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# Threads and atomics

```cplus
import "stdlib/thread" as thread;

fn worker() -> i32 { return 42; }

fn main() -> i32 {
    let h: thread::JoinHandle[i32] = thread::spawn::[i32](worker);
    return h.join();
}
```

For runnable proof recipes, see [Threads and atomics](/examples/threads-atomics).
It documents the checked `parallel_sum` partition-and-join recipe and the
`concurrent_counter` atomic shared-counter recipe from the C+ source tree.

## Passing data into the worker

For non-Copy input, use `spawn_with`, which moves the value into the thread:

```cplus
import "stdlib/text" as text;

fn proc(move s: text::Text) -> i32 { return s.len() as i32; }

let s = "hello".to_text();   // an owned Text
let h = thread::spawn_with::[text::Text, i32](s, proc);
let n = h.join();
```

`Text` is `Send`, so it is a valid `spawn_with` payload.

## The safe pattern: partition and join

This is the first pattern to reach for. Race-freedom is mechanical, because there is no shared memory:

```cplus
struct Range { start: i64, end: i64 }

fn sum_range(r: Range) -> i64 {
    let mut total: i64 = 0 as i64;
    let mut i: i64 = r.start;
    while i < r.end { total = total +% i; i = i +% (1 as i64); }
    return total;
}

let h1 = thread::spawn_with::[Range, i64](left,  sum_range);
let h2 = thread::spawn_with::[Range, i64](right, sum_range);
let total: i64 = h1.join() +% h2.join();
```

Note that `Rc[T]` is `!Send` and is rejected at `spawn`; share across threads with `Arc[T]`. See [Real-time](/docs/realtime) for the `Send` / `Sync` rules (**E0502**).

## `Send` and `Sync`

`Send` means a value can move to another thread; `Sync` means it can be shared across threads. Both are enforced structurally. A nominal type that transitively hides a raw pointer is `!Send` and `!Sync` by default, so moving or sharing it across a `Send` / `Sync` bound (such as `thread::spawn`) is rejected with **E0502**. A bare `*T` used directly stays `Send`; the default applies to types that *wrap* one.

When you have audited a type and know it is safe to send or share, vouch for it with a manual marker. The impl must carry `unsafe`, because the compiler is taking your word for it:

```cplus
struct Handle { opaque ptr: *u8 }

unsafe impl Send for Handle {}
unsafe impl Sync for Handle {}
```

The conditional, generic form carries its requirement as bounds, so the marker holds only when the element type also qualifies:

```cplus
unsafe impl Send for Arc[T: Send + Sync] {}
```

`Arc`, `Mutex`, and `Channel` already carry the right conditional impls, so you rarely write these by hand outside of FFI wrappers. A `Send` or `Sync` impl that omits `unsafe` is **E0860**; `unsafe` on any other impl is **E0861**.

## Atomics

For the rare cases that cannot partition:

```cplus
import "stdlib/atomic" as atomic;

let p: *u64 = unsafe { ... };       // pointer to a shared u64
unsafe {
    atomic::atomic_fetch_add_u64(p, 1 as u64, atomic::Ordering::Relaxed);
}
```

Ordering values: `Relaxed`, `Acquire`, `Release`, `AcqRel`, `SeqCst`. Widths: `i32` / `i64` / `u32` / `u64`.

## Mutex

The mutex is internally refcounted, so a `clone` shares it across threads (C+ has no `&T` to make `Arc[Mutex[T]]` work):

```cplus
import "stdlib/mutex" as mutex;

let m = mutex::new::[i32](10);
let m2 = m.clone();              // share across threads
{
    let mut g = m.lock();
    g.set(g.get() +% 1);
}                                 // the guard's Drop releases
```

Two guards in the same scope **deadlock**: the borrow checker does not yet prevent this, so use block scopes to bound each guard's lifetime.