book: indirection for faster message passing

examples/pool.rs

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+//! examples/pool.rs
+
+#![deny(unsafe_code)]
+#![deny(warnings)]
+#![no_main]
+#![no_std]
+
+extern crate panic_semihosting;
+
+use cortex_m_semihosting::{debug, hprintln};
+use heapless::{
+    pool,
+    pool::singleton::{Box, Pool},
+};
+use lm3s6965::Interrupt;
+use rtfm::app;
+
+// Declare a pool of 128-byte memory blocks
+pool!(P: [u8; 128]);
+
+#[app(device = lm3s6965)]
+const APP: () = {
+    #[init]
+    fn init(_: init::Context) {
+        static mut MEMORY: [u8; 512] = [0; 512];
+
+        // Increase the capacity of the memory pool by ~4
+        P::grow(MEMORY);
+
+        rtfm::pend(Interrupt::I2C0);
+    }
+
+    #[interrupt(priority = 2, spawn = [foo, bar])]
+    fn I2C0(c: I2C0::Context) {
+        // claim a memory block, leave it uninitialized and ..
+        let x = P::alloc().unwrap().freeze();
+
+        // .. send it to the `foo` task
+        c.spawn.foo(x).ok().unwrap();
+
+        // send another block to the task `bar`
+        c.spawn.bar(P::alloc().unwrap().freeze()).ok().unwrap();
+    }
+
+    #[task]
+    fn foo(_: foo::Context, x: Box<P>) {
+        hprintln!("foo({:?})", x.as_ptr()).unwrap();
+
+        // explicitly return the block to the pool
+        drop(x);
+
+        debug::exit(debug::EXIT_SUCCESS);
+    }
+
+    #[task(priority = 2)]
+    fn bar(_: bar::Context, x: Box<P>) {
+        hprintln!("bar({:?})", x.as_ptr()).unwrap();
+
+        // this is done automatically so we can omit the call to `drop`
+        // drop(x);
+    }
+
+    extern "C" {
+        fn UART0();
+        fn UART1();
+    }
+};