move key toggling into a thread

Instead of using the key event hook to re-issue a caps lock key event,
move this responsibility to another thread. This can help this utility
play nicely with other applications that monitor/manipulate key events
on a system by letting the initial key event do its thing and then
gracefully inject another caps lock event afterwards.

Signed-off-by: James Knight <[email protected]>

src/capsblock/main.c

@@ -7,62 +7,107 @@
 #endif
 #include <windows.h>
 
-// state to suppress hook events when generating new caps-lock events
-volatile bool gSkip = false;
+// semaphore used to signal the toggle-key thread
+HANDLE gCapsEventSem;
+
+// shutdown flag for the thread
+volatile bool gShutdown = false;
 
 // invoked when a low-level keyboard event occurs
 LRESULT CALLBACK LowLevelKeyboardProc(int code, WPARAM msg, LPARAM ctx)
 {
     // listen for key-up events of the caps-lock key
-    if (!gSkip && !code >= 0 && msg == WM_KEYUP) {
+    if (!code >= 0 && msg == WM_KEYUP) {
         KBDLLHOOKSTRUCT* kbd = (KBDLLHOOKSTRUCT*)ctx;
         if (kbd->vkCode == VK_CAPITAL) {
             // check if we have caps-lock enabled in our current state
             bool hasCapsLock = ((GetKeyState(VK_CAPITAL) & 0x0001) != 0);
             if (hasCapsLock) {
-                // before we attempt to toggle off the caps-lock state,
-                // flag our hook as disabled, to ensure we do not attempt
-                // to process the state of any caps-lock keys issued from
-                // our hook
-                gSkip = true;
+                // signal to the toggle-key thread to check/enforce
+                // the key state
+                ReleaseSemaphore(gCapsEventSem, 1, NULL);
+            }
+        }
+    }
+
+    return CallNextHookEx(NULL, code, msg, ctx);
+}
 
+// thread used to wait for events to check/enforce the key state
+// of the caps lock key
+#pragma warning(disable: 4100)
+DWORD WINAPI ToggleKeyThread(void* data)
+{
+    while (!gShutdown) {
+        DWORD signal = WaitForSingleObject(gCapsEventSem, INFINITE);
+        switch (signal) {
+        case WAIT_OBJECT_0:
+            // check (again) if we have caps-lock enabled in our current state
+            bool hasCapsLock = ((GetKeyState(VK_CAPITAL) & 0x0001) != 0);
+            if (hasCapsLock) {
                 // generate two additional key events:
                 ///
-                //  1) a key-up event to "complete" the toggled on state
-                //  2) a key-down event to trigger a new caps-lock event
-                //
-                // do not consume this active event, to complete the final
-                // key-up event
-                INPUT input[2] = {0};
+                //  1) a key-down event to trigger a new caps-lock event
+                //  2) a key-up event to "complete" the event
+                INPUT input[2] = { 0 };
                 input[0].type = input[1].type = INPUT_KEYBOARD;
-                input[0].ki.wVk  = input[1].ki.wVk = VK_CAPITAL;
-                input[0].ki.dwFlags = KEYEVENTF_KEYUP;
+                input[0].ki.wVk = input[1].ki.wVk = VK_CAPITAL;
+                input[1].ki.dwFlags = KEYEVENTF_KEYUP;
                 SendInput(2, input, sizeof(INPUT));
-
-                // after issuing our key changes, re-enable this hook
-                gSkip = false;
             }
+            break;
+
+        case WAIT_TIMEOUT:
+            // spurious wakeup
+            break;
+
+        default:
+            // broken state
+            gShutdown = true;
+            break;
         }
     }
 
-    return CallNextHookEx(NULL, code, msg, ctx);
+    return 0;
 }
 
 #pragma warning(disable: 4100)
 int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
         PWSTR pCmdLine, int nCmdShow)
 {
+    // create a signal between that can be issued by the keyboard hook to
+    // out toggle-key thread
+    gCapsEventSem = CreateSemaphore(NULL, 1, 1, NULL);
+    if (!gCapsEventSem) {
+        fprintf(stderr, "unable to create keyboard hook\n");
+        return 1;
+    }
+
     // create a hook to listen for keyboard events
     HHOOK hook = SetWindowsHookEx(
         WH_KEYBOARD_LL, LowLevelKeyboardProc, GetModuleHandle(NULL), 0);
     if (!hook) {
         fprintf(stderr, "unable to create keyboard hook\n");
-        return 1;
+        return 2;
+    }
+
+    // build a thread reposible for toggling the cap-lock key off
+    HANDLE tkt = CreateThread(NULL, 0, ToggleKeyThread, NULL, 0, NULL);
+    if (!tkt) {
+        fprintf(stderr, "unable to create toggle-key thread\n");
+        return 3;
     }
 
     // consume all messages
     MSG msg;
     while(GetMessage(&msg, NULL, 0, 0) != 0);
 
+    // cleanup
+    gShutdown = true;
+    ReleaseSemaphore(gCapsEventSem, 1, NULL);
+    WaitForSingleObject(tkt, INFINITE);
+    CloseHandle(tkt);
+    CloseHandle(gCapsEventSem);
+
     return 0;
 }