hotcorner.c

#define WIN32_LEAN_AND_MEAN
#include <stdlib.h>
#include <windows.h>

#pragma comment(lib, "USER32")
#pragma comment(linker, "/SUBSYSTEM:WINDOWS")

#define KEYDOWN(k) ((k) & 0x80)

// This is a **very** minimal hotcorner app, written in C. Maybe its not the
// optimal way to do this, but it works for me.
//
// Zero state is stored anywhere, no registry keys or configuration files.
//
// - If you want to configure something, edit the code.
// - If you want to uninstall it, just delete it.
//
// Tavis Ormandy <taviso@cmpxchg8b.com> December, 2016
//
// https://github.com/taviso/hotcorner
//

// If the mouse enters this rectangle, activate the hot corner function.
// There are some hints about changing corners here
//      https://github.com/taviso/hotcorner/issues/7#issuecomment-269367351
static const RECT kHotCorner = {
    .top    = -20,
    .left   = -20,
    .right  = +20,
    .bottom = +20,
};

// Input to inject when corner activated (Win+Tab by default).
static const INPUT kCornerInput[] = {
    { INPUT_KEYBOARD, .ki = { VK_LWIN, .dwFlags = 0 }},
    { INPUT_KEYBOARD, .ki = { VK_TAB,  .dwFlags = 0 }},
    { INPUT_KEYBOARD, .ki = { VK_TAB,  .dwFlags = KEYEVENTF_KEYUP }},
    { INPUT_KEYBOARD, .ki = { VK_LWIN, .dwFlags = KEYEVENTF_KEYUP }},
};

// How long cursor has to linger in the kHotCorner RECT to trigger input.
static const DWORD kHotDelay = 300;

// You can exit the application using the hot key CTRL+ALT+C by default, if it
// interferes with some application you're using (e.g. a full screen game).
static const DWORD kHotKeyModifiers = MOD_CONTROL | MOD_ALT;
static const DWORD kHotKey = 'C';

static HANDLE CornerThread = INVALID_HANDLE_VALUE;

// This thread runs when the cursor enters the hot corner, and waits to see if the cursor stays in the corner.
// If the mouse leaves while we're waiting, the thread is just terminated.
static DWORD WINAPI CornerHotFunc(LPVOID lpParameter)
{
    BYTE KeyState[256];
    POINT Point;

    Sleep(kHotDelay);

    // Check if a mouse putton is pressed, maybe a drag operation?
    if (GetKeyState(VK_LBUTTON) < 0 || GetKeyState(VK_RBUTTON) < 0) {
        return 0;
    }

    // Check if any modifier keys are pressed.
    if (GetKeyboardState(KeyState)) {
        if (KEYDOWN(KeyState[VK_SHIFT]) || KEYDOWN(KeyState[VK_CONTROL])
          || KEYDOWN(KeyState[VK_MENU]) || KEYDOWN(KeyState[VK_LWIN])
          || KEYDOWN(KeyState[VK_RWIN])) {
            return 0;
        }
    }

    // Verify the corner is still hot
    if (GetCursorPos(&Point) == FALSE) {
        return 1;
    }

    // Check co-ordinates.
    if (PtInRect(&kHotCorner, Point)) {
        #pragma warning(suppress : 4090)
        if (SendInput(_countof(kCornerInput), kCornerInput, sizeof(INPUT)) != _countof(kCornerInput)) {
            return 1;
        }
    }

    return 0;
}

static LRESULT CALLBACK MouseHookCallback(int nCode, WPARAM wParam, LPARAM lParam)
{
    MSLLHOOKSTRUCT *evt = (MSLLHOOKSTRUCT *) lParam;

    // If the mouse hasn't moved, we're done.
    if (wParam != WM_MOUSEMOVE)
        goto finish;

    // Check if the cursor is hot or cold.
    if (!PtInRect(&kHotCorner, evt->pt)) {

        // The corner is cold, and was cold before.
        if (CornerThread == INVALID_HANDLE_VALUE)
            goto finish;

        // The corner is cold, but was previously hot.
        TerminateThread(CornerThread, 0);

        CloseHandle(CornerThread);

        // Reset state.
        CornerThread = INVALID_HANDLE_VALUE;

        goto finish;
    }

    // The corner is hot, check if it was already hot.
    if (CornerThread != INVALID_HANDLE_VALUE) {
        goto finish;
    }

    // Check if a mouse putton is pressed, maybe a drag operation?
    if (GetKeyState(VK_LBUTTON) < 0 || GetKeyState(VK_RBUTTON) < 0) {
        goto finish;
    }

    // The corner is hot, and was previously cold. Here we start a thread to
    // monitor if the mouse lingers.
    CornerThread = CreateThread(NULL, 0, CornerHotFunc, NULL, 0, NULL);

finish:
    return CallNextHookEx(NULL, nCode, wParam, lParam);
}

int CALLBACK WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    MSG Msg;
    HHOOK MouseHook;

    if (!(MouseHook = SetWindowsHookEx(WH_MOUSE_LL, MouseHookCallback, NULL, 0)))
        return 1;

    RegisterHotKey(NULL, 1, kHotKeyModifiers, kHotKey);

    while (GetMessage(&Msg, NULL, 0, 0)) {
        if (Msg.message == WM_HOTKEY) {
            break;
        }
        DispatchMessage(&Msg);
    }

    UnhookWindowsHookEx(MouseHook);

    return Msg.wParam;
}