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# Real-time

Audio callbacks, control loops, and frame hot paths cannot afford a hidden allocation or a lock. C+ lets you mark such a function and have the compiler **prove** the property holds across the entire transitive call graph. A violation is a compile error, not a lint you can ignore.

The near-term target is **soft real-time**: audio callbacks, game frame loops, robotics control loops, market-data hot paths, and embedded-style firmware on a normal OS. Hard real-time needs scheduler, interrupt, and hardware guarantees that live below the language.

## The `#[realtime]` bundle

`#[realtime]` is sugar for a bundle of smaller contracts:

- `#[no_alloc]` — no heap allocation
- `#[no_block]` — no blocking call or lock wait
- `#[bounded_recursion]` — no unbounded recursion

Bounded stack usage is checked alongside it. Each smaller attribute is available on its own, because many code paths need only one part of the contract.

```cplus
#[realtime]
fn process_frame(input: f32x4[], output: f32x4[]) {
    let n: usize = output.len();
    let mut i: usize = 0 as usize;
    while i < n {
        output[i] = input[i] * GAIN;   // pure SIMD math
        i = i +% (1 as usize);
    }
    // a Vec::new, lock, channel recv, sleep, or unknown extern here
    // would not compile (E0901 / E0907)
    return;
}
```

## `#[no_alloc]` — proven allocation-free (E0901)

A `#[no_alloc]` function rejects any path that reaches the heap. Direct `malloc`, `calloc`, `realloc`, `aligned_alloc`, and `free` reject, as do calls through `#[link_name = "malloc"]`. The check is resolved from sema information, not text, so it also catches allocation through a method call, through string interpolation (`"...${x}..."`), and through the `Vec` / `HashMap` / `Box` / `Arc` / `string` constructors. The blessed `to_string()` is rejected at its call site because it allocates.

The contract is **transitive**: a function called from a `#[no_alloc]` body must itself be `#[no_alloc]`, and an unknown extern is treated as "not proven" and rejected. The escape hatch is to vouch for a known-safe extern explicitly:

```cplus
#[no_alloc]
extern fn sinf(x: f32) -> f32;
```

The check reaches **implicit drop-glue** too. If a `#[no_alloc]` function lets a value drop at scope exit and that destructor would allocate or free — a `string` / `Vec` / `Box` local, or a type whose `drop` is not itself `#[no_alloc]` — it is rejected (**E0901**), reaching through struct fields, enum payloads, and array elements. So a hidden free at the closing brace is caught the same way a direct `free` call is.

## `#[no_block]` — proven non-blocking (E0907)

A `#[no_block]` function rejects mutex locks, condition-variable waits, thread joins, sleep and timer waits, blocking file and socket I/O, unbounded channel `recv`, and unknown externs. It allows plain arithmetic, stack memory, atomics, `atomic_thread_fence`, `#cpu_relax`, and nonblocking try-style APIs. Like `#[no_alloc]`, it composes transitively.

## `#[max_stack(N)]` — bounded frame size (E0908)

```cplus
#[max_stack(4096)]
fn callback(...) { ... }
```

The checker estimates the frame from the parameters and typed locals across all nested blocks, using ABI-accurate sizes for primitives, pointers, arrays, structs, enums, and SIMD types, and reports **E0908** when the estimate exceeds `N`. Large local arrays and by-value temporaries become visible in the diagnostic.

## Project-wide: `[profile.realtime]`

Rather than annotate every function, a project can opt in globally from its manifest:

```toml
[profile.realtime]
deny_alloc = true
deny_block = true
deny_unknown_extern = true
stack_limit = 4096
```

When present, the driver synthesizes the matching contracts onto every function defined in **this** package (dependencies are exempt), and the same `E0901` / `E0907` / `E0908` diagnostics do the enforcement. `cpc check` runs the whole-project front-end gate without codegen, the fast CI check, and `cpc check --diagnostics=json` emits one machine-readable diagnostic per line for editor and CI tooling.

## Threads: `Send` and `Sync` (E0502)

The marker types are enforced structurally. `Rc[T]` is `!Send` and `!Sync`, and `MutexGuard[T]` is `!Send`, so handing one to a `Send`-bounded site such as `thread::spawn` is **E0502**. `Arc[T]` stays `Send + Sync` as the thread-safe sibling.

A nominal type that transitively hides a raw pointer is `!Send` and `!Sync` by default, and the same `!Send` propagates structurally through a struct that holds an `Rc`. When a type is in fact safe to move or share, opt back in with a manual `unsafe impl Send` / `unsafe impl Sync` — see [Threads & atomics](/docs/threads) for the marker syntax and its conditional, generic form.

## Building blocks

The contracts say what a hot path may not do; these packages give it allocation-free, lock-free tools to work with:

- [rt](/docs/packages/rt) — a lock-free SPSC ring (`SpscRingU64`) and a fixed-capacity object pool (`FixedPoolU64`). Every method is `#[no_alloc]` and `#[no_block]`, so they are callable from inside a `#[realtime]` body.
- [static-arena](/docs/packages/static-arena) — a fixed-size, stack-resident arena with zero `malloc` and zero `free`.
- `rt_darwin` (see the [rt page](/docs/packages/rt)) — the macOS platform controls: a monotonic clock, audio-QoS thread priority, and page locking, each returning `Result`. `rt_linux` and `rt_posix` will mirror it.