The critical section created by the `lock` API is based on dynamic priorities: it temporarily raises the dynamic priority of the context to a *ceiling* priority that prevents other tasks from preempting the critical section. This synchronization protocol is known as the [Immediate Ceiling Priority Protocol
(ICPP)][icpp], and complies with [Stack Resource Policy(SRP)](srp) based scheduling of RTIC.
In the ex ample below we have three interrupt handlers with priorities ranging from one to three. The two handlers with the lower priorities contend for the `shared` resource and need to lock the resource for accessing the data. The highest priority handler, which do nat access the `shared` resource, is free to preempt the critical section created by the
Late resources are resources that are not given an initial value at compile time using the `#[init]` attribute but instead are initialized at runtime using the `init::LateResources` values returned by the `init` function.
The example below uses late resources to establish a lockless, one-way channel between the `UART0` interrupt handler and the `idle` task. A single producer single consumer [`Queue`] is used as the channel. The queue is split into consumer and producer end points in `init` and then each end point is stored in a different resource; `UART0` owns the producer resource and `idle` owns the consumer resource.
By default the framework assumes that all tasks require exclusive access (`&mut-`) to resources but it is possible to specify that a task only requires shared access (`&-`) to a resource using the `&resource_name` syntax in the `resources` list.
The advantage of specifying shared access (`&-`) to a resource is that no locks are required to access the resource even if the resource is contended by several tasks running at different priorities. The downside is that the task only gets a shared reference (`&-`) to the resource, limiting the operations it can perform on it, but where a shared reference is enough this approach reduces the number of required locks. In addition to simple immutable data, this shared access can be useful where the resource type safely implements interior mutability, with
Note that in this release of RTIC it is not possible to request both exclusive access (`&mut-`) and shared access (`&-`) to the *same* resource from different tasks. Attempting to do so will result in a compile error.
In the example below a key (e.g. a cryptographic key) is loaded (or created) at runtime and then used from two tasks that run at different priorities without any kind of lock.
