Qt Network Internals

Windows:
In the file qtbase\src\corelib\kernel\qeventdispatcher_win.cpp

 QEventDispatcherWin32Private::doWsaAsyncSelect

UNIX:
qtbase\src\corelib\kernel\qeventdispatcher_unix.cpp

   QEventDispatcherUNIXPrivate::doSelect

GLib:
qtbase\src\corelib\kernel\qeventdispatcher_glib.cpp

socketNotifierSourceDispatch