read_compressed_resource(): Remove potential aliasing violation

[wimlib] / README

                                  INTRODUCTION

This is wimlib version 1.4.0 (May 2013).  wimlib is a C library for creating,
modifying, extracting, and mounting files in the Windows Imaging Format (WIM
files).  These files are normally created by using the `imagex.exe' utility on
Windows, but wimlib is distributed with a free implementation of ImageX called
"wimlib-imagex" for both UNIX and Windows.

                                  INSTALLATION

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 (with Linux being the primary
supported and tested platform), you must compile it from the source code.  At
some point I might start posting RPMs and Debian packages for convenience.

                                    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 two compression formats: LZX and
XPRESS.  Both 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
images.

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

                             IMAGEX IMPLEMENTATION

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 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 UNIX, 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,
    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 or LZX compressed WIM archives.
Currently, the XPRESS compression ratio is slightly better than that provided by
Microsoft's software, while the LZX compression ratio is approaching that of
Microsoft's software but is not quite there yet.  Running time is as good as or
better than Microsoft's software, especially with multithreaded compression,
available in wimlib v1.1.0 and later.

The following tables compare the compression ratio and performance for creating
a compressed x86_64 Windows PE image.  Note: these timings were done on Windows
7 so that the times would be fully comparable; however, wimlib-imagex may have
even better performance on Linux.

        Table 1. WIM size

                                           XPRESS Compression      LZX Compression
        wimlib-imagex (v1.4.0):            165,301,379 bytes       155,254,385 bytes
        Microsoft imagex.exe:              167,212,939 bytes       149,973,212 bytes

        Table 2. Time to create WIM

                                           XPRESS Compression      LZX Compression
        wimlib-imagex (v1.4.0, 2 threads): 18 sec                  51 sec
        Microsoft imagex.exe:              25 sec                  93 sec

                                  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 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.

                                   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 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 `WinPE.cab' file, which you can extract natively on
Windows, or on UNIX 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 to ease the
process of creating and customizing a bootable Windows PE image.

                                  DEPENDENCIES

This section documents the dependencies of wimlib and the programs distributed
with it, when building for UNIX 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).

                                 CONFIGURATION

This section documents the most important options that may be passed to the
"configure" script when building the UNIX version 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
        apply or capture images directly to/from NTFS volumes.

--without-fuse
        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.

--without-libcrypto
        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-xattr, --disable-xattr
        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.

--disable-multithreaded-compression
        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.

--enable-ssse3-sha1
        Use a very fast assembly language implementation of SHA1 from Intel.
        Only use this if the build target supports the SSSE3 instructions.

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

--disable-assertions
        Remove assertions included by default.

                                  PORTABILITY

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').

wimlib has not been tested on big-endian CPU architectures.

                                   REFERENCES

The WIM file format is specified in a document that can be found in the
Microsoft Download Center.  There is a similar document that specifies the LZX
compression format, and a document that specifies the XPRESS compression format.
However, many parts of these formats are poorly documented, and some parts have
no documentation whatsoever.  Some particularly poorly documented parts of the
formats have had comments added in various places in the library code.  Please
see the code and/or ask me if you have any questions about the WIM file format
as it exists in reality and not as it exists in Microsoft's poorly written
documentation.

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.

lzx-decompress.c, the code to decompress WIM file resources that are compressed
using LZX compression, was originally based on code from the cabextract project
(http://www.cabextract.org.uk) but has been rewritten.

lzx-compress.c, the code to compress WIM file resources using LZX compression,
was originally based on code written by Matthew Russotto (www.russotto.net/chm/)
but has been rewritten.

lz77.c, the code to find LZ77 matches (used for both XPRESS and LZX
compression), is based on code from zlib but has been rewritten.

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
    imagex.exe, 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
http://www.ultimatedeployment.org/.

You can see the documentation about Microsoft's version of ImageX at
http://technet.microsoft.com/en-us/library/cc749447(v=ws.10).aspx, so you can
see how it compares to wimlib-imagex.

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/).

                                    LICENSE

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, 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
functionality.

                                   DISCLAIMER

wimlib comes with no warranty whatsoever.  Use Microsoft's `imagex.exe' if you
want to make sure your WIM files are made "correctly" (but beware: Microsoft's
version contains some bugs).

Please submit a bug report (to ebiggers3@gmail.com) 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.