Hacker's Corner: A Windows Task Killer

by Duncan Pearson

Adrian Smith came to me the other day and said “If I start a Windows program from Dyalog with ⎕CMD how can I kill it again from within APL ?”. So we dived for the manuals and found out a few interesting things.

First of all the ⎕CMD function, when used to start a Windows task, returns a number.

      +⎕CMD 'notepad.exe' 'minimised'
12345

“Aha !” say we, “this looks like a task handle. Why not send a Windows ‘QUIT’ message to the task and see if it kills itself.” If you are a Windows task then getting a WM_QUIT is like being locked in a room with a bottle of whiskey and a loaded revolver and being told to get on with it. It looked like a good bet.

So we did. We used ⎕NA to associate the PostAppMessage windows function

⎕NA 'I user.P16|PostAppMessage U U I I4'

and we posted the task a WM_QUIT (message number 18)

PostAppMessage 12345 18 0 0

Hello DOS prompt, goodbye Windows - "bummer!" said Pooh!

Apparently ⎕CMD returns something called an hinstance which is not a task handle but an instance handle. So is there a way from one to the other ? As a matter of fact there is but you have to look for it. There is a pair of functions in toolhelp.dll with which one can go round all the tasks in the Windows event-queue collecting information from them. One of the bits of information is the hinstance and another is the task-handle.

We associate them thus.

⎕NA 'I toolhelp.P16|TaskFirst ={I4 U U U U I I I I I I U C[10] I U}'
<PRE class=aplu>
⎕NA 'I toolhelp.P16|TaskNext ={I4 U U U U I I I I I I U C[10] I U}'
</PRE>

They are pretty much the same. Each one takes a horrible structure called a TASKENTRY and returns it (the = means take and return the argument). We pass TaskFirst an empy structure and it fills it with details of the first task in the queue. We pass the filled structure to TaskNext and it replaces the information about the first task with information about the next task. We carry on calling TaskNext until we find a task with the hinstance that was returned, get the task handle of that task from the structure and send it a WM_QUIT.

What I omitted to say was what the horrible structure meant. We get back twelve numbers, a ten character text vector and two more numbers. The text string is the task name, the second number is the task handle and the fourth is the hinstance. I‘m sure that the rest is very useful, but not right now so I will ignore it.

>>>>>>>>>> taskfns + examples of Kill_Handle and Kill_Name