c-gull 0.22.3

A libc implementation in Rust
Documentation 
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//! Utilities to check against C signatures.

#![allow(unused_macros)]
#![allow(dead_code)]

/// Cast the given pointer to another type, similarly to calling `cast()` on
/// it, while verifying that the layout of the pointee stays the same after the
/// cast.
macro_rules! checked_cast {
    ($ptr:expr) => {{
        let src_ptr = $ptr;
        let target_ptr = src_ptr.cast();

        #[allow(unnecessary_transmutes)]
        if false {
            let target = crate::use_libc::Pad::new(core::ptr::read(target_ptr));

            // Uses the fact that the compiler checks for size equality,
            // when transmuting between types.
            let size_check = core::mem::transmute(core::ptr::read(src_ptr));
            target.compare_size(size_check);
            let align_check =
                core::mem::transmute(crate::use_libc::Pad::new(core::ptr::read(src_ptr)));
            target.compare_alignment(align_check);
        }

        target_ptr
    }};
}

/// A macro for ensuring that the `libc` crate signature for a function matches
/// the signature that our implementation of it is using.
///
/// # Example
///
/// ```no_compile
/// #[no_mangle]
/// unsafe extern "C" fn strlen(s: *const c_char) -> usize {
///     libc!(libc::strlen(s));
///     // ...
/// }
/// ```
///
/// This will elicit a compile-time error if the signature doesn't match.
macro_rules! libc {
    ($e:expr) => {
        #[allow(unreachable_code)]
        #[allow(clippy::diverging_sub_expression)]
        if false {
            #[allow(unused_imports)]
            use crate::use_libc::*;
            return $e;
        }
    };
}

/// This is used to check that our type definition matches the layout of the
/// corresponding libc type.
#[cfg(all(feature = "take-charge", feature = "thread"))]
macro_rules! libc_type {
    ($name:ident, $libc:ident) => {
        const _: () = {
            if core::mem::size_of::<$name>() != core::mem::size_of::<libc::$libc>() {
                panic!();
            }
            if core::mem::align_of::<$name>() != core::mem::align_of::<libc::$libc>() {
                panic!();
            }
        };
    };
}

/// A struct that adds `align_of<T>` padding bytes to type `T`.
///
/// Based on the rules of C struct alignment:
/// * `align_of<Pad<T>> == max(align_of<T>, align_of<u8>) == align_of<T>`
/// * `size_of<Pad<T>> / align_of<Pad<T>> == ciel( (size_of<T> + size_of<u8>) /
///   align_of<Pad<T>>)`
/// * `size_of<T> % align_of<T> == 0`
///
/// Therefore `size_of<Pad<T>> == size_of<T> + align_of<T>`.
#[repr(C)]
pub(crate) struct Pad<T> {
    field: T,
    force_padding: u8,
}

impl<T> Pad<T> {
    pub unsafe fn new(v: T) -> Self {
        Pad {
            field: v,
            force_padding: 0,
        }
    }

    /// Used to check that `size_of<T> == size_of<U>` with `transmute`.
    pub fn compare_size(&self, _v: T) {}

    /// Used to check that `size_of<Pad<T>> == size_of<Pad<U>>` with
    /// `transmute`.
    ///
    /// Since `size_of<Pad<T>> == size_of<T> + align_of<T>`,
    /// if `size_of<T> == size_of<U>` then `align_of<T> == align_of<U>`.
    pub fn compare_alignment(&self, _pad: Pad<T>) {}
}