windows 7 - where and how can I store machine user independent data?

Question

I have an app (32 bit c++) running under XP that I need to adapt to run under Windows 7 and Vista. It needs to store a few dozen bytes of data someplace independent of User. Under XP, I stored the data in the registry under HKEY_LOCAL_MACHINE\Software. When I run the app on Windows 7 the registry entries are virtual-ized and each user gets a separate copy of the data.

The non-virtual-ized registry seems like a logical place for the data but I have no idea how to go about doing so. I note that there are a multitude of applications that actually store data there; how do they go about doing so?

I'm also willing to store the data else where, is there is some well know global repository for it? A single small file is all I need.

I'm more or less ignorant of the whole rights/privileges business so any hints, pointers, etc much appreciated.

Answer 1

One of the design goals of Windows 7 is to isolate user data and applications. This is to improve privacy, security, and customization. In fact, standard users in Win 7 cannot change data of other users.

The standard places to store application data is returned by the System.Environment.SpecialFolder enumeration. Note that not all folders are readable or writable by all users. For example, CommonApplicationData is readable by all users, but only writable by those with an appropriate policy, like Admins.

If you absolutely must have data that is shared among users, an Admin or one with permission must install it to a shared location. If users need to update this data, they should copy it to a location they can write to, such as ApplicationData, and update their own private copies. This private copy cannot be changed by other users. You should not install data to shared locations unless your application won't work otherwise.

In fact, in Win7 you should install all applications and data to the logged-on user's application and data folders, not to shared locations. If multiple users install the application, each user will get their own copy of the application and data. This is almost always what you want. If multiple users are running an application or game, you do not want one user changing everyone else. If multiple users really do need the same change, let each user update their private copy when they need it. If one user's account is hacked or turns evil, you do not want it destroying everyone else's applications and data.

Also be aware that in Win7, users can log on to a machine remotely, so it is not a good idea to store machine-specific data, like screen resolutions or IP addresses, by user. Instead, check this every time your app runs.