Locking a Windows Session

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

Please note! If you're having difficulties compiling the C source code presented below, please see my post: Compiling C from the Command Line with Pelles C

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

Views expressed in this blog are those of the author and do not necessary reflect those of the author's employer. Views expressed in the comments are those of the responding individual.

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