How can 32-bit application find the location of 64-bit Program Files directory on Windows Vista 64-bit?

I'm struggling with a problem of how to determine the location of the 64-bit Program Files directory on 64-bit Windows Vista from a 32-bit application.

Calls to SHGetKnownFolderPath(FOLDERID_ProgramFilesX64) do not return anything. The MSDN article KNOWNFOLDERID also states that this particular call with FOLDERID_ProgramFilesX64 is not supported for a 32-bit application.

I would like to avoid as much as possible hardcoding the path to "C:\Program Files". Doing something like GetWindowsDirectory(), extracting the drive from the return value and adding "\Program Files" to it is not appealing either.

How can a 32-bit application properly get the location of the folder from 64-bit Windows Vista?

Background

Our application has a service component which is supposed to launch other processes based on requests from user-session-specific component. The applications launched can be 32-bit or 64-bit. We do this is via CreateProcessAsUser() by passing in a token from initiating user-s开发者_如何学运维ession process. For call to CreateProcessAsUser, we create an environment block via the CreateEnvironmentBlock() API. The problem is that CreateEnvironmentBlock(), using the token of the user-session application, creates a block with ProgramW6432="C:\Program Files (x86)", which is a problem for 64-bit applications. We need to override it with the proper value.

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As you mentioned, using SHGetKnownFolderPath from a 32-bit application will not work on a 64-bit operating system. This is because Wow64 emulation is in effect.

You can however use RegOpenKeyEx passing in the flag KEY_WOW64_64KEY and then read the program files directory from registry.

The location in registry:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion

You are interested in the string value:

ProgramFilesDir

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If you read that page carefully you will see that FOLDERID_ProgramFilesX64 is supported for 32 bits applications on a 64-bit OS. It's NOT supported on a 32-bit OS, which makes complete sense.

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You can also query the environment variable ProgramW6432. It obviously exists only in 64-bit Windows, but it should return the real 64-bit Program Files directory, and it seems to be defined for both 64-bit and 32-bit programs. At least it worked for me (C#, GetEnvironmentVariable)...

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FOLDERID_ProgramFilesX64 is supported...

MSDN says it is supported, but Microsoft's "WOW64" best practices document says it is not. See http://download.microsoft.com/download/A/F/7/AF7777E5-7DCD-4800-8A0A-B18336565F5B/wow64_bestprac.docx

To quote:

• Some variables work only if the process is 64-bit. For example, FOLDERID_ProgramFilesX64 does not work for 32-bit callers. In versions of Windows earlier than Windows 7, %ProgramW6432% did not work in the context of 32-bit processes. An application must determine whether it is running in a 64-bit process before it uses these variables.

Under Windows 7 x64, running a 32-bit app in the Visual Studio debugger, I also get a return code of 0x80070002 (and a NULL pointer). Running the same code compiled as 64-bit returns the value S_OK and the path is properly filled in.

I've used the registry hack as listed above since I can't find any other workaround.