|
@@ -0,0 +1,54 @@
|
|
|
+/// Macro to create a mutable reference to a statically allocated value
|
|
|
+///
|
|
|
+/// This macro returns a value with type `Option<&'static mut $ty>`. `Some($expr)` will be returned
|
|
|
+/// the first time the macro is executed; further calls will return `None`. To avoid `unwrap`ping a
|
|
|
+/// `None` variant the caller must ensure that the macro is called from a function that's executed
|
|
|
+/// at most once in the whole lifetime of the program.
|
|
|
+///
|
|
|
+/// # Note
|
|
|
+/// this macro is unsound on multi-core systems
|
|
|
+///
|
|
|
+/// # Example
|
|
|
+///
|
|
|
+/// ``` no_run
|
|
|
+/// use riscv::singleton;
|
|
|
+///
|
|
|
+/// fn main() {
|
|
|
+/// // OK if `main` is executed only once
|
|
|
+/// let x: &'static mut bool = singleton!(: bool = false).unwrap();
|
|
|
+///
|
|
|
+/// let y = alias();
|
|
|
+/// // BAD this second call to `alias` will definitively `panic!`
|
|
|
+/// let y_alias = alias();
|
|
|
+/// }
|
|
|
+///
|
|
|
+/// fn alias() -> &'static mut bool {
|
|
|
+/// singleton!(: bool = false).unwrap()
|
|
|
+/// }
|
|
|
+/// ```
|
|
|
+#[macro_export]
|
|
|
+macro_rules! singleton {
|
|
|
+ (: $ty:ty = $expr:expr) => {
|
|
|
+ $crate::interrupt::free(|_| {
|
|
|
+ static mut VAR: Option<$ty> = None;
|
|
|
+
|
|
|
+ #[allow(unsafe_code)]
|
|
|
+ let used = unsafe { VAR.is_some() };
|
|
|
+ if used {
|
|
|
+ None
|
|
|
+ } else {
|
|
|
+ let expr = $expr;
|
|
|
+
|
|
|
+ #[allow(unsafe_code)]
|
|
|
+ unsafe {
|
|
|
+ VAR = Some(expr)
|
|
|
+ }
|
|
|
+
|
|
|
+ #[allow(unsafe_code)]
|
|
|
+ unsafe {
|
|
|
+ VAR.as_mut()
|
|
|
+ }
|
|
|
+ }
|
|
|
+ })
|
|
|
+ };
|
|
|
+}
|