How do certain Windows DLL apparently manage to support both 32 and 64 bit?

Question

I seem to be able to access the functions in C:\WINDOWS\system32\opengl32.dll (and likewise C:\WINDOWS\system32\glu32.dll) from either a 32-bit or a 64-bit application (for what it's worth, I'm doing this from separate 32-bit and 64-bit Python 2.7 interpreters, via the ctypes module).

With glut32.dll, things are different. It only happens to be on my Path as part of a 32-bit installation of GraphViz. From 32-bit Python, I can link dynamically to it, but from 64-bit Python I get [Error 193] %1 is not a valid Win32 application.

Now, this error does not surprise me, because I had always thought DLLs were obliged to commit to one architecture or another on Windows (in particular, this recent question and its answers seem to say so). What surprises me is the lack of an error in the first case... How does opengl32.dll do it, and how can I replicate this behaviour when building my own DLLs?

https://msdn.microsoft.com/en-us/library/windows/desktop/aa384187(v=vs.85).aspx — Hans Passant, Oct 06 '17 at 03:16

score 3 · Accepted Answer · answered Oct 06 '17 at 05:27

On 64 bit system only 64 bit processes use c:\Windows\System32\opengl32.dll. For 32 bit processes system redirects c:\Windows\System32\opengl32.dll to c:\Windows\SysWOW64\opengl32.dll.

File System Redirector

In most cases, whenever a 32-bit application attempts to access %windir%\System32, the access is redirected to %windir%\SysWOW64. Access to %windir%\lastgood\system32 is redirected to %windir%\lastgood\SysWOW64. Access to %windir%\regedit.exe is redirected to %windir%\SysWOW64\regedit.exe.

How do certain Windows DLL apparently manage to support both 32 and 64 bit?

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