Windows installer MSI format

Question

I cannot find the MSI format specification. It says that its an open standard, but I don't see any documentation - just bits and pieces of information here and there.

I want to modify some .msi installer, but I want to understand first what it is doing.

Answer 1

Don't forget MakeMSI which is a free app to create MSI files.

Answer 2

Sounds like you got a misunderstanding of Microsoft's promise.

Open Specification Promise

Published: September 12, 2006. Revised: February 15, 2007

Microsoft irrevocably promises not to assert any Microsoft Necessary Claims against you for making, using, selling, offering for sale, importing or distributing any implementation to the extent it conforms to a Covered Specification (“Covered Implementation”), subject to the following. This is a personal promise directly from Microsoft to you, and you acknowledge as a condition of benefiting from it that no Microsoft rights are received from suppliers, distributors, or otherwise in connection with this promise. If you file, maintain or voluntarily participate in a patent infringement lawsuit against a Microsoft implementation of such Covered Specification, then this personal promise does not apply with respect to any Covered Implementation of the same Covered Specification made or used by you. To clarify, “Microsoft Necessary Claims” are those claims of Microsoft-owned or Microsoft-controlled patents that are necessary to implement only the required portions of the Covered Specification that are described in detail and not merely referenced in such Specification. “Covered Specifications” are listed below.

This promise is not an assurance either (i) that any of Microsoft issued patent claims covers a Covered Implementation or are enforceable or (ii) that a Covered Implementation would not infringe patents or other intellectual property rights of any third party. No other rights except those expressly stated in this promise shall be deemed granted, waived or received by implication, exhaustion, estoppel, or otherwise.

The full list of the open specifications can be found at

http://www.microsoft.com/openspecifications/en/us/programs/osp/default.aspx

Since MSI is not listed within, you can safely assume that it is a private standard and won't be shared to general public.

Answer 3

Are you sure that .MSI database format is required to modify some installer? Have you heard about the Orca tool? If not, this MS KB article is for you:

http://support.microsoft.com/kb/255905

Answer 4

MSI files are COM structured storage, which is described here: Structured Storage (Windows).

Answer 5

The binary format is not open, I wouldn't even go as far as calling it a standard.

However I assume it's not the file format you're concerned with (which is undocumented), but rather the tables, actions, sequences, etc. that are visible when interfacing with the MSI via the Windows Installer SDK, API, Orca, etc.

Everything is covered in the SDK documentation, however for some reason search engines don't rank the documentation very highly. Also you can no longer download the Windows Installer SDK separately, it is now bundled as part of the Windows SDK.

• Windows Installer SDK Documentation
• Windows SDK Download

Answer 6

The MSI file format is not open and is in fact undocumented, however the API required to access these files is part of the Windows Installer SDK. That link describes both the API functions required to access and modify MSI databases (files), and the contents of the tables in an MSI database.

That said if you want to modify or view the contents of an MSI then the easiest way to do it is probably just to use an editor that someone else has made (which will in turn use these functions). The ones that I know of are:

• ORCA
• InstEd
• InstallShield also have an MSI editor

If you really want to know more about the details of the internals of the MSI file format then take a read through of these two blog articles by Rob Mensching (the author of WiX)

• Inside the MSI file format
• Inside the MSI file format, again