don't run if an instance is already running

README.md

@@ -11,7 +11,7 @@ _Stellate is in usable state but is not finished. That is why I haven't provided
 
 1. Clone this repository
 1. Install golang
-1. Run `go build -ldflags "-H=windowsgui"` in the project root. A file called stellate.exe will be built.
+1. Run `go build -ldflags "-H=windowsgui"` in the project root (this may take few mins to complete). A file called stellate.exe will be built.
 1. Open Windows Explorer and copy this file. Then press <kbd>Win</kbd>+<kbd>R</kbd> key and type `shell:startup`. Right click and Paste Shortcut.
 1. Double click on this shortcut. The program will run. 
 

main.go

@@ -18,6 +18,10 @@ const (
 var quit = false // probably ok that it is not protected by a mutex
 
 func main() {
+	if !checkSingleInstance() {
+		os.Exit(1)
+	}
+
 	startSystray, stopSystray := systray.RunWithExternalLoop(func() {
 		iconBytes, err := os.ReadFile("icon.ico")
 		if err != nil {

single-instance_windows.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+	"syscall"
+	"unsafe"
+)
+
+var (
+	kernel32        = syscall.NewLazyDLL("kernel32.dll")
+	procCreateMutex = kernel32.NewProc("CreateMutexW")
+)
+
+// from https://claude.ai/chat/38e56e68-e64a-4a1b-8272-7ac1a5e7ba82
+func checkSingleInstance() bool {
+	mutexName, err := syscall.UTF16PtrFromString("Global\\StellateBrightnessUtilityMutex")
+	if err != nil {
+		fmt.Println("Error creating mutex name:", err)
+		return false
+	}
+
+	handle, _, lastErr := procCreateMutex.Call(
+		0,
+		0,
+		uintptr(unsafe.Pointer(mutexName)),
+	)
+
+	if handle == 0 {
+		fmt.Println("Error creating mutex:", lastErr)
+		return false
+	}
+
+	// Check if the mutex already exists
+	if lastErr == syscall.ERROR_ALREADY_EXISTS {
+		syscall.CloseHandle(syscall.Handle(handle))
+		fmt.Println("Another instance is already running")
+		return false
+	}
+
+	// Keep the mutex handle open until the program exits
+	return true
+}