Unauthorized Windows 95 Update -- W16LOCK

This page last updated January 12, 1996

• W16LOCK.C -- Source code for a small Win32 program which demonstrates the Win16Lock in Windows 95. This version is corrected from the version that appears in Unauthorized Windows 95, page 553.
  After the book was written, KERNEL32.DLL stopped exporting the undocumented GetpWin16Lock, _EnterSysLevel, and _LeaveSysLevel Win32 functions by name. These functions are still present, and are exported-but by number, not name. (GetpWin16Lock is KERNEL32.93, _EnterSysLevel is KERNEL32.97, and _LeaveSysLevel is KERNEL32.98.) However, KERNEL32 does not allow imports by ordinal. To access these functions, W16LOCK.C now uses the GetK32ProcAddress function from K32EXP.C.
  Note: The Win16Lock is definitely not a mutex, even though Microsoft now calls it the "Win16Mutex." As can be seen from this program, it's a single DWORD, located down in conventional memory (below 1MB) no less. Matt Pietrek informs me that Win16Lock is "really a critical section (KERNEL32 object type 4). It just happens to be a global critical section, rather than task specific."
  An interesting Win95 phenomenon, which I believe has to do with Win16Lock: close all Win16 programs, so you're only running Win32 code; run some Win32 program that updates the display frequently, such as WinBezMT or the Win32 version of Clock; click down on any window titlebar a titlebar and hold the mouse down (you can move the window around if you want; just keep the mouse clicked dow); notice that all display stops while the mouse is clicked down in the titlebar! Interesting, enabling Control Panel | Display | Plus! | "Show window contents while dragging" has the side effect of fixing this nasty behavior.
  
• W16LOCK.EXE -- A small Win32 program which demonstrates how easy it is to grab the Win16Lock in Windows 95, and thus stop the entire system. The Win16Lock is grabbed with the undocumented _EnterSysLevel Win32 API, and released with _LeaveSysLevel.

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Unauthorized Windows 95 Update