# Thread

Registered with `register_addon_thread(engine)`.

Three addon types for thread synchronization: `mutex`, `lock_guard` (RAII over a mutex), and `cond_var`. `mutex` is backed by `std::shared_mutex`, so the same handle supports both exclusive (writer) and shared (reader) locks. `cond_var` is a thin wrapper around `std::condition_variable_any`.

## mutex

```cpp
mutex m;
m.lock();
// critical section (exclusive)
m.unlock();

bool got = m.try_lock();          // non-blocking exclusive
if (got) m.unlock();

// Reader-writer usage:
m.lock_shared();                  // multiple shared holders allowed
// read-only critical section
m.unlock_shared();
```

### Methods

```cpp
void m.lock()             // exclusive (writer)
void m.unlock()
bool m.try_lock()

void m.lock_shared()      // shared (reader); concurrent shared holders allowed
void m.unlock_shared()
bool m.try_lock_shared()
```

Use `lock` / `unlock` for plain mutual exclusion; the shared variants when multiple readers can safely run together but writers need to be alone (`std::shared_mutex` semantics).

## lock\_guard

RAII wrapper: constructor locks, destructor unlocks. Copies are rejected at runtime because two guards can't both own the same lock.

```cpp
mutex m;
{
    lock_guard g = lock_guard(m);
    // m is held here
}   // scope exit -> dtor -> m is released
```

Attempting to copy raises a runtime error:

```cpp
lock_guard a = lock_guard(m);
lock_guard b = a;       // runtime: "lock_guard is non-copyable"
```

## cond\_var

`cond_var` is `std::condition_variable_any`, which waits on any mutex. The mutex must already be held by the caller when `wait` is invoked; it's released during the wait and reacquired before returning.

```cpp
mutex m;
cond_var cv;

// Consumer:
m.lock();
while (!queue_has_items()) {
    cv.wait(cast<int64>(/* mutex handle */));
}
// drain queue...
m.unlock();

// Producer:
m.lock();
queue_push(x);
cv.notify_one();    // or notify_all()
m.unlock();
```

### Methods

```cpp
void cv.wait(int64 mutex_handle)
void cv.notify_one()
void cv.notify_all()
```

## Free helpers

```cpp
sleep_us(1000)              // sleep for N microseconds
yield_cpu()                 // hint scheduler to yield this quantum
int64 n = hardware_threads() // platform's reported core count
```

## Cross-thread usage

For a real multi-thread scenario, the host spawns threads from native code and shares the mutex handle across threads. Mutex/cond\_var handles can be passed across threads safely.


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