Retrofitting the WM_COPY­DATA message onto Windows 3.1

# Retrofitting the WM_COPY­DATA message onto Windows 3.1 - The Old New Thing Source: [https://devblogs.microsoft.com/oldnewthing/20260616-00?p=112430](https://devblogs.microsoft.com/oldnewthing/20260616-00?p=112430) Some time ago, I talked about[how to return results back from the`WM\_COPY­DATA`message](https://devblogs.microsoft.com/oldnewthing/20251114-00/?p=111792)\. Which reminded me of a clever bit of history\. The`WM\_COPY­DATA`message was introduced in 32\-bit Windows\. There was no need for it in 16\-bit Windows because all 16\-bit programs ran in the same address space\. A far pointer in one process was good in any process\. You could put it in the`lParam`of a window message and send it to any other window, same process or different process, doesn’t matter\. But 32\-bit programs ran in separate address spaces, so this trick didn’t work\. Hence the need for`WM\_COPY­DATA`to pass data not only between 32\-bit programs, but also between 32\-bit programs and 16\-bit programs\. How did this message get retrofitted into 16\-bit Windows so that Win32s could support it? Easy: It was already implemented, unwittingly\. If the source and destination windows are both 16\-bit windows, then the pointer to the`COPY­DATA­STRUCT`is already valid in both processes, as is the pointer inside the`COPY­DATA­STRUCT`\. And the window handle in the`wParam`is also the same for both processes\. Therefore, doing absolutely nothing with the`wParam`and`lParam`and simply allowing it to pass from a 16\-bit program to another 16\-bit program will still behave as expected\. And it so happens that Windows 3\.1 already did that: Windows 3\.1 always passed the`wParam`and`lParam`unmodified, even when the message sender and receiver are in different processes, because all programs shared the same address space\. It was just a sneaky trick to design the`WM\_COPY­DATA`message in such a way that the null marshaler is the correct behavior when it is sent between 16\-bit programs\. ### Category ### Topics ## Author  Raymond has been involved in the evolution of Windows for more than 30 years\. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie\-jeebies\. The Web site spawned a book, coincidentally also titled The Old New Thing \(Addison Wesley 2007\)\. He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information\.