Issue with generating left click in win32 api?

Question

Below is my code to generate left click using win32 api. The problem is that it gets stuck and does not return to main. When i press Ctrl+c, then it returns to main. BUT when I call it twice, to simulate double click then it is fine. Is there anything wrong with this code? Thank you.

void LeftClick(void)
{  
  INPUT    Input={0};

  // left down 
  Input.type      = INPUT_MOUSE; /*The event is a mouse event. Use the mi structure of the union.*/
  Input.mi.dwFlags  = MOUSEEVENTF_LEFTDOWN;
  SendInput(1,&Input,sizeof(INPUT));

  // left up
  ZeroMemory(&Input,sizeof(INPUT));
  Input.type      = INPUT_MOUSE;
  Input.mi.dwFlags  = MOUSEEVENTF_LEFTUP;
  SendInput(1,&Input,sizeof(INPUT));
}

When you run in the debugger, on what line does it get stuck? — Ben Voigt, Mar 28 '11 at 00:23
When i do debugging it is fine, i do not see it getting stuck any where. But when i run it then it gets stuck! so confusing. — infinitloop, Mar 28 '11 at 00:39
You are clicking on stuff, it could have any kinds of side effects. Depending on where the mouse is at. — Hans Passant, Mar 28 '11 at 01:15
@rashid: Try printf debugging then. Once you isolate where it gets stuck, you can start to understand why. — Ben Voigt, Mar 28 '11 at 01:16
@Ben, thanks for the advice, doing that pointed the problem to ZeroMemory(&Input,sizeof(INPUT)); — infinitloop, Mar 28 '11 at 01:46
@rashid: That's very unusual. Do you still have a problem if you use `memset` instead? Or you could use an array of 2 INPUT structures and send both with a single `SendInput` call. — Ben Voigt, Mar 28 '11 at 01:56
ok so sometimes it gets stuck at SendInput(1,&Input,sizeof(INPUT)); and sometimes at that ZeroMemory(...). I have no idea. — infinitloop, Mar 28 '11 at 02:05
The ZeroMemory can't be to blame. That said it's pointless, but it can't explain this problem. — David Heffernan, Mar 28 '11 at 15:40

score 1 · Accepted Answer · answered Mar 28 '11 at 16:43

If you're not committed to using SendInput, I've had success in the past using SendMessage on the desired hWnd with WM_LBUTTONDOWN then again with WM_LBUTTONUP.

Most buttons also simulate a click with keyboard entry. You can use SendMessage to your desired hWnd with WM_KEYDOWN and wParam VK_SPACE, then WM_KEYUP with VK_SPACE to complete the space bar keypress simulation.

Issue with generating left click in win32 api?

1 Answers1