Interprocess Communication Between 32-bit and 64-bit Applications

The following techniques can be used for communication between 32-bit and 64-bit applications:

• 64-bit versions of Windows use 32-bit handles for interoperability. When sharing a handle between 32-bit and 64-bit applications, only the lower 32 bits are significant, so it is safe to truncate the handle (when passing it from 64-bit to 32-bit) or sign-extend the handle (when passing it from 32-bit to 64-bit). Handles that can be shared include handles to user objects such as windows (HWND), handles to GDI objects such as pens and brushes (HBRUSH and HPEN), and handles to named objects such as mutexes, semaphores, and file handles.
• Remote procedure calls (RPC) can be used.
• COM LocalServers can be used if both 32-bit and 64-bit proxy/stub DLLs are registered for all interfaces used.
• Shared memory can be used if pointer-dependent types are converted properly (or avoided).
• The CreateProcess and ShellExecute functions can launch 32-bit and 64-bit processes from either 32-bit or 64-bit processes with certain limitations.

A 64-bit executable file located under %windir%\System32 cannot be launched from a 32-bit process, because the file system redirector redirects the path. Do not disable redirection to accomplish this; use %windir%\Sysnative instead. For more information, see File System Redirector.