1/ If there is a dll been loaded, I want to unload it. I have to write code like below:
QLibrary lib(path);
if (lib.load())
lib.unload();
I don't know why I should load it first.

2/ the dll does not unload immediately

3/ User can't handle load reasons like thread attach or detach

this solution works correctly on windows
on the place you want to load:
HMODULE pHnd = LoadLibraryW((wchar_t*)QDir::toNativeSeparators(path).utf16());
if (!pHnd)

DisableThreadLibraryCalls(pHnd);

on the place you want to unload:
FreeLibrary((HMODULE)pHnd);