Create a simple memory resident program

Question

I was always intrigued on how TSR programs worked so I decided to begin creating an extremely primitive one.

This C program reads keyboard keystrokes and prints them on console screen unless you type a then b then c then a message box will tell you that you typed abc and exits also q will cause the program to quit.

Some notes, the program is taking a least 15% of my CPU in task manager, why? I tried comparing the keystroke to 27 as in ESC but it didn't work.

#include <stdio.h>
#include <conio.h>
#include <Windows.h>

int main() {
  char i;

  while (1) {
    if (kbhit()) {
      i = getch();
      if (i == 'q')
        return 0;
      if (i == 'a') {
        i = getch();
        if (i == 'b') {
          i = getch();
          if (i == 'c') {
            int ans = MessageBox(NULL, "You typed abc\nPress OK to exit", "TSR confirmation", MB_ICONINFORMATION | MB_OK);
            if (ans == 1)
              return 0;
          }
        }
      }
      printf("You pressed %c\n", i);
    }
  }
  return 0;
}

My questions: The console still shows, I tried to compile it as a Windows program, it loaded itself in memory and I lost it without the console and had to use Task Manager to kill it, is there a way to make it resident in memory and always monitor my keystrokes for the sequence without the console.

Is there a better way to listen to a longer sequence of characters without nested ifs, it becomes ugly with longer sequences.

On Windows (and other multitasking operating systems) each process has its own input queue. You'll need to use a keyboard hook to monitor input that occurs when your program does not have focus. — 500 - Internal Server Error, Jan 08 '21 at 11:49
TSR is a MS-DOS thing, the concept makes little sense in Windows. It's how to make a background task in a single-tasking system. If no key is pressed, your loop is basically a `while (1) {}` which eats CPU. What are you trying to achieve anyway? — , Jan 08 '21 at 11:53
Just trying to understand how a process can still monitor what is happening even when present in the background. — user10191234, Jan 08 '21 at 12:00
The DOS answer to that was to install an interrupt handler. Windows won't let you do that. — , Jan 08 '21 at 12:03
The Win32 API has calls to install callbacks on several events, and so also on key events. Please visit the MSDN and start reading. @dratenik This way a key logger is really quite simple. Done that, with AutoIt3. — the busybee, Jan 08 '21 at 13:10

the busybee · Accepted Answer · 2021-01-08T16:45:05.397

Why is the program taking a least 15% of my CPU in task manager?

Because your program restlessly executes the loop. And it seems that you have a CPU with 6 or 8 (virtual) cores. Actually your program takes about 100% of one core.

You can reduce this "load" a lot by inserting a Sleep(1) in the loop. This inserts a "sleeping" phase, which is so much longer than the runtime of a single turn of the loop, that the load will drop to nearly 0%.

Is there a way to make it resident in memory and always monitor my keystrokes for the sequence without the console?

As Windows is a multitasking system, each process that is not stopped is a resident process. You can simply program an endless loop, as you already did.

To monitor all keystrokes, you can try to use a so-called callback function, which is called for each keyboard event. Start at the MSDN page about SetWindowsHookEx() to learn more. Note, this is not a beginner's task.

Is there a better way to listen to a longer sequence of characters without nested ifs?

Sure, use a finite state machine. Since an explanation will take much more room than appropriate, please learn about this yourself. You might start at the Wikipedia page on the matter.

Create a simple memory resident program

1 Answers1