This is wimlib version 1.6.2 (March 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
(http://sourceforge.net/projects/wimlib/), 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 http://sourceforge.net/projects/wimlib/. 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.
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.
wimlib itself is a C library, and it provides a documented public API (See:
http://wimlib.sourceforge.net) 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.
wimlib (and wimlib-imagex) can create XPRESS, LZX, or LZMS compressed WIM
archives. wimlib includes its own compression codecs and does not use the
compression API available on some versions of Windows. The below table provides
the results (file size, in bytes, and time to create, in seconds) of capturing a
WIM containing an x86 Windows PE image, using various compression types and
options. When applicable, the results with the equivalent Microsoft
implementation in WIMGAPI, which is the library used by ImageX and Dism, are
| Compression type || wimlib (v1.6.1) | WIMGAPI (Windows 8) |
| None  || 531,979,435 in 18s | 531,980,333 in 24s |
| XPRESS  || 207,369,912 in 22s | 209,886,010 in 39s |
| LZX (quick)  || 194,876,901 in 29s | N/A |
| LZX (normal)  || 187,962,713 in 158s | 188,163,523 in 125s |
| LZX (slow)  || 186,913,423 in 358s | N/A |
| LZMS (non-solid)  || 176,880,594 in 182s | N/A |
| LZMS (solid)  || 136,507,304 in 494s | 126,735,608 in 623s |
 '--compress=none' for wimlib-imagex;
'/compress none' or no option for ImageX.
 '--compress=fast' or '--compress=XPRESS' for wimlib-imagex;
'/compress fast' or no option for ImageX.
Compression chunk size is 32768 (the default for XPRESS).
 No compression option specified to wimlib-imagex; no known equivalent for
WIMGAPI (ImageX uses XPRESS compression if no option specified).
Compression chunk size is 32768 (the default for LZX).
 '--compress=maximum' or '--compress=LZX' for wimlib-imagex;
'/compress maximum' for ImageX.
Compression chunk size is 32768 (the default for LZX).
 '--compress=maximum --compress-slow' for wimlib-imagex;
no known equivalent for WIMGAPI.
Compression chunk size is 32768 (the default for LZX).
 '--compress=recovery' or '--compress=LZMS' for wimlib-imagex;
no known way to create the equivalent with WIMGAPI.
Compression chunk size is 131072 (the default for LZMS). Note: this
compression type is not generally recommended due to its limited
compatibility with the MS implementations.
 '--compress=recovery --solid' or '--compress=LZMS --solid' for
wimlib-imagex; WIMCreateFile with WIM_COMPRESSION_LZMS and flag
0x20000000 for WIMGAPI. Compression chunk size in packed resources is
33554432 for wimlib, 67108864 for WIMGAPI. Note: this compression type
is not generally recommended due to its limited compatibility with the MS
implementations. Also, due to the large chunk size, wimlib uses about
500MB of memory per thread when compressing in this format.
The above timings were done on Windows 8 (x86) so that side-by-side comparisons
with the Microsoft implementation would be possible; however, wimlib may have
even better performance on other operating systems such as Linux. The system
had 2 CPUs and 2 GiB of memory available. All times were done with the page
cache warmed, so the times primarily measure the performance of the compression
algorithms and not the time to read data from disk, which presumably is similar
in each implementation.
Below are results for compressing the Canterbury corpus using wimlib (v1.6.1),
WIMGAPI (Windows 8), and some other formats/programs, including the archive size
only. Note that the Canterbury corpus includes no duplicate files or hard
links, which WIM handles better than most other formats by storing only distinct
| 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) | 639,464 |
| WIM (wimlib, LZX slow) | 633,144 |
| WIM (wimlib, LZMS non-solid) | 590,252 |
| tar.bz2 (bzip, default) | 565,008 |
| tar.bz2 (bzip, -9) | 565,008 |
| WIM (wimlib, LZMS solid) | 534,218 |
| WIM (wimlib, LZMS solid, slow) | 529,904 |
| 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 |
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
* 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
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 a 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.
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
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 `WinPE.cab' file, which you can extract natively on
Windows, or on UNIX-like systems if you install either the `cabextract' or
In addition, Windows installations and recovery partitions frequently contain a
WIM containing an image of the Windows Recovery Environment, which is similar to
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 http://xmlsoft.org/.
* 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 http://fuse.sourceforge.net/. 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' (http://www.cdrkit.org). Making a disk image
requires `mtools' (http://www.gnu.org/software/mtools) and `syslinux'
(http://www.syslinux.org). Retrieving files from the Windows Automated
Installation Kit requires `cabextract' (http://www.cabextract.org.uk).
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.
Enable or disable support for the extended-attributes interface to NTFS
alternate data streams in mounted WIMs. To support these, wimlib
requires that the setxattr() function and the attr/xattr.h header are
available. The default is to autodetect whether support is possible.
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
http://www.tuxera.com/community/ntfs-3g-download/ for more information.
The LZX decompressor (lzx-decompress.c) was originally based on code from the
cabextract project (http://www.cabextract.org.uk) but has been rewritten.
The LZX compressor (lzx-compress.c) was originally based on code written by
Matthew Russotto (www.russotto.net/chm/) but has been rewritten. It now uses
suffix array construction code from divsufsort
(https://code.google.com/p/libdivsufsort/) and algorithms from 7-Zip as well as
several published papers.
lz_hash.c contains a hash-table-based LZ77 matchfinder that is based on code
from zlib but has been rewritten. This code is applicable to XPRESS, LZX, and
LZMS, all of which are partly based on LZ77 compression.
A limited number of other free programs can handle some parts of the WIM
* 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 LZX or XPRESS compressed WIMs, and the
correct handling of security descriptors and hard links.
* ImagePyX (https://github.com/maxpat78/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.
A very early version of wimlib is being used to deploy Windows 7 from the
Ultimate Deployment Appliance. For more information see
If you are looking for a UNIX archive format that provides features similar to
WIM, I recommend you take a look at SquashFS (http://squashfs.sourceforge.net/).
As of version 1.0.0, wimlib and all programs and scripts distributed with it are
released under the GNU GPL version 3.0 or later.
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
firstname.lastname@example.org) 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.