// Copyright (c) 2017 Gilad Naaman | |
// | |
// Permission is hereby granted, free of charge, to any person obtaining a copy | |
// of this software and associated documentation files (the "Software"), to deal | |
// in the Software without restriction, including without limitation the rights | |
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | |
// copies of the Software, and to permit persons to whom the Software is | |
// furnished to do so, subject to the following conditions: | |
// | |
// The above copyright notice and this permission notice shall be included in all | |
// copies or substantial portions of the Software. | |
// | |
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | |
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | |
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | |
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | |
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | |
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE | |
// SOFTWARE. | |
/// Macro to create a local `base_ptr` raw pointer of the given type, avoiding UB as | |
/// much as is possible currently. | |
#[cfg(memoffset_maybe_uninit)] | |
#[macro_export] | |
#[doc(hidden)] | |
macro_rules! _memoffset__let_base_ptr { | |
($name:ident, $type:path) => { | |
// No UB here, and the pointer does not dangle, either. | |
// But we have to make sure that `uninit` lives long enough, | |
// so it has to be in the same scope as `$name`. That's why | |
// `let_base_ptr` declares a variable (several, actually) | |
// instad of returning one. | |
let uninit = $crate::mem::MaybeUninit::<$type>::uninit(); | |
let $name = uninit.as_ptr(); | |
}; | |
} | |
#[cfg(not(memoffset_maybe_uninit))] | |
#[macro_export] | |
#[doc(hidden)] | |
macro_rules! _memoffset__let_base_ptr { | |
($name:ident, $type:path) => { | |
// No UB right here, but we will later dereference this pointer to | |
// offset into a field, and that is UB when the pointer is dangling. | |
let $name = $crate::mem::align_of::<$type>() as *const $type; | |
}; | |
} | |
/// Deref-coercion protection macro. | |
#[macro_export] | |
#[doc(hidden)] | |
macro_rules! _memoffset__field_check { | |
($type:path, $field:tt) => { | |
// Make sure the field actually exists. This line ensures that a | |
// compile-time error is generated if $field is accessed through a | |
// Deref impl. | |
let $type { $field: _, .. }; | |
}; | |
} | |
/// Calculates the offset of the specified field from the start of the struct. | |
/// | |
/// ## Examples | |
/// ``` | |
/// #[macro_use] | |
/// extern crate memoffset; | |
/// | |
/// #[repr(C, packed)] | |
/// struct Foo { | |
/// a: u32, | |
/// b: u64, | |
/// c: [u8; 5] | |
/// } | |
/// | |
/// fn main() { | |
/// assert_eq!(offset_of!(Foo, a), 0); | |
/// assert_eq!(offset_of!(Foo, b), 4); | |
/// } | |
/// ``` | |
#[macro_export(local_inner_macros)] | |
macro_rules! offset_of { | |
($parent:path, $field:tt) => {{ | |
_memoffset__field_check!($parent, $field); | |
// Get a base pointer. | |
_memoffset__let_base_ptr!(base_ptr, $parent); | |
// Get the field address. This is UB because we are creating a reference to | |
// the uninitialized field. | |
// Crucially, we know that this will not trigger a deref coercion because | |
// of the `field_check!` we did above. | |
#[allow(unused_unsafe)] // for when the macro is used in an unsafe block | |
let field_ptr = unsafe { &(*base_ptr).$field as *const _ }; | |
let offset = (field_ptr as usize) - (base_ptr as usize); | |
offset | |
}}; | |
} | |
#[cfg(test)] | |
mod tests { | |
#[test] | |
fn offset_simple() { | |
#[repr(C)] | |
struct Foo { | |
a: u32, | |
b: [u8; 2], | |
c: i64, | |
} | |
assert_eq!(offset_of!(Foo, a), 0); | |
assert_eq!(offset_of!(Foo, b), 4); | |
assert_eq!(offset_of!(Foo, c), 8); | |
} | |
#[test] | |
#[cfg(not(miri))] // this creates unaligned references | |
fn offset_simple_packed() { | |
#[repr(C, packed)] | |
struct Foo { | |
a: u32, | |
b: [u8; 2], | |
c: i64, | |
} | |
assert_eq!(offset_of!(Foo, a), 0); | |
assert_eq!(offset_of!(Foo, b), 4); | |
assert_eq!(offset_of!(Foo, c), 6); | |
} | |
#[test] | |
fn tuple_struct() { | |
#[repr(C)] | |
struct Tup(i32, i32); | |
assert_eq!(offset_of!(Tup, 0), 0); | |
assert_eq!(offset_of!(Tup, 1), 4); | |
} | |
#[test] | |
fn path() { | |
mod sub { | |
#[repr(C)] | |
pub struct Foo { | |
pub x: u32, | |
} | |
} | |
assert_eq!(offset_of!(sub::Foo, x), 0); | |
} | |
} |