linked with
libcrypto (OpenSSL), configure with --without-libcrypto.  This replaces the SHA1
implementation with built-in code and there will be no difference in

wimlib comes with no warranty whatsoever.  Please submit a bug report (to if you find a bug in wimlib and/or wimlib-imagex.

Be aware that some parts of the WIM file format are poorly documented or even
completely undocumented, so I've just had to do the best I can to read and write
WIMs that appear to be compatible with Microsoft's software.