| // 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, $type:tt) => { |
| // No UB here, and the pointer does not dangle, either. |
| // But we have to make sure that `uninit` lives long enough, |
| // so it has to be in the same scope as `$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:tt) => { |
| // No UB right here, but we will later offset into a field |
| // of this pointer, and that is UB when the pointer is dangling. |
| let non_null = $crate::ptr::NonNull::<$type>::dangling(); |
| let $name = non_null.as_ptr() as *const $type; |
| }; |
| } |
| |
| /// Deref-coercion protection macro. |
| #[macro_export] |
| #[doc(hidden)] |
| macro_rules! _memoffset__field_check { |
| ($type:tt, $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:tt, $field:tt) => {{ |
| _memoffset__field_check!($parent, $field); |
| |
| // Get a base pointer. |
| _memoffset__let_base_ptr!(base_ptr, $parent); |
| // Get the field address. This is UB because we are creating a reference to |
| // the uninitialized field. |
| #[allow(unused_unsafe)] // for when the macro is used in an unsafe block |
| let field_ptr = unsafe { &(*base_ptr).$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); |
| } |
| } |