Why is the SetupDiCallClassInstaller function restricted to 64 bit programs?

Question

Attempting to call SetupDiCallClassInstaller from a program compiled in 32 bit mode fails on 64 bit Windows.

Apparently this is by design, but I'd like to know the reason.

Answer 1

According to MSDN:

Device Installations on 64-Bit Systems:

The 32-bit version of the application must check the value returned by UpdateDriverForPlugAndPlayDevices. If the return value is ERROR_IN_WOW64, the 32-bit application is executing on a 64-bit platform and cannot update inbox drivers. Instead, it must call CreateProcess (described in the Windows SDK documentation) to start the 64-bit version of the application. The 64-bit version can then call UpdateDriverForPlugAndPlayDevices, specifying a FullInfPath parameter that identifies the location of the 64-bit versions of all files.

So it looks like any API that is designed to report ERROR_IN_WOW64 is specifically intended NOT to work in WOW64, a 32bit process has to invoke a 64bit process to call the API.