error[E0793]: reference to packed field is unaligned
--> C:\Users\lbc15\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ntapi-0.3.7\src\ntexapi.rs:2783:52
|
2783 | *tick_count.QuadPart_mut() = read_volatile(&(*USER_SHARED_DATA).u.TickCountQuad);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
= note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
= help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
error[E0793]: reference to packed field is unaligned
--> C:\Users\lbc15\.cargo\registry\src\index.crates.io-6f17d22bba15001f\ntapi-0.3.7\src\ntexapi.rs:2807:25
|
2807 | ((read_volatile(&(*USER_SHARED_DATA).u.TickCountQuad)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
= note: creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
= help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)
Compiling tui v0.19.0
Compiling failure v0.1.8
For more information about this error, try `rustc --explain E0793`.
error: could not compile `ntapi` (lib) due to 2 previous errors
warning: build failed, waiting for other jobs to finish...
PS D:\projects\rust_demos\RustPlayer> rustc --explain E0793
An unaligned references to a field of a [packed] struct got created.
Erroneous code example:
[repr(packed)]
pub struct Foo {
field1: u64,
field2: u8,
}
unsafe {
let foo = Foo { field1: 0, field2: 0 };
// Accessing the field directly is fine.
let val = foo.field1;
// A reference to a packed field causes a error.
let val = &foo.field1; // ERROR
// An implicit & is added in format strings, causing the same error.
println!("{}", foo.field1); // ERROR
}
Creating a reference to an insufficiently aligned packed field is
[undefined behavior] and therefore disallowed. Using an `unsafe` block does not
change anything about this. Instead, the code should do a copy of the data in
the packed field or use raw pointers and unaligned accesses.
[repr(packed)]
pub struct Foo {
field1: u64,
field2: u8,
}
unsafe {
let foo = Foo { field1: 0, field2: 0 };
// Instead of a reference, we can create a raw pointer...
let ptr = std::ptr::addr_of!(foo.field1);
// ... and then (crucially!) access it in an explicitly unaligned way.
let val = unsafe { ptr.read_unaligned() };
// This would *NOT* be correct:
// let val = unsafe { *ptr }; // Undefined Behavior due to unaligned load!
// For formatting, we can create a copy to avoid the direct reference.
let copy = foo.field1;
println!("{}", copy);
// Creating a copy can be written in a single line with curly braces.
// (This is equivalent to the two lines above.)
println!("{}", { foo.field1 });
}
### Additional information
Note that this error is specifically about *references* to packed fields.
Direct by-value access of those fields is fine, since then the compiler has
enough information to generate the correct kind of access.
See [issue #82523] for more information.
[packed]: https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers
[undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
[issue #82523]: https://github.com/rust-lang/rust/issues/82523
windows11
[repr(packed)]
pub struct Foo { field1: u64, field2: u8, }
unsafe { let foo = Foo { field1: 0, field2: 0 }; // Accessing the field directly is fine. let val = foo.field1; // A reference to a packed field causes a error. let val = &foo.field1; // ERROR // An implicit
&
is added in format strings, causing the same error. println!("{}", foo.field1); // ERROR }[repr(packed)]
pub struct Foo { field1: u64, field2: u8, }
unsafe { let foo = Foo { field1: 0, field2: 0 };
}