Why is GWLP_HWNDPARENT (-8) undocumented in SetWindowLongPtr, and is it safe to use?

[Zakariathr22]

Hello everyone,

I’m working with Win32 APIs and trying to understand how SetWindowLongPtr works with GWLP_HWNDPARENT (-8). While researching, I found that some developers use:

SetWindowLongPtr(hWndChild, -8, (LONG_PTR)hWndParent); // -8 = GWLP_HWNDPARENT

However, GWLP_HWNDPARENT (-8) is not documented in official Microsoft documentation, yet it seems to work for setting window ownership.

My Questions:

1. Is GWLP_HWNDPARENT (-8) officially supported?
2. Why is it undocumented, and does Microsoft internally recognize it?
3. Are there any risks in using it (e.g., compatibility, stability, or future Windows updates breaking it)?
4. What is the correct Win32 alternative for setting a window's owner, if not this method?

Sorry, I know this is a bit off-topic, but I’d really appreciate any insights from experienced developers. (My use case is for modal windows in WinUI.)

Thanks in advance!

Edit:
If this isn't the place to ask this question, could you please direct me to the right one?

[AArnott]

The microsoft/win32metadata repo discussions tab may be a more appropriate forum, as that is staffed by win32 folks, whereas this one is not so much.