# How OpenLogi Handles Input Hooking Across Windows, macOS, and Linux

> Discover how OpenLogi achieves cross-platform input hooking on Windows, macOS, and Linux. Learn about its OS-specific low-level hooks and unified Backend API.

- Repository: [Xuan Zhang/OpenLogi](https://github.com/AprilNEA/OpenLogi)
- Tags: how-to-guide
- Published: 2026-09-10

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**OpenLogi implements OS-specific low-level input hooks in the `openlogi-hook` crate, using `SetWindowsHookExW` on Windows, `CGEventTap` on macOS, and `evdev`/`uinput` on Linux, unified behind a platform-agnostic `Backend` API.**

OpenLogi is an open-source input management framework that abstracts the complexities of cross-platform input hooking. The `openlogi-hook` crate contains the platform-specific implementations that capture mouse and keyboard events while exposing a common Rust API through conditional compilation via `cfg(target_os = "...")` directives.

## Windows Input Hooking Implementation

### Low-Level System Hooks

On Windows, OpenLogi utilizes `SetWindowsHookExW` with **low-level hooks** (`WH_MOUSE_LL` and `WH_KEYBOARD_LL`) to capture system-wide input events. The implementation in [`crates/openlogi-hook/src/windows/hook.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/crates/openlogi-hook/src/windows/hook.rs) spawns a dedicated thread that runs a standard Win32 message loop, ensuring the hooks operate in a single-threaded environment as required by the Windows API.

The hook installation follows this pattern: the dedicated thread calls `SetWindowsHookExW`, forwards captured events to the consumer callback, and ensures clean teardown via `UnhookWindowsHookEx` when the `Backend` handle is dropped. This approach prevents the application from requiring elevated privileges while still capturing global input.

### Foreground Application Monitoring

Beyond raw input capture, the Windows module tracks the currently active application using a **`WinEventHook`** foreground observer. Located in [`crates/openlogi-hook/src/windows/foreground.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/crates/openlogi-hook/src/windows/foreground.rs), this component monitors window focus changes through the `EVENT_SYSTEM_FOREGROUND` event, allowing OpenLogi to contextually adjust input handling based on which application currently has focus.

## macOS Input Hooking Implementation

### CoreGraphics Event Taps

The macOS implementation in [`crates/openlogi-hook/src/macos.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/crates/openlogi-hook/src/macos.rs) leverages **`CGEventTap`**, the same CoreGraphics primitive used by commercial software like Logitech Options+. This creates a tap on the HID event stream that intercepts system input events before they reach applications.

The implementation creates the event tap via `CGEventTapCreate`, runs it on a private `CFRunLoop` thread to avoid blocking the main thread, and translates native `CGEvent` objects into the unified OpenLogi event format. When `CGEventTapCreate` returns `null`, the error indicates missing **Accessibility permissions**, which users must grant in Security & Privacy settings.

### Tap Health Monitoring

macOS automatically disables event taps when the application lacks focus or exhibits performance issues. To handle this, [`crates/openlogi-hook/src/macos/watchdog.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/crates/openlogi-hook/src/macos/watchdog.rs) implements a watchdog thread that monitors the tap's health and automatically re-arms the tap when the OS disables it. This ensures continuous input capture even when the system attempts to suspend the tap.

## Linux Input Hooking Implementation

### evdev Event Capture

On Linux, OpenLogi takes a direct approach by reading from **`/dev/input/event*`** devices using the `evdev` protocol. The implementation in [`crates/openlogi-hook/src/linux.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/crates/openlogi-hook/src/linux.rs) opens physical input devices with appropriate permissions and reads raw kernel input events through the `evdev` crate.

### uinput Injection and Loop Prevention

For output functionality, the Linux backend creates a synthetic device via **`uinput`**. The implementation uses a specific device name prefix (`"openlogi-"`) when creating the virtual input device. The enumeration logic explicitly ignores any devices carrying this prefix when scanning for input sources, preventing recursive input loops where OpenLogi would capture its own injected events.

## Platform-Agnostic API Usage

All three implementations expose a common interface through `openlogi_hook::Backend`, allowing dependent crates (agent, GUI, overlay) to consume input events without platform-specific code. The [`lib.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/lib.rs) file documents this public interface, while platform modules compile conditionally based on the target OS.

The following example demonstrates installing a hook that works identically across all supported platforms:

```rust
use openlogi_hook::{Backend, HookError};

fn main() -> Result<(), HookError> {
    let callback = |event| {
        println!("Received event: {:?}", event);
    };

    // Automatically selects the correct OS implementation
    let hook = Backend::new(callback)?;
    
    // Hook runs in background; keep handle alive
    std::thread::sleep(std::time::Duration::from_secs(10));
    Ok(())
}

```

OpenLogi also provides example programs in `crates/openlogi-hook/examples/` for debugging and development:

```rust
// Print raw hook events
// Usage: cargo run --example print_events
fn main() {
    openlogi_hook::print_events::run();
}

```

```rust
// List active input taps
// Usage: cargo run --example list_taps
fn main() {
    openlogi_hook::list_taps::run();
}

```

## Summary

- **Windows** uses `SetWindowsHookExW` with low-level mouse and keyboard hooks in a dedicated message-loop thread, plus `WinEventHook` for foreground tracking in [`foreground.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/foreground.rs).
- **macOS** relies on `CGEventTap` running on a private `CFRunLoop`, with a watchdog thread in [`watchdog.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/watchdog.rs) to handle OS-enforced tap disabling and re-arming.
- **Linux** implements raw `evdev` reading from `/dev/input/event*` and `uinput` injection with a `"openlogi-"` prefix to prevent recursive event loops.
- All platforms unify behind the `Backend` API in [`lib.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/lib.rs), enabling the rest of the OpenLogi ecosystem to remain platform-agnostic.

## Frequently Asked Questions

### Does OpenLogi require elevated privileges or root access to capture input?

On Windows and macOS, OpenLogi does not require administrator privileges for basic input hooking, though macOS requires granting Accessibility permissions in Security & Privacy settings. On Linux, reading from `/dev/input/event*` devices typically requires membership in the `input` group or root privileges, depending on system udev rules.

### How does OpenLogi prevent capturing its own injected events on Linux?

The Linux implementation in [`linux.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/linux.rs) filters devices by name, specifically ignoring any input devices prefixed with `"openlogi-"`. Since synthetic events injected via `uinput` use this prefix, the hook logic skips these devices during enumeration, effectively breaking potential feedback loops.

### What happens if macOS disables the CGEventTap during operation?

According to the source code in [`macos/watchdog.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/macos/watchdog.rs), a dedicated watchdog thread monitors the event tap's state. If macOS disables the tap—commonly due to timeout or security policies—the watchdog detects the failure and re-arms the tap by recreating it, ensuring continuous operation without user intervention.

### Can I use the hook functionality without the rest of the OpenLogi application?

Yes. The `openlogi-hook` crate is designed as a standalone library with no dependencies on the OpenLogi agent or GUI. You can import the crate directly and use `Backend::new()` to receive platform-agnostic input events in any Rust application, as demonstrated in the [`print_events.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/print_events.rs) and [`list_taps.rs`](https://github.com/AprilNEA/OpenLogi/blob/main/list_taps.rs) examples.