Locking a Windows Session

Originally published on: Wed, 02 Dec 2009 03:11:19 +0000

Several years ago, a friend and I discussed the topic of a program that would, after a period of inactivity, issue the equivalent function of locking one's Windows session via CTRL-ALT-DEL followed by a click on the appropriate "lock" button. The screen-saver was supposed to fulfill this role, but the thought-process was that someone could disable their screen-saver.

I did a little investigation into the matter and could not find a way to lock the session via a program. Years later, a function was added to user32.dll that could be invoked to provide this functionality. The C program below attempts to find this function in User32 dynamically and then invokes it. If not found, an error message will be displayed.

lock.c

// Lock a Windows session.
//
// License: MIT / X11
// Copyright (c) 2009 by James K. Lawless
// jimbo@radiks.net http://www.radiks.net/~jimbo
// http://www.mailsend-online.com
//
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.

#include <windows.h>

#pragma comment(lib,"user32.lib")

int main(int argc,char **argv) {
   FARPROC lockWorkStation;
   HANDLE user32;
   user32=LoadLibrary("user32.dll");
   lockWorkStation=GetProcAddress(user32,"LockWorkStation");
   if(lockWorkStation!=NULL)
      (*lockWorkStation)();
   else
      MessageBox(NULL,
         "You don't have LockWorkStation in User32",
         "",MB_OK);

}

The source and sample executable file for lock can be downloaded in a single archive at: http://www.mailsend-online.com/wp/lock.zip

Unless otherwise noted, all code and text entries are Copyright Š2009 by James K. Lawless

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