# Singleton LiteBox Instance: How to Enforce Single-Instance Constraints in Rust

> Enforce single instance constraints in Rust with the optional singleton LiteBox instance compile-time flag. Learn how to prevent multiple LiteBox object creations.

- Repository: [Microsoft/litebox](https://github.com/microsoft/litebox)
- Tags: internals
- Published: 2026-02-16

---

**The singleton LiteBox instance feature is an optional compile-time flag that restricts a process to creating exactly one `LiteBox` object by panicking on subsequent construction attempts.**

In the `microsoft/litebox` repository, this feature provides a safety mechanism for applications that require a global, unique synchronization primitive provider. When enabled, the `enforce_singleton_litebox_instance` Cargo feature injects an atomic check into the `LiteBox::new` constructor, ensuring that only the first instantiation succeeds.

## What Is the Singleton LiteBox Instance Feature?

The **singleton LiteBox instance** feature is a conditional compilation option defined in [`litebox/Cargo.toml`](https://github.com/microsoft/litebox/blob/main/litebox/Cargo.toml). It addresses scenarios where multiple `LiteBox` instances could cause logical errors, such as when subsystems expect a single global source of truth for lock-tracing or descriptor tables.

By default, this feature is **disabled**, allowing developers to create multiple independent `LiteBox` objects. Each instance maintains its own `Arc<LiteBoxX>` internal state, making them safe for concurrent use in multi-instance configurations.

## How the Singleton Enforcement Works

When the `enforce_singleton_litebox_instance` feature is active, the `LiteBox::new` method in [`litebox/src/litebox.rs`](https://github.com/microsoft/litebox/blob/main/litebox/src/litebox.rs) executes a runtime atomic check before completing construction.

### The AtomicBool Check in LiteBox::new

The implementation uses a static `AtomicBool` named `LITEBOX_SINGLETON_INITIALIZED`. The constructor performs a `fetch_or` operation with `Ordering::SeqCst` to atomically set the flag and retrieve the previous value:

```rust
#[cfg(feature = "enforce_singleton_litebox_instance")]
{
    static LITEBOX_SINGLETON_INITIALIZED: core::sync::atomic::AtomicBool =
        core::sync::atomic::AtomicBool::new(false);

    let previously_initialized =
        LITEBOX_SINGLETON_INITIALIZED.fetch_or(true, core::sync::atomic::Ordering::SeqCst);
    assert!(
        !previously_initialized,
        "In this configuration, there should be only one LiteBox instance ever made.  Failing to make second instance.",
    );
}

```

If `previously_initialized` is `true`, the `assert!` macro triggers a panic, preventing the second instantiation.

### Feature Flag Configuration

The feature is declared in [`litebox/Cargo.toml`](https://github.com/microsoft/litebox/blob/main/litebox/Cargo.toml) as an empty feature flag, meaning it enables conditional compilation without additional dependencies:

```toml
[features]
enforce_singleton_litebox_instance = []

```

## When to Use the Singleton LiteBox Instance

Enable the **singleton LiteBox instance** feature when your architecture assumes a single global synchronization provider. Common use cases include:

- **Global lock-tracing systems** that require a unified instrumentation point across the entire process.
- **Descriptor table management** where multiple instances could lead to resource leaks or inconsistent state.
- **Shim layers** that intercept system calls and expect a single `LiteBox` to coordinate primitive creation.

If your application requires isolated synchronization domains or testing with multiple configurations, leave the feature disabled to permit multiple instances.

## Code Examples

### Enabling the Feature

Add the feature to your dependency declaration in [`Cargo.toml`](https://github.com/microsoft/litebox/blob/main/Cargo.toml):

```toml
[dependencies]
litebox = { path = "../litebox", features = ["enforce_singleton_litebox_instance"] }

```

### Creating the First Instance

The first call to `LiteBox::new` succeeds and initializes the singleton flag:

```rust
use litebox::LiteBox;
use my_platform::MyPlatform;

static PLATFORM: MyPlatform = MyPlatform::new();

let box1 = LiteBox::new(&PLATFORM); // ✅ Succeeds

```

### Attempting a Second Instance (Panic)

Any subsequent construction attempt triggers the assertion and panics:

```rust
let box2 = LiteBox::new(&PLATFORM); // ❌ Panics with:
// "In this configuration, there should be only one LiteBox instance ever made..."

```

### Multiple Instances Without the Feature

With the default configuration, you can create independent instances:

```toml
[dependencies]
litebox = { path = "../litebox" } # No singleton feature

```

```rust
let box1 = LiteBox::new(&PLATFORM);
let box2 = LiteBox::new(&PLATFORM); // ✅ Both succeed

```

## Summary

- The **singleton LiteBox instance** feature restricts a process to exactly one `LiteBox` object via the `enforce_singleton_litebox_instance` Cargo flag.
- When enabled, `LiteBox::new` in [`litebox/src/litebox.rs`](https://github.com/microsoft/litebox/blob/main/litebox/src/litebox.rs) uses a static `AtomicBool` to detect and panic on duplicate construction attempts.
- The feature is **disabled by default**, allowing multiple independent instances for applications requiring isolated synchronization domains.
- Enable this feature only when subsystems require a global, unique `LiteBox` for coordination, such as in lock-tracing or descriptor table management.

## Frequently Asked Questions

### What happens if I try to create a second LiteBox with the singleton feature enabled?

The application will **panic** at runtime. The `LiteBox::new` constructor checks a static `AtomicBool` flag, and if it finds that an instance already exists, it triggers an `assert!` with the message: *"In this configuration, there should be only one LiteBox instance ever made."*

### Is the singleton LiteBox instance feature enabled by default?

**No.** The `enforce_singleton_litebox_instance` feature is **disabled by default** in [`litebox/Cargo.toml`](https://github.com/microsoft/litebox/blob/main/litebox/Cargo.toml). You must explicitly enable it in your dependency declaration to activate the singleton enforcement behavior.

### Can I disable the singleton check after enabling it?

**No.** The singleton check is a **compile-time configuration**. Once you enable the `enforce_singleton_litebox_instance` feature and compile your project, the atomic check is baked into the `LiteBox::new` method. To allow multiple instances, you must recompile without the feature flag.

### Why would I need only one LiteBox instance?

Certain architectures require a **global synchronization provider** to maintain consistency across subsystems. Common scenarios include **lock-tracing instrumentation** that must see all synchronization operations in one place, **global descriptor tables** that would leak resources if split across instances, and **shim layers** that intercept system calls and expect a single coordination point.