Add `NullTerminatedFormatter`, a formatter that writes a null terminated
string to an array or slice buffer. Because this type needs to manage the
trailing null marker, the existing formatters cannot be used to implement
this type.
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
rust/kernel/str.rs | 55 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 55 insertions(+)
diff --git a/rust/kernel/str.rs b/rust/kernel/str.rs
index 46cdc85dad63..e84f62c01815 100644
--- a/rust/kernel/str.rs
+++ b/rust/kernel/str.rs
@@ -871,6 +871,61 @@ fn write_str(&mut self, s: &str) -> fmt::Result {
}
}
+/// A mutable reference to a byte buffer where a string can be written into.
+///
+/// The buffer will be automatically null terminated after the last written character.
+///
+/// # Invariants
+///
+/// * The first byte of `buffer` is always zero.
+/// * The length of `buffer` is at least 1.
+pub(crate) struct NullTerminatedFormatter<'a> {
+ buffer: &'a mut [u8],
+}
+
+impl<'a> NullTerminatedFormatter<'a> {
+ /// Create a new [`Self`] instance.
+ pub(crate) fn new(buffer: &'a mut [u8]) -> Option<NullTerminatedFormatter<'a>> {
+ *(buffer.first_mut()?) = 0;
+
+ // INVARIANT:
+ // - We wrote zero to the first byte above.
+ // - If buffer was not at least length 1, `buffer.first_mut()` would return None.
+ Some(Self { buffer })
+ }
+
+ #[expect(dead_code)]
+ pub(crate) fn from_array<const N: usize>(
+ buffer: &'a mut [crate::ffi::c_char; N],
+ ) -> Option<NullTerminatedFormatter<'a>> {
+ Self::new(buffer)
+ }
+}
+
+impl Write for NullTerminatedFormatter<'_> {
+ fn write_str(&mut self, s: &str) -> fmt::Result {
+ let bytes = s.as_bytes();
+ let len = bytes.len();
+
+ // We want space for a zero. By type invariant, buffer length is always at least 1, so no
+ // underflow.
+ if len > self.buffer.len() - 1 {
+ return Err(fmt::Error);
+ }
+
+ let buffer = core::mem::take(&mut self.buffer);
+ // We break the zero start invariant for a short while.
+ buffer[..len].copy_from_slice(bytes);
+ // INVARIANT: We checked above that buffer will have size at least 1 after this assignment.
+ self.buffer = &mut buffer[len..];
+
+ // INVARIANT: We write zero to the first byte of the buffer.
+ self.buffer[0] = 0;
+
+ Ok(())
+ }
+}
+
/// An owned string that is guaranteed to have exactly one `NUL` byte, which is at the end.
///
/// Used for interoperability with kernel APIs that take C strings.
--
2.47.2