Implement a global hotkey in golang?

This is possible on most operating systems with simple system calls, in Go you can use package syscall to do system calls without any extra C code or cgo compiler.

Note that the online official documentation of the syscall package shows only the linux interface. Other operating systems have slightly different interface, e.g. on Windows the syscall package also contains a syscall.DLL type, syscall.LoadDLL() and syscall.MustLoadDLL() functions, among others. To view these, run the godoc tool locally, e.g.

godoc -http=:6060

This starts a webserver that hosts a web page similar to godoc.org, navigate to http://localhost:6060/pkg/syscall/. You can also see platform-specific doc online, for details, see How to access platform specific package documentation?

Here I present a complete, runnable Windows solution in pure Go. The complete example application is available on the Go Playground. It doesn't run on the playground, download it and run it locally (on Windows).

Let's define the type to describe hotkeys we want to use. This is not Windows-specific, just to make our code nicer. The Hotkey.String() method provides a human-friendly display name of the hotkey such as "Hotkey[Id: 1, Alt+Ctrl+O]".

const (
    ModAlt = 1 << iota
    ModCtrl
    ModShift
    ModWin
)

type Hotkey struct {
    Id        int // Unique id
    Modifiers int // Mask of modifiers
    KeyCode   int // Key code, e.g. 'A'
}

// String returns a human-friendly display name of the hotkey
// such as "Hotkey[Id: 1, Alt+Ctrl+O]"
func (h *Hotkey) String() string {
    mod := &bytes.Buffer{}
    if h.Modifiers&ModAlt != 0 {
        mod.WriteString("Alt+")
    }
    if h.Modifiers&ModCtrl != 0 {
        mod.WriteString("Ctrl+")
    }
    if h.Modifiers&ModShift != 0 {
        mod.WriteString("Shift+")
    }
    if h.Modifiers&ModWin != 0 {
        mod.WriteString("Win+")
    }
    return fmt.Sprintf("Hotkey[Id: %d, %s%c]", h.Id, mod, h.KeyCode)
}

The windows user32.dll contains functions for global hotkey management. Let's load it:

user32 := syscall.MustLoadDLL("user32")
defer user32.Release()

It has a RegisterHotkey() function for registering global hotkeys:

reghotkey := user32.MustFindProc("RegisterHotKey")

Using this, let's register some hotkeys, namely ALT+CTRL+O, ALT+SHIFT+M, ALT+CTRL+X (which will be used to exit from the app):

// Hotkeys to listen to:
keys := map[int16]*Hotkey{
    1: &Hotkey{1, ModAlt + ModCtrl, 'O'},  // ALT+CTRL+O
    2: &Hotkey{2, ModAlt + ModShift, 'M'}, // ALT+SHIFT+M
    3: &Hotkey{3, ModAlt + ModCtrl, 'X'},  // ALT+CTRL+X
}

// Register hotkeys:
for _, v := range keys {
    r1, _, err := reghotkey.Call(
        0, uintptr(v.Id), uintptr(v.Modifiers), uintptr(v.KeyCode))
    if r1 == 1 {
        fmt.Println("Registered", v)
    } else {
        fmt.Println("Failed to register", v, ", error:", err)
    }
}

We need a way to "listen" to events of pressing those hotkeys. For this, user32.dll contains the PeekMessage() function:

peekmsg := user32.MustFindProc("PeekMessageW")

PeekMessage() stores the message into an MSG struct, let's define it:

type MSG struct {
    HWND   uintptr
    UINT   uintptr
    WPARAM int16
    LPARAM int64
    DWORD  int32
    POINT  struct{ X, Y int64 }
}

Now here's our listening loop which listens and acts on global key presses (here just simply print the pressed hotkey to the console, and if CTRL+ALT+X is pressed, exit from the app):

for {
    var msg = &MSG{}
    peekmsg.Call(uintptr(unsafe.Pointer(msg)), 0, 0, 0, 1)

    // Registered id is in the WPARAM field:
    if id := msg.WPARAM; id != 0 {
        fmt.Println("Hotkey pressed:", keys[id])
        if id == 3 { // CTRL+ALT+X = Exit
            fmt.Println("CTRL+ALT+X pressed, goodbye...")
            return
        }
    }

    time.Sleep(time.Millisecond * 50)
}

And we're done!

Starting the above application prints:

Registered Hotkey[Id: 1, Alt+Ctrl+O]
Registered Hotkey[Id: 2, Alt+Shift+M]
Registered Hotkey[Id: 3, Alt+Ctrl+X]

Now let's press some of the registered hotkeys (with any app being in focus, not necessarily our app), we'll see on the console:

Hotkey pressed: Hotkey[Id: 1, Alt+Ctrl+O]
Hotkey pressed: Hotkey[Id: 1, Alt+Ctrl+O]
Hotkey pressed: Hotkey[Id: 2, Alt+Shift+M]
Hotkey pressed: Hotkey[Id: 3, Alt+Ctrl+X]
CTRL+ALT+X pressed, goodbye...

It can be done. For example gaming engine Azul3d can do that https://github.com/azul3d/engine/tree/master/keyboard. Your OSX snippet doesn't compile because AppDelegate.h file is missing. You can either write it by hand or first compile project with XCode which do bookkeeping for you and then look at AppDelegate.h and AppDelegate.m. On linux to log keystroke stream you can just read /dev/input/event$ID file which is done here https://github.com/MarinX/keylogger. On windows you will need some syscalls like in this code https://github.com/eiannone/keyboard. Both on nix and win you even need not cgo and can be happy with syscalls and pure Go.

icza's answer is awesome and was of great help to me, but I found that using the PeekMessageW API call to be more considerably expensive than the GetMessageW API call and less reliable.

As per the Microsoft documentation:

Unlike GetMessage, the PeekMessage function does not wait for a message to be posted before returning.

From my limited understanding of Windows internals, that means that PeekMessageW is continually checking the message queue and returning "0" until an event is found, resulting in unnecessary compute.

Simply changing the code to:

getmsg := user32.MustFindProc("GetMessageW")

for {
    var msg = &MSG{}
    getmsg.Call(uintptr(unsafe.Pointer(msg)), 0, 0, 0)

Fixed this for me. That means you can also get rid of the timeout without a notable performance hit:

time.Sleep(time.Millisecond * 50)

I do wonder if the reliability issues comes from the fact that I'm running the hotkey loop in a goroutine. I'm not experienced enough with Go or the Windows API to know but hope this helps someone! I also haven't experienced any sort of memory leak with this change, though I cannot verify if a memory leak existed with the original code as Programming4life suggests.