# lib.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/volatile-0.3.0/src/lib.rs`.
Keywords: rust, rustlang, rust-lang

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    #![cfg_attr(feature = "const_fn", feature(const_fn))]

    //! Provides wrapper types `Volatile`, `ReadOnly`, `WriteOnly`, `ReadWrite`, which wrap any copy-able type and allows for
    //! volatile memory access to wrapped value. Volatile memory accesses are never optimized away by
    //! the compiler, and are useful in many low-level systems programming and concurrent contexts.
    //!
    //! The wrapper types *do not* enforce any atomicity guarantees; to also get atomicity, consider
    //! looking at the `Atomic` wrapper type found in `libcore` or `libstd`.
    //!
    //! These wrappers do not depend on the standard library and never panic.
    //!
    //! # Dealing with Volatile Pointers
    //!
    //! Frequently, one may have to deal with volatile pointers, eg, writes to specific memory
    //! locations. The canonical way to solve this is to cast the pointer to a volatile wrapper
    //! directly, eg:
    //!
    //! ```rust
    //! use volatile::Volatile;
    //!
    //! let mut_ptr = 0xFEE00000 as *mut u32;
    //!
    //! let volatile_ptr = mut_ptr as *mut Volatile<u32>;
    //! ```
    //!
    //! and then perform operations on the pointer as usual in a volatile way. This method works as all
    //! of the volatile wrapper types are the same size as their contained values.
    #![no_std]

    use core::ptr;

    /// A wrapper type around a volatile variable, which allows for volatile reads and writes
    /// to the contained value. The stored type needs to be `Copy`, as volatile reads and writes
    /// take and return copies of the value.
    ///
    /// The size of this struct is the same as the size of the contained type.
    #[derive(Debug, Default)]
    #[repr(transparent)]
    pub struct Volatile<T: Copy>(T);

    impl<T: Copy> Volatile<T> {
        /// Construct a new volatile instance wrapping the given value.
        ///
        /// ```rust
        /// use volatile::Volatile;
        ///
        /// let value = Volatile::new(0u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This method never panics.
        #[cfg(feature = "const_fn")]
        pub const fn new(value: T) -> Volatile<T> {
            Volatile(value)
        }

        /// Construct a new volatile instance wrapping the given value.
        ///
        /// ```rust
        /// use volatile::Volatile;
        ///
        /// let value = Volatile::new(0u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This method never panics.
        #[cfg(not(feature = "const_fn"))]
        pub fn new(value: T) -> Volatile<T> {
            Volatile(value)
        }

        /// Performs a volatile read of the contained value, returning a copy
        /// of the read value. Volatile reads are guaranteed not to be optimized
        /// away by the compiler, but by themselves do not have atomic ordering
        /// guarantees. To also get atomicity, consider looking at the `Atomic` wrapper type.
        ///
        /// ```rust
        /// use volatile::Volatile;
        ///
        /// let value = Volatile::new(42u32);
        ///
        /// assert_eq!(value.read(), 42u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This method never panics.
        pub fn read(&self) -> T {
            // UNSAFE: Safe, as we know that our internal value exists.
            unsafe { ptr::read_volatile(&self.0) }
        }

        /// Performs a volatile write, setting the contained value to the given value `value`. Volatile
        /// writes are guaranteed to not be optimized away by the compiler, but by themselves do not
        /// have atomic ordering guarantees. To also get atomicity, consider looking at the `Atomic`
        /// wrapper type.
        ///
        /// ```rust
        /// use volatile::Volatile;
        ///
        /// let mut value = Volatile::new(0u32);
        ///
        /// value.write(42u32);
        ///
        /// assert_eq!(value.read(), 42u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This method never panics.
        pub fn write(&mut self, value: T) {
            // UNSAFE: Safe, as we know that our internal value exists.
            unsafe { ptr::write_volatile(&mut self.0, value) };
        }

        /// Performs a volatile read of the contained value, passes a mutable reference to it to the
        /// function `f`, and then performs a volatile write of the (potentially updated) value back to
        /// the contained value.
        ///
        /// ```rust
        /// use volatile::Volatile;
        ///
        /// let mut value = Volatile::new(21u32);
        ///
        /// value.update(|val_ref| *val_ref *= 2);
        ///
        /// assert_eq!(value.read(), 42u32);
        /// ```
        ///
        /// # Panics
        ///
        /// Ths method never panics.
        pub fn update<F>(&mut self, f: F)
        where
            F: FnOnce(&mut T),
        {
            let mut value = self.read();
            f(&mut value);
            self.write(value);
        }
    }

    impl<T: Copy> Clone for Volatile<T> {
        fn clone(&self) -> Self {
            Volatile(self.read())
        }
    }

    /// A volatile wrapper which only allows read operations.
    ///
    /// The size of this struct is the same as the contained type.
    #[derive(Debug, Clone, Default)]
    pub struct ReadOnly<T: Copy>(Volatile<T>);

    impl<T: Copy> ReadOnly<T> {
        /// Construct a new read-only volatile wrapper wrapping the given value.
        ///
        /// ```rust
        /// use volatile::ReadOnly;
        ///
        /// let value = ReadOnly::new(42u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This function never panics.
        #[cfg(feature = "const_fn")]
        pub const fn new(value: T) -> ReadOnly<T> {
            ReadOnly(Volatile::new(value))
        }

        /// Construct a new read-only volatile wrapper wrapping the given value.
        ///
        /// ```rust
        /// use volatile::ReadOnly;
        ///
        /// let value = ReadOnly::new(42u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This function never panics.
        #[cfg(not(feature = "const_fn"))]
        pub fn new(value: T) -> ReadOnly<T> {
            ReadOnly(Volatile::new(value))
        }

        /// Perform a volatile read of the contained value, returning a copy of the read value.
        /// Functionally equivalent to `Volatile::read`.
        ///
        /// ```rust
        /// use volatile::ReadOnly;
        ///
        /// let value = ReadOnly::new(42u32);
        /// assert_eq!(value.read(), 42u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This function never panics.
        pub fn read(&self) -> T {
            self.0.read()
        }
    }

    /// A volatile wrapper which only allows write operations.
    ///
    /// The size of this struct is the same as the contained type.
    #[derive(Default)]
    pub struct WriteOnly<T: Copy>(Volatile<T>);

    impl<T: Copy> WriteOnly<T> {
        /// Constructs a new write only volatile wrapper around the given value.
        ///
        /// ```rust
        /// use volatile::WriteOnly;
        ///
        /// let value = WriteOnly::new(0u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This function never panics.
        #[cfg(feature = "const_fn")]
        pub const fn new(value: T) -> WriteOnly<T> {
            WriteOnly(Volatile::new(value))
        }

        /// Constructs a new write only volatile wrapper around the given value.
        ///
        /// ```rust
        /// use volatile::WriteOnly;
        ///
        /// let value = WriteOnly::new(0u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This function never panics.
        #[cfg(not(feature = "const_fn"))]
        pub fn new(value: T) -> WriteOnly<T> {
            WriteOnly(Volatile::new(value))
        }

        /// Performs a volatile write of value `value` into the contained value. Functionally identical
        /// to `Volatile::write`.
        ///
        /// ```rust
        /// use volatile::WriteOnly;
        ///
        /// let mut value = WriteOnly::new(0u32);
        ///
        /// value.write(42u32);
        /// ```
        ///
        /// # Panics
        ///
        /// This method never panics.
        pub fn write(&mut self, value: T) {
            self.0.write(value)
        }
    }

    /// A volatile wrapper which allows both read and write operations;
    /// functionally equivalent to the `Volatile` type, as it is a type
    /// alias for it.
    ///
    /// The size of this struct is the same as the contained type.
    pub type ReadWrite<T> = Volatile<T>;

    #[cfg(test)]
    mod tests {
        use super::Volatile;

        #[test]
        fn test_read() {
            assert_eq!(Volatile(42).read(), 42);
        }

        #[test]
        fn test_write() {
            let mut volatile = Volatile(42);
            volatile.write(50);
            assert_eq!(volatile.0, 50);
        }

        #[test]
        fn test_update() {
            let mut volatile = Volatile(42);
            volatile.update(|v| *v += 1);
            assert_eq!(volatile.0, 43);
        }

        #[test]
        fn test_pointer_recast() {
            let mut target_value = 0u32;

            let target_ptr: *mut u32 = &mut target_value;
            let volatile_ptr = target_ptr as *mut Volatile<u32>;

            // UNSAFE: Safe, as we know the value exists on the stack.
            unsafe {
                (*volatile_ptr).write(42u32);
            }

            assert_eq!(target_value, 42u32);
        }
    }