dlsym 0.0.2

Memory-safe bindings around the platform's dynamic library loading primitives
Documentation 
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use std::ffi::c_void;
use std::fmt;
use std::marker::PhantomData;
use std::sync::Arc;

use crate::{FnPtr, Library, SymbolType};

/// A symbol borrowed from a [`Library`].
///
/// The `'lib` lifetime ties the symbol to its library, so the library cannot
/// be dropped while the symbol is reachable:
///
/// ```compile_fail,E0505
/// # use dlsym::Library;
/// # unsafe {
/// let lib = Library::open("libm.so.6").unwrap();
/// let cos = lib.get::<unsafe extern "C" fn(f64) -> f64>(c"cos").unwrap();
/// drop(lib); // error[E0505]: cannot move out of `lib` because it is borrowed
/// cos.call((0.0,));
/// # }
/// ```
///
/// Unlike a `Deref<Target = T>` design, the raw pointer is only reachable
/// through `unsafe` methods ([`with`](Symbol::with),
/// [`into_inner`](Symbol::into_inner)), so it cannot be copied out and
/// outlive the library by accident.
pub struct Symbol<'lib, T> {
    inner: T,
    _lib: PhantomData<&'lib Library>,
}

impl<T: SymbolType> Symbol<'_, T> {
    pub(crate) fn new(inner: T) -> Self {
        Self {
            inner,
            _lib: PhantomData,
        }
    }

    /// Returns the symbol's address.
    pub fn as_ptr(&self) -> *mut c_void {
        self.inner.to_address()
    }

    /// Runs `f` with the raw symbol.
    ///
    /// This is how variadic functions are called:
    ///
    /// ```no_run
    /// # use dlsym::Library;
    /// # use std::ffi::{c_char, c_int};
    /// # let libc = unsafe { Library::open("libc.so.6") }.unwrap();
    /// let printf = unsafe {
    ///     libc.get::<unsafe extern "C" fn(*const c_char, ...) -> c_int>(c"printf")
    /// }
    /// .unwrap();
    /// // SAFETY: the format string matches the arguments, and `f` is not
    /// // kept past the closure.
    /// unsafe { printf.with(|f| f(c"%d\n".as_ptr(), 42)) };
    /// ```
    ///
    /// # Safety
    ///
    /// `f` must not let the raw symbol escape, since it would no longer be
    /// tied to the library's lifetime.
    pub unsafe fn with<R>(&self, f: impl FnOnce(T) -> R) -> R {
        f(self.inner)
    }

    /// Detaches the symbol from its library's lifetime.
    ///
    /// # Safety
    ///
    /// The returned value must not be used after the library is unloaded.
    pub unsafe fn into_inner(self) -> T {
        self.inner
    }
}

impl<T: FnPtr> Symbol<'_, T> {
    /// Calls the function. Arguments are passed as a tuple.
    ///
    /// # Safety
    ///
    /// The call must satisfy the foreign function's contract.
    pub unsafe fn call(&self, args: T::Args) -> T::Output {
        // SAFETY: the library outlives `self`; the rest is the caller's.
        unsafe { self.inner.call(args) }
    }
}

impl<'lib, T> Symbol<'lib, *const T> {
    /// Dereferences a data symbol.
    ///
    /// # Safety
    ///
    /// The address must be non-null, aligned, and point to a valid `T` that
    /// is not mutated for as long as the reference is alive.
    pub unsafe fn as_ref(&self) -> &'lib T {
        // SAFETY: forwarded to the caller; the library outlives `'lib`.
        unsafe { &*self.inner }
    }
}

impl<'lib, T> Symbol<'lib, *mut T> {
    /// Dereferences a data symbol.
    ///
    /// # Safety
    ///
    /// The address must be non-null, aligned, and point to a valid `T` that
    /// is not mutated for as long as the reference is alive.
    pub unsafe fn as_ref(&self) -> &'lib T {
        // SAFETY: forwarded to the caller; the library outlives `'lib`.
        unsafe { &*self.inner }
    }

    /// Mutably dereferences a data symbol.
    ///
    /// # Safety
    ///
    /// The address must be non-null, aligned, and point to a valid `T`, and
    /// nothing else (including other copies of this `Symbol` and the library
    /// itself) may access it while the reference is alive.
    #[allow(clippy::mut_from_ref)]
    pub unsafe fn as_mut(&self) -> &'lib mut T {
        // SAFETY: forwarded to the caller; the library outlives `'lib`.
        unsafe { &mut *self.inner }
    }
}

impl<T: SymbolType> Clone for Symbol<'_, T> {
    fn clone(&self) -> Self {
        *self
    }
}

impl<T: SymbolType> Copy for Symbol<'_, T> {}

impl<T: SymbolType> fmt::Debug for Symbol<'_, T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("Symbol").field(&self.as_ptr()).finish()
    }
}

/// A symbol that keeps its [`Library`] alive through an [`Arc`].
///
/// Obtained from [`Library::get_arc`]. The library is unloaded only once every
/// `ArcSymbol` and every other `Arc<Library>` clone is dropped.
pub struct ArcSymbol<T> {
    inner: T,
    lib: Arc<Library>,
}

impl<T: SymbolType> ArcSymbol<T> {
    pub(crate) fn new(inner: T, lib: Arc<Library>) -> Self {
        Self { inner, lib }
    }

    /// Returns the library this symbol keeps alive.
    pub fn library(&self) -> &Arc<Library> {
        &self.lib
    }

    /// Returns the symbol's address.
    pub fn as_ptr(&self) -> *mut c_void {
        self.inner.to_address()
    }

    /// Runs `f` with the raw symbol. See [`Symbol::with`].
    ///
    /// # Safety
    ///
    /// `f` must not let the raw symbol escape.
    pub unsafe fn with<R>(&self, f: impl FnOnce(T) -> R) -> R {
        f(self.inner)
    }

    /// Detaches the symbol from its library.
    ///
    /// # Safety
    ///
    /// The returned value must not be used after the library is unloaded.
    pub unsafe fn into_inner(self) -> T {
        self.inner
    }
}

impl<T: FnPtr> ArcSymbol<T> {
    /// Calls the function. Arguments are passed as a tuple.
    ///
    /// # Safety
    ///
    /// The call must satisfy the foreign function's contract.
    pub unsafe fn call(&self, args: T::Args) -> T::Output {
        // SAFETY: `self.lib` keeps the library loaded; the rest is the
        // caller's.
        unsafe { self.inner.call(args) }
    }
}

impl<T> ArcSymbol<*const T> {
    /// Dereferences a data symbol. See [`Symbol::as_ref`].
    ///
    /// # Safety
    ///
    /// Same as [`Symbol::as_ref`].
    pub unsafe fn as_ref(&self) -> &T {
        // SAFETY: forwarded to the caller; `self.lib` keeps the data alive.
        unsafe { &*self.inner }
    }
}

impl<T> ArcSymbol<*mut T> {
    /// Dereferences a data symbol. See [`Symbol::as_ref`].
    ///
    /// # Safety
    ///
    /// Same as [`Symbol::as_ref`].
    pub unsafe fn as_ref(&self) -> &T {
        // SAFETY: forwarded to the caller; `self.lib` keeps the data alive.
        unsafe { &*self.inner }
    }
}

impl<T: SymbolType> Clone for ArcSymbol<T> {
    fn clone(&self) -> Self {
        Self {
            inner: self.inner,
            lib: Arc::clone(&self.lib),
        }
    }
}

impl<T: SymbolType> fmt::Debug for ArcSymbol<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("ArcSymbol").field(&self.as_ptr()).finish()
    }
}