Making program use less CPU when waiting in an infinite loop

Question

Setup : I have this program in c++:

#include <windows.h>
using namespace std;
int main(){
    HWND window;
    AllocConsole();
    window = FindWindowA("ConsoleWindowClass", NULL);
    ShowWindow(window,0);
while (1){  
if (GetKeyState('A'))
        {
    system("start love.mp3");
    return 0;
        }
    }
return 0;
}

So the program runs as a process and waits until the key A is pressed. And then it plays the love.mp3 file :)

However, when the program is waiting it uses up 25% of the CPU usage.

Qustion : Is there a way to reduce this so that the program doesn't consume so much of the CPU?

Answer 1

Your program is guilty of busy-waiting .

Try hooking your keyboard calls by using SetWindowsHookEx .

Answer 2

Calling Sleep in the loop is sort of effective, but IMO, it's clearly the wrong way to go.

Instead, I'd do a blocking read, then check if the result was A , and do your thing when it is:

while ((ch = getch()) != EOF)
    if (ch == 'A') {
        system("start love.mp3");
        break;
    }

This won't miss keystrokes, and it'll use even less CPU time than a loop that calls Sleep .

Answer 3

You can stick a call to the Windows Sleep() function into the loop pretty easily. The argument is in milliseconds, so this will check the keyboard roughly five times a second:

#include <windows.h>
// ...
while (1){  
    if (GetKeyState('A'))
    {
        system("start love.mp3");
        return 0;
    }
    Sleep(200);
}

Answer 4

The way to do this would be to use a Windows hook, probably of type WH_KEYBOARD or WH_KEYBOARD_LL hook (although WH_GETMESSAGE would work as well) and do your processing there. 方法是使用Windows挂钩，可能是WH_KEYBOARD或WH_KEYBOARD_LL挂钩类型（尽管WH_GETMESSAGE也可以工作）并在那里进行处理。

Look at the documentation for SetWindowsHookEx at http://msdn.microsoft.com/en-us/library/windows/desktop/ms644990(v=vs.85).aspx

I haven't tested this - just whipped it up on the fly - but here's a simple example that should do what you want when the 'A' key is pressed.

LRESULT __stdcall CALLBACK LoveProc(int nCode, WPARAM wParam, LPARAM lParam)
{
    static int love = 0;

    if((nCode == HC_ACTION) && 
       (wParam == 'A') &&      /* the key pressed was 'A' */
       (lParam & 0x40000000))  /* trigger when the key is pressed */
    {        
        if(love == 0)
            play_romantic_love_song();

        /* but don't overdo it because "Too Much Love Will Kill You" */
        love = 1;
    }

    return CallNextHookEx(hOldKeyHook, nCode, wParam, lParam );
}

You may also want to google for "*SetWindowsHookEx WH_KEYBOARD*" as I'm pretty sure there's at least a couple of articles that explain this on CodeProject. I'd include the links, but I'm typing this from my iPhone and it's being... difficult.