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Add repr(C) attribute to ScreenChar

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see phil-opp/blog_os#8
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Philipp Oppermann
2015-10-25 15:52:22 +01:00
parent d017999722
commit 07fdd581d8
1 changed files with 3 additions and 3 deletions
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6
_posts/2015-10-23-printing-to-screen.md
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@@ -84,6 +84,7 @@ The `ColorCode` contains the full color byte, containing foreground and backgrou
Now we can add structures to represent a screen character and the text buffer:
```rust
#[repr(C)]
struct ScreenChar {
ascii_character: u8,
color_code: ColorCode,
@@ -96,10 +97,9 @@ struct Buffer {
chars: [[ScreenChar; BUFFER_WIDTH]; BUFFER_HEIGHT],
}
```
To ensure that `ScreenChar` is exactly 16-bits, one might be tempted to use the [repr(packed)] attribute. But Rust does not insert any padding around two `u8` values, so it's not needed here. And using `repr(packed)` is generally discouraged because it can [cause undefined behavior][repr(packed) issue].
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.
[repr(packed)]: https://doc.rust-lang.org/nightly/nomicon/other-reprs.html#reprpacked
[repr(packed) issue]: https://github.com/rust-lang/rust/issues/27060
[repr(C)]: https://doc.rust-lang.org/nightly/nomicon/other-reprs.html#reprc
To actually write to screen, we now create a writer type: