Introduction

This is wimlib version 1.14.3 (September 2023). wimlib is a C library for creating, modifying, extracting, and mounting files in the Windows Imaging Format (WIM files). wimlib and its command-line frontend wimlib-imagex provide a free and cross-platform alternative to Microsoft's WIMGAPI, ImageX, and DISM.

For the release notes, see the NEWS file.

Table of Contents

• Installation
• WIM files
• ImageX implementation
• Compression
• NTFS support
• Windows PE
• Dependencies
• Configuration
• Portability
• References
• History
• Notices

Installation

To install wimlib and wimlib-imagex on UNIX-like systems, you can compile from source (e.g. ./configure && make && sudo make install). Alternatively, check if a package has already been prepared for your operating system.

To install wimlib and wimlib-imagex on Windows, just download and extract the ZIP file containing the latest binaries. For more details, see README.WINDOWS.md.

All official wimlib releases are available from wimlib.net.

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 archives support multiple compression formats, including LZX, XPRESS, and LZMS. All these formats are supported by wimlib.

A WIM archive contains one or more "images", each of which is a logically independent directory tree. Each image has a 1-based index and usually a name.

WIM archives provide data deduplication at the level of full file contents. In other words, each unique "file contents" is only stored once in the archive, regardless of how many files have that contents across all images.

A WIM archive may be either stand-alone or split into multiple parts.

An update of the WIM format --- first added by Microsoft for Windows 8 --- supports solid-mode compression. This refers to files being compressed together (e.g. as in a .tar.xz or .7z archive) rather than separately (e.g. as in a .zip archive). This usually produces a much better compression ratio. Solid archives are sometimes called "ESD files" by Microsoft and may have the ".esd" file extension rather than ".wim". They are supported in wimlib since v1.6.0.

ImageX implementation

wimlib itself is a C library, and it provides a documented public API 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. Note that the commands have both long and short forms, e.g. wimlib-imagex apply is equivalent to wimapply. 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 started.

• If you have downloaded the Windows binary distribution, you will find the documentation for wimlib-imagex in PDF format in the doc directory. Note that although the documentation is written in the style of UNIX manual pages, it does document Windows-specific behavior when relevant.

Compression

wimlib (and wimlib-imagex) can create XPRESS, LZX, and LZMS compressed WIM archives. wimlib's compression codecs usually outperform and outcompress their closed-source Microsoft equivalents. Multiple compression levels and chunk sizes as well as solid mode compression are supported. Compression is multithreaded by default. Detailed benchmark results and descriptions of the algorithms used can be found at wimlib.net.

NTFS support

WIM images may contain data, such as named 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 or later) 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

wimlib can also be used to create customized images of Windows PE on either UNIX-like systems or Windows. Windows PE (Preinstallation Environment) is a lightweight version of Windows that runs entirely from memory and can be used to perform maintenance or to install Windows. It is the operating system that runs when you boot from the Windows installation media.

A copy of Windows PE can be found on the installation media for Windows (Vista or later) as the file sources/boot.wim, or in the Windows Automated Installation Kit (WAIK), which is free to download from Microsoft.

A shell script mkwinpeimg is provided with wimlib on UNIX-like systems to simplify the process of creating and customizing a bootable Windows PE image, sourcing the needed files from the Windows installation media or from the WAIK.

Dependencies

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.

• libfuse3 (optional but recommended)

  Unless configured --without-fuse, wimlib requires libfuse3, for mounting WIM images using FUSE. Most Linux distributions already include this, but make sure you have the fuse3 package installed, and also libfuse3-dev if your distribution distributes development files separately. FUSE also requires a kernel module. If the kernel module is available it should automatically be loaded if you try to mount a WIM image.

• libntfs-3g (optional but recommended)

  Unless configured --without-ntfs-3g, wimlib requires the library and headers for libntfs-3g to be installed. The minimum required version is 2011-4-12, but newer versions contain important bug fixes.

• 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.

Configuration

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

• --without-ntfs-3g

  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 capture or apply WIM images directly from/to NTFS volumes.

  The default is --with-ntfs-3g when building for any UNIX-like system, and --without-ntfs-3g when building for Windows.

• --without-fuse

  The --without-fuse option disables support for mounting WIM images. This removes dependencies on libfuse and librt. The wimmount, wimmountrw, and wimunmount commands will not work.

  The default is --with-fuse when building for Linux, and --without-fuse otherwise.

Portability

wimlib works on both UNIX-like systems (Linux, Mac OS X, FreeBSD, etc.) and Windows (Vista and later).

As much code as possible is shared among all supported platforms, but there necessarily are some differences in what features are supported on each platform and how they are implemented. Most notable is that file tree scanning and extraction are implemented separately for Windows, UNIX, and UNIX (NTFS-3G mode), to ensure a fast and feature-rich implementation of each platform/mode.

wimlib is mainly used on x86 and x86_64 CPUs, but it should also work on a number of other GCC-supported 32-bit or 64-bit architectures. It has been tested on the ARM and MIPS architectures.

Currently, gcc and clang are the only supported compilers. A few nonstandard extensions are used in the code.

References

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. It also does not cover later extensions of the format, such as solid resources.

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 in two different documents, neither of which is completely applicable to its use in the WIM format, and the first of which contains multiple 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 algorithms used by wimlib's compression and decompression codecs are inspired by a variety of sources, including open source projects and computer science papers.

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).

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, the correct handling of security descriptors and hard links, and support for LZMS compression.

• ImagePyX is a Python program that provides some capabilities of wimlib-imagex, with the help of external compression codecs.

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 supports storing standard UNIX file permissions (owners, groups, and modes); special files such as device nodes and FIFOs; and extended attributes. Actually, I use it to back up my own files on Linux!

History

wimlib was originally a project started by Carl Thijssen for use on Linux in the Ultimate Deployment Appliance. Since then the code has been entirely rewritten and improved (main author: Eric Biggers). Windows support has been available since version 1.3.0 (March 2013). A list of version-to-version changes can be found in the NEWS file.

Notices

wimlib is free software that comes with NO WARRANTY, to the extent permitted by law. For full details, see the COPYING file.

Bug reports, suggestions, and other contributions are appreciated and should be posted to the forums.

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.