VPatch 3.1


VPatch allows to create a patch file to update previous versions of your software. The GenPat utility generates the patch file. The plug-in can use the patch to update a file. Using a patch, you can reduce the download size of your updates because only the differences between the files are included in the patch file.

How to use

Generate the patch file

Make sure you have the source file (original version) and the target file (version to update to). For example, DATA.DTA (currently on user system) and DATA_20.DTA (version 2.0 of this data file). Now call the command line tool GenPat.exe:

GENPAT oldfile.txt newfile.txt patch.pat

Now, the patch will be generated, this will take some time.

Using the /B=(BlockSize) parameter of the GenPat utility (put it after the filenames), you can use a different block size. A smaller block size may result in a smaller patch, but the generation will take more time (the default blocksize is 64).

If you have trouble using this command-line utility, you can download a GUI (graphical user interface) for VPatch from its own website: http://www.tibed.net/vpatch.

Update the file during installation

Use the VPatch plug-in to update a file using a patch file:

vpatch::vpatchfile "patch.pat" "oldfile.txt" "temporary_newfile.txt"

The result of the patch operating will be added to the stack and can be one of the following texts:

  • OK
  • OK, new version already installed
  • An error occurred while patching
  • Patch data is invalid or corrupt
  • No suitable patches were found

Check example.nsi for an example. You should check whether the stack string starts with "OK" because then the patch has succeeded and you can rename "temporary_newfile.txt" to "oldfile.txt" to replace the original, if you want.

Multiple patches in one file

GenPat appends a patch to the file you specified. If there is already a patch for the same original file, with the same CRC/MD5, in the patch file, the patch will be replaced. For example, if you want to be able to upgrade version 1 and 2 to version 3, you can put a 1 > 3 and 2 > 3 patch in one file.

You can also put patches for different files in one patch file, for example, a patch from file A version 1 to file A version 2 and a patch from file B version 1 to file B version 2. Just call the plug-in multiple times with the same patch file. It will automatically select the right patch (based on the file CRC).

Patch generator (GenPat) exit codes

In version 3 the following exit codes (known as error levels in the DOS period) can be returned by GenPat. GenPat will return an exit code based on success of the patch generation. Here is a list of the possible exit codes:

Exit code Description
0 Success
1 Arguments missing
2 Other error
3 Source file already has a patch in specified patch file (ERROR), use /R switch to override

These exit codes can be useful when you generate patch files through a NSIS script.

Source code

Source code is available in the original package and in the SVN repository of NSIS.

NSIS plug-in (C++)

The source of the NSIS plug-in that applies patches can be found in the Source\Plugin folder.

Patch Generator (C++)

The most interesting part of VPatch, the actual patch generation algorithm, can be found in Source\GenPat\PatchGenerator.cpp. The header of that file contains a brief explanation of the algorithm as well.

User interface (Delphi)

A user interface is included as well, which you will have to build yourself because the GUI executable was too large to include. Besides Borland Delphi 6 or higher (you can use the freely available Personal edition), you will also need to install the VirtualTreeView component by Mike Lischke.

Version history

  • 3.1
    • GenPat now compiles on POSIX platforms (MinGW/GCC), Visual C++ 6 and Borland C++.
    • More test cases to verify functionality of GenPat.
  • 3.0
    • Final: Updates to the GUI, installer
    • RC8: GenPat will now flag replacement of a patch (e.g. the source file has the same contents as a previous patch inside a patch file) as an error. You can specifically allow it using the /R switch. Added license to source files.
    • RC7: Fixed critical bug in GenPat with multiple patches in a single file. Fixed serious bug in stand-alone EXE runtime: process kept on running forever. Included case testing through a Python script to test common usage (and prevent bugs like the one in GenPat in the future).
    • RC6: Upgraded to non-beta compiler. Added /A switch to change block match limit and /O to deactivate the limit. Updated GUI to support the /O switch.
    • RC4a to RC5a: input block size is now checked for power of 2 and fixed if incorrect. When patch file does not yet exist, no longer forgets to create the header. No longer tries to allocate memory when there are no chunks. Fixed memory leaks.
    • Target file date is now preserved inside a patch and restored on the user system.
    • MD5 checksums are now used instead of CRC32 checksums, unless existing patches in a file already are in CRC32 mode.
    • The patch generator, GenPat, has been completely rewritten in C++. It no longer needs to keep the entire files in memory, instead memory usage is a certain percentage of the source file size. The percentage is based on the block size, larger block sizes will reduce memory usage.
    • All runtimes now share a common codebase, perform proper error checking and don't leave behind files if the input file was already up to date.
    • Bug Fix: The patch generator algorithm no longer reduces to a quadratic runtime if there are many blocks with the same content in the files to patch.
    • Bug Fix: The documentation of the command-line utilities was incorrect and no warnings would be given by the runtimes, causing the patch not to work (this does not apply to NSIS patches).
  • 2.1
    • Added argument checking and error handling to GenPat. Now returns exit codes as well to indicate success/failure (and the reason for failure). Only GenPat has changed in this version compared to 2.0 final.
    • Bug Fix: GenPat no longer gives an Access Violation when attempting to patch a file smaller than 64 bytes into a file larger than 64 bytes.
  • 2.0 final
    • Cleaned up source code for the patch generator, which is now included (this code is written in Borland Delphi 6 and compiles with the freely available Personal edition).
  • 2.0 beta 2
    • All new algorithm used in the patch generator: much faster (up to 90%) while using smaller block sizes (higher compression)
    • Created a NSIS 2 plugin
    • Works with small files
    • Replaces existing patch in file if original file CRC is identical


Written by Koen van de Sande
C plug-in initially by Edgewize, updated by Koen van de Sande
New documentation and example by Joost Verburg and Koen van de Sande


Copyright (C) 2001-2005 Koen van de Sande / Van de Sande Productions

This software is provided 'as-is', without any express or implied
warranty. In no event will the authors be held liable for any damages
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Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute
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