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) and handles to GDI objects such as pens and brushes (HBRUSH and HPEN).
• Named objects created from a 32-bit process can be accessed by 64-bit processes and vice versa (subject to the usual security rules). Named objects include 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.