How should my program decide to install under “Program Files (x86)”?

Question

Just out of curiosity, if I am creating a program installer, how should I decide in which "Program Files" directory to install to? On 32-bit systems, the environmental variable "%programfiles%" is good enough. However, on 64-bit systems, 32-bit programs should not install to that folder and instead to "%programfiles(x86)%", which as I understand points to "C:\Program Files (x86)". My question is: How should the installer decide which environment variable to use? Will the value of "%programfiles%" change for a 32-bit application, or should I always check first whether "%programfiles(x86)%" exists before using "%programfiles%", or should I do something entirely different?

Thanks! This is just out of my own curiosity, as I try to get used to 64-bit operating systems.

Answer 1

When the 32-bit program (installer in your case) asks the system to resolve the ProgramFilePath constant (check the exact name in MSDN), the system does not return C:\Program files, but C:\Program files(x86). So it's the system that decides, not the application.

Answer 2

I'm pretty certain that I read somewhere that Windows did this for you automagically. In other words, if your installer was 32-bits, it would be routed to the x86 directory variant even though you were trying to install into Program Files.

I'm sure I read this on The Old New Thing but here's a link that supports the contention until I can find that one.

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Ah, yes, here it is, from the ever useful Raymond Chen.

Commenter Koro is writing an installer in the form of a 32-bit program that detects that it's running on a 64-bit system and wants to copy files (and presumably set registry entries and do other installery things) into the 64-bit directories, but the emulation layer redirects the operations into the 32-bit locations. The question is "What is the way of finding the x64 Program Files directory from a 32-bit application?"

The answer is "It is better to work with the system than against it." If you're a 32-bit program, then you're going to be fighting against the emulator each time you try to interact with the outside world. Instead, just recompile your installer as a 64-bit program. Have the 32-bit installer detect that it's running on a 64-bit system and launch the 64-bit installer instead. The 64-bit installer will not run in the 32-bit emulation layer, so when it tries to copy a file or update a registry key, it will see the real 64-bit file system and the real 64-bit registry.