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macros.rs
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macros.rs
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/// 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 requires a `critical-section` implementation to be set. For most single-hart systems,
/// you can enable the `critical-section-single-hart` feature for this crate. For other systems, you
/// have to provide one from elsewhere, typically your chip's HAL crate.
///
/// # 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) => {
$crate::_export::critical_section::with(|_| {
static mut VAR: Option<$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()
}
}
})
};
}