[windows] how to get program files x86 env variable?

On a 64-bit Windows system, the reading of the various environment variables and some Windows Registry keys is redirected to different sources, depending whether the process doing the reading is 32-bit or 64-bit.

The table below lists these data sources:

X = HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion
Y = HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion
Z = HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
     
READING ENVIRONMENT VARIABLES:    Source for 64-bit process               Source for 32-bit process
-------------------------------|----------------------------------------|--------------------------------------------------------------
                %ProgramFiles% :  X\ProgramW6432Dir                       X\ProgramFilesDir (x86)
           %ProgramFiles(x86)% :  X\ProgramFilesDir (x86)                 X\ProgramFilesDir (x86)
                %ProgramW6432% :  X\ProgramW6432Dir                       X\ProgramW6432Dir
     
          %CommonProgramFiles% :  X\CommonW6432Dir                        X\CommonFilesDir (x86)
     %CommonProgramFiles(x86)% :  X\CommonFilesDir (x86)                  X\CommonFilesDir (x86)
          %CommonProgramW6432% :  X\CommonW6432Dir                        X\CommonW6432Dir
     
                 %ProgramData% :  Z\ProgramData                           Z\ProgramData


      READING REGISTRY VALUES:    Source for 64-bit process               Source for 32-bit process
-------------------------------|----------------------------------------|--------------------------------------------------------------
             X\ProgramFilesDir :  X\ProgramFilesDir                       Y\ProgramFilesDir
       X\ProgramFilesDir (x86) :  X\ProgramFilesDir (x86)                 Y\ProgramFilesDir (x86)
            X\ProgramFilesPath :  X\ProgramFilesPath = %ProgramFiles%     Y\ProgramFilesPath = %ProgramFiles(x86)%
             X\ProgramW6432Dir :  X\ProgramW6432Dir                       Y\ProgramW6432Dir
     
              X\CommonFilesDir :  X\CommonFilesDir                        Y\CommonFilesDir
        X\CommonFilesDir (x86) :  X\CommonFilesDir (x86)                  Y\CommonFilesDir (x86)
              X\CommonW6432Dir :  X\CommonW6432Dir                        Y\CommonW6432Dir
     

So for example, for a 32-bit process, the source of the data for the %ProgramFiles% and %ProgramFiles(x86)% environment variables is the Registry value HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\ProgramFilesDir (x86).

However, for a 64-bit process, the source of the data for the %ProgramFiles% environment variable is the Registry value HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\ProgramW6432Dir ...and the source of the data for the %ProgramFiles(x86)% environment variable is the Registry value HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\ProgramFilesDir (x86)

Most default Windows installation put a string like C:\Program Files (x86) into the Registry value HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\ProgramFilesDir (x86) but this (and others) can be changed.

Whatever is entered into these Windows Registry values will be read by Windows Explorer into respective Environment Variables upon login and then copied to any child process that it subsequently spawns.

The registry value HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\ProgramFilesPath is especially noteworthy because most Windows installations put the string %ProgramFiles% into it, to be read by 64-bit processes. This string refers to the environment variable %ProgramFiles% which in turn, takes its data from the Registry value HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\ProgramW6432Dir ...unless some program changes the value of this environment variable apriori.

I have written a small utility, which displays these environment variables for 64-bit and 32-bit processes. You can download it here.
The source code for VisualStudio 2017 is included and the compiled 64-bit and 32-bit binary executables are in the directories ..\x64\Release and ..\x86\Release, respectively.