1 // SPDX-License-Identifier: GPL-2.0
2
3 use core::fmt::{self, Write};
4
5 use crate::error::Result;
6 use crate::prelude::EINVAL;
7
8 /// A mutable reference to a byte buffer where a string can be written into.
9 ///
10 /// # Invariants
11 ///
12 /// `buffer` is always null terminated.
13 pub(crate) struct RawWriter<'a> {
14 buffer: &'a mut [u8],
15 pos: usize,
16 }
17
18 impl<'a> RawWriter<'a> {
19 /// Create a new `RawWriter` instance.
20 fn new(buffer: &'a mut [u8]) -> Result<RawWriter<'a>> {
21 *(buffer.last_mut().ok_or(EINVAL)?) = 0;
22
23 // INVARIANT: We null terminated the buffer above.
24 Ok(Self { buffer, pos: 0 })
25 }
26
27 pub(crate) fn from_array<const N: usize>(
28 a: &'a mut [core::ffi::c_char; N],
29 ) -> Result<RawWriter<'a>> {
30 Self::new(
31 // SAFETY: the buffer of `a` is valid for read and write as `u8` for
32 // at least `N` bytes.
33 unsafe { core::slice::from_raw_parts_mut(a.as_mut_ptr().cast::<u8>(), N) },
34 )
35 }
36 }
37
38 impl Write for RawWriter<'_> {
39 fn write_str(&mut self, s: &str) -> fmt::Result {
40 let bytes = s.as_bytes();
41 let len = bytes.len();
42
43 // We do not want to overwrite our null terminator
44 if self.pos + len > self.buffer.len() - 1 {
45 return Err(fmt::Error);
46 }
47
48 // INVARIANT: We are not overwriting the last byte
49 self.buffer[self.pos..self.pos + len].copy_from_slice(bytes);
50
51 self.pos += len;
52
53 Ok(())
54 }
55 }
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