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Use repr(transparent) for vga_buffer::{ColorCode, Buffer}

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Philipp Oppermann
2019-02-05 10:53:55 +01:00
parent 87d542f7a1
commit 47092acf8c
1 changed files with 6 additions and 2 deletions
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8
blog/content/second-edition/posts/03-vga-text-buffer/index.md
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@@ -114,6 +114,7 @@ To represent a full color code that specifies foreground and background color, w
// in src/vga_buffer.rs // in src/vga_buffer.rs
#[derive(Debug, Clone, Copy, PartialEq, Eq)] #[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(transparent)]
struct ColorCode(u8); struct ColorCode(u8);
impl ColorCode { impl ColorCode {
@@ -122,7 +123,9 @@ impl ColorCode {
} }
} }
``` ```
The `ColorCode` contains the full color byte, containing foreground and background color. Like before, we derive the `Copy` and `Debug` traits for it. The `ColorCode` struct contains the full color byte, containing foreground and background color. Like before, we derive the `Copy` and `Debug` traits for it. To ensure that the `ColorCode` has the exact same data layout as an `u8`, we use the [`repr(transparent)`] attribute.
[`repr(transparent)`]: https://doc.rust-lang.org/nomicon/other-reprs.html#reprtransparent
### Text Buffer ### Text Buffer
Now we can add structures to represent a screen character and the text buffer: Now we can add structures to represent a screen character and the text buffer:
@@ -140,11 +143,12 @@ struct ScreenChar {
const BUFFER_HEIGHT: usize = 25; const BUFFER_HEIGHT: usize = 25;
const BUFFER_WIDTH: usize = 80; const BUFFER_WIDTH: usize = 80;
#[repr(transparent)]
struct Buffer { struct Buffer {
chars: [[ScreenChar; BUFFER_WIDTH]; BUFFER_HEIGHT], chars: [[ScreenChar; BUFFER_WIDTH]; BUFFER_HEIGHT],
} }
``` ```
Since the field ordering in default structs is undefined in Rust, we need the [`repr(C)`] attribute. It guarantees that the struct's fields are laid out exactly like in a C struct and thus guarantees the correct field ordering. Since the field ordering in default structs is undefined in Rust, we need the [`repr(C)`] attribute. It guarantees that the struct's fields are laid out exactly like in a C struct and thus guarantees the correct field ordering. For the `Buffer` struct, we use [`repr(transparent)`] again to ensure that it has the same memory layout as its single field.
[`repr(C)`]: https://doc.rust-lang.org/nightly/nomicon/other-reprs.html#reprc [`repr(C)`]: https://doc.rust-lang.org/nightly/nomicon/other-reprs.html#reprc