Source code
Revision control
Copy as Markdown
Other Tools
// 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(maybe_uninit)]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset__let_base_ptr {
($name:ident, $type:ty) => {
// No UB here, and the pointer does not dangle, either.
// But we have to make sure that `uninit` lives long enough,
// so it has to be in the same scope as `$name`. That's why
// `let_base_ptr` declares a variable (several, actually)
// instead of returning one.
let uninit = $crate::__priv::mem::MaybeUninit::<$type>::uninit();
let $name: *const $type = uninit.as_ptr();
};
}
#[cfg(not(maybe_uninit))]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset__let_base_ptr {
($name:ident, $type:ty) => {
// No UB right here, but we will later dereference this pointer to
// offset into a field, and that is UB because the pointer is dangling.
let $name = $crate::__priv::mem::align_of::<$type>() as *const $type;
};
}
/// Macro to compute the distance between two pointers.
#[cfg(any(feature = "unstable_const", stable_const))]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset_offset_from_unsafe {
($field:expr, $base:expr) => {{
let field = $field; // evaluate $field outside the `unsafe` block
let base = $base; // evaluate $base outside the `unsafe` block
// Compute offset, with unstable `offset_from` for const-compatibility.
// (Requires the pointers to not dangle, but we already need that for `raw_field!` anyway.)
unsafe { (field as *const u8).offset_from(base as *const u8) as usize }
}};
}
#[cfg(not(any(feature = "unstable_const", stable_const)))]
#[macro_export]
#[doc(hidden)]
macro_rules! _memoffset_offset_from_unsafe {
($field:expr, $base:expr) => {
// Compute offset.
($field as usize) - ($base as usize)
};
}
#[cfg(not(feature = "unstable_offset_of"))]
#[macro_export(local_inner_macros)]
#[doc(hidden)]
macro_rules! _memoffset__offset_of_impl {
($parent:path, $field:tt) => {{
// Get a base pointer (non-dangling if rustc supports `MaybeUninit`).
_memoffset__let_base_ptr!(base_ptr, $parent);
// Get field pointer.
let field_ptr = raw_field!(base_ptr, $parent, $field);
// Compute offset.
_memoffset_offset_from_unsafe!(field_ptr, base_ptr)
}};
}
#[cfg(feature = "unstable_offset_of")]
#[macro_export]
#[doc(hidden)]
#[allow_internal_unstable(offset_of)]
macro_rules! _memoffset__offset_of_impl {
($parent:path, $field:tt) => {{
$crate::__priv::mem::offset_of!($parent, $field)
}};
}
/// Calculates the offset of the specified field from the start of the named struct.
///
/// ## Examples
/// ```
/// use memoffset::offset_of;
///
/// #[repr(C, packed)]
/// struct Foo {
/// a: u32,
/// b: u64,
/// c: [u8; 5]
/// }
///
/// fn main() {
/// assert_eq!(offset_of!(Foo, a), 0);
/// assert_eq!(offset_of!(Foo, b), 4);
/// }
/// ```
///
/// ## Notes
/// Rust's ABI is unstable, and [type layout can be changed with each
///
/// Using `offset_of!` with a `repr(Rust)` struct will return the correct offset of the
/// specified `field` for a particular compilation, but the exact value may change
/// based on the compiler version, concrete struct type, time of day, or rustc's mood.
///
/// As a result, the value should not be retained and used between different compilations.
#[macro_export(local_inner_macros)]
macro_rules! offset_of {
($parent:path, $field:tt) => {
_memoffset__offset_of_impl!($parent, $field)
};
}
/// Calculates the offset of the specified field from the start of the tuple.
///
/// ## Examples
/// ```
/// use memoffset::offset_of_tuple;
///
/// fn main() {
/// assert!(offset_of_tuple!((u8, u32), 1) >= 0, "Tuples do not have a defined layout");
/// }
/// ```
#[cfg(tuple_ty)]
#[macro_export(local_inner_macros)]
macro_rules! offset_of_tuple {
($parent:ty, $field:tt) => {{
// Get a base pointer (non-dangling if rustc supports `MaybeUninit`).
_memoffset__let_base_ptr!(base_ptr, $parent);
// Get field pointer.
let field_ptr = raw_field_tuple!(base_ptr, $parent, $field);
// Compute offset.
_memoffset_offset_from_unsafe!(field_ptr, base_ptr)
}};
}
#[cfg(not(feature = "unstable_offset_of"))]
#[macro_export(local_inner_macros)]
#[doc(hidden)]
macro_rules! _memoffset__offset_of_union_impl {
($parent:path, $field:tt) => {{
// Get a base pointer (non-dangling if rustc supports `MaybeUninit`).
_memoffset__let_base_ptr!(base_ptr, $parent);
// Get field pointer.
let field_ptr = raw_field_union!(base_ptr, $parent, $field);
// Compute offset.
_memoffset_offset_from_unsafe!(field_ptr, base_ptr)
}};
}
#[cfg(feature = "unstable_offset_of")]
#[macro_export(local_inner_macros)]
#[doc(hidden)]
#[allow_internal_unstable(offset_of)]
macro_rules! _memoffset__offset_of_union_impl {
($parent:path, $field:tt) => {{
$crate::__priv::mem::offset_of!($parent, $field)
}};
}
/// Calculates the offset of the specified union member from the start of the union.
///
/// ## Examples
/// ```
/// use memoffset::offset_of_union;
///
/// #[repr(C, packed)]
/// union Foo {
/// foo32: i32,
/// foo64: i64,
/// }
///
/// fn main() {
/// assert!(offset_of_union!(Foo, foo64) == 0);
/// }
/// ```
///
/// ## Note
/// Due to macro_rules limitations, this macro will accept structs with a single field as well as unions.
/// This is not a stable guarantee, and future versions of this crate might fail
/// on any use of this macro with a struct, without a semver bump.
#[macro_export(local_inner_macros)]
macro_rules! offset_of_union {
($parent:path, $field:tt) => {{
_memoffset__offset_of_union_impl!($parent, $field)
}};
}
#[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_attr(miri, ignore)] // 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 offset_union() {
// Since we're specifying repr(C), all fields are supposed to be at offset 0
#[repr(C)]
union Foo {
a: u32,
b: [u8; 2],
c: i64,
}
assert_eq!(offset_of_union!(Foo, a), 0);
assert_eq!(offset_of_union!(Foo, b), 0);
assert_eq!(offset_of_union!(Foo, c), 0);
}
#[test]
fn path() {
mod sub {
#[repr(C)]
pub struct Foo {
pub x: u32,
}
}
assert_eq!(offset_of!(sub::Foo, x), 0);
}
#[test]
fn inside_generic_method() {
struct Pair<T, U>(T, U);
fn foo<T, U>(_: Pair<T, U>) -> usize {
offset_of!(Pair<T, U>, 1)
}
assert_eq!(foo(Pair(0, 0)), 4);
}
#[cfg(tuple_ty)]
#[test]
fn test_tuple_offset() {
let f = (0i32, 0.0f32, 0u8);
let f_ptr = &f as *const _;
let f1_ptr = &f.1 as *const _;
assert_eq!(
f1_ptr as usize - f_ptr as usize,
offset_of_tuple!((i32, f32, u8), 1)
);
}
#[test]
fn test_raw_field() {
#[repr(C)]
struct Foo {
a: u32,
b: [u8; 2],
c: i64,
}
let f: Foo = Foo {
a: 0,
b: [0, 0],
c: 0,
};
let f_ptr = &f as *const _;
assert_eq!(f_ptr as usize + 0, raw_field!(f_ptr, Foo, a) as usize);
assert_eq!(f_ptr as usize + 4, raw_field!(f_ptr, Foo, b) as usize);
assert_eq!(f_ptr as usize + 8, raw_field!(f_ptr, Foo, c) as usize);
}
#[cfg(tuple_ty)]
#[test]
fn test_raw_field_tuple() {
let t = (0u32, 0u8, false);
let t_ptr = &t as *const _;
let t_addr = t_ptr as usize;
assert_eq!(
&t.0 as *const _ as usize - t_addr,
raw_field_tuple!(t_ptr, (u32, u8, bool), 0) as usize - t_addr
);
assert_eq!(
&t.1 as *const _ as usize - t_addr,
raw_field_tuple!(t_ptr, (u32, u8, bool), 1) as usize - t_addr
);
assert_eq!(
&t.2 as *const _ as usize - t_addr,
raw_field_tuple!(t_ptr, (u32, u8, bool), 2) as usize - t_addr
);
}
#[test]
fn test_raw_field_union() {
#[repr(C)]
union Foo {
a: u32,
b: [u8; 2],
c: i64,
}
let f = Foo { a: 0 };
let f_ptr = &f as *const _;
assert_eq!(f_ptr as usize + 0, raw_field_union!(f_ptr, Foo, a) as usize);
assert_eq!(f_ptr as usize + 0, raw_field_union!(f_ptr, Foo, b) as usize);
assert_eq!(f_ptr as usize + 0, raw_field_union!(f_ptr, Foo, c) as usize);
}
#[cfg(any(
feature = "unstable_const",
feature = "unstable_offset_of",
stable_const
))]
#[test]
fn const_offset() {
#[repr(C)]
struct Foo {
a: u32,
b: [u8; 2],
c: i64,
}
assert_eq!([0; offset_of!(Foo, b)].len(), 4);
}
#[cfg(feature = "unstable_const")]
#[test]
fn const_offset_interior_mutable() {
#[repr(C)]
struct Foo {
a: u32,
b: core::cell::Cell<u32>,
}
assert_eq!([0; offset_of!(Foo, b)].len(), 4);
}
#[cfg(any(
feature = "unstable_const",
feature = "unstable_offset_of",
stable_const
))]
#[test]
fn const_fn_offset() {
const fn test_fn() -> usize {
#[repr(C)]
struct Foo {
a: u32,
b: [u8; 2],
c: i64,
}
offset_of!(Foo, b)
}
assert_eq!([0; test_fn()].len(), 4);
}
}