Windows does automatic parameter marshaling for system messages (0 to WM_USER-1), which WM_SETTEXT is a part of. Since the meaning of the parameters is well-est

Windows does automatic parameter marshaling for system messages (0 to WM_USER-1), which WM_SETTEXT is a part of. Since the meaning of the parameters is well-established, the system can allocate memory in the target process and replace the pointers with meaningful values for the target wndproc.