rustix/
buffer.rs

1//! Utilities for functions that return data via buffers.
2
3#![allow(unsafe_code)]
4
5#[cfg(feature = "alloc")]
6use alloc::vec::Vec;
7use core::mem::MaybeUninit;
8use core::slice;
9
10/// A memory buffer that may be uninitialized.
11///
12/// There are three types that implement the `Buffer` trait, and the type you
13/// use determines the return type of the functions that use it:
14///
15/// | If you pass a…           | You get back a… |
16/// | ------------------------ | --------------- |
17/// | `&mut [u8]`              | `usize`, indicating the number of elements initialized. |
18/// | `&mut [MaybeUninit<u8>]` | `(&mut [u8], &mut [MaybeUninit<u8>])`, holding the initialized and uninitialized subslices. |
19/// | [`SpareCapacity`]        | `usize`, indicating the number of elements initialized. And the `Vec` is extended. |
20///
21/// # Examples
22///
23/// Passing a `&mut [u8]`:
24///
25/// ```
26/// # use rustix::io::read;
27/// # fn example(fd: rustix::fd::BorrowedFd) -> rustix::io::Result<()> {
28/// let mut buf = [0_u8; 64];
29/// let nread = read(fd, &mut buf)?;
30/// // `nread` is the number of bytes read.
31/// # Ok(())
32/// # }
33/// ```
34///
35/// Passing a `&mut [MaybeUninit<u8>]`:
36///
37/// ```
38/// # use rustix::io::read;
39/// # use std::mem::MaybeUninit;
40/// # fn example(fd: rustix::fd::BorrowedFd) -> rustix::io::Result<()> {
41/// let mut buf = [MaybeUninit::<u8>::uninit(); 64];
42/// let (init, uninit) = read(fd, &mut buf)?;
43/// // `init` is a `&mut [u8]` with the initialized bytes.
44/// // `uninit` is a `&mut [MaybeUninit<u8>]` with the remaining bytes.
45/// # Ok(())
46/// # }
47/// ```
48///
49/// Passing a [`SpareCapacity`], via the [`spare_capacity`] helper function:
50///
51/// ```
52/// # use rustix::io::read;
53/// # use rustix::buffer::spare_capacity;
54/// # fn example(fd: rustix::fd::BorrowedFd) -> rustix::io::Result<()> {
55/// let mut buf = Vec::with_capacity(64);
56/// let nread = read(fd, spare_capacity(&mut buf))?;
57/// // `nread` is the number of bytes read.
58/// // Also, `buf.len()` is now `nread` elements longer than it was before.
59/// # Ok(())
60/// # }
61/// ```
62///
63/// # Guide to error messages
64///
65/// Sometimes code using `Buffer` can encounter non-obvious error messages.
66/// Here are some we've encountered, along with ways to fix them.
67///
68/// If you see errors like
69/// "cannot move out of `self` which is behind a mutable reference"
70/// and
71/// "move occurs because `x` has type `&mut [u8]`, which does not implement the `Copy` trait",
72/// replace `x` with `&mut *x`. See `error_buffer_wrapper` in
73/// examples/buffer_errors.rs.
74///
75/// If you see errors like
76/// "type annotations needed"
77/// and
78/// "cannot infer type of the type parameter `Buf` declared on the function `read`",
79/// you may need to change a `&mut []` to `&mut [0_u8; 0]`. See
80/// `error_empty_slice` in examples/buffer_errors.rs.
81///
82/// If you see errors like
83/// "the trait bound `[MaybeUninit<u8>; 1]: Buffer<u8>` is not satisfied",
84/// add a `&mut` to pass the array by reference instead of by value. See
85/// `error_array_by_value` in examples/buffer_errors.rs.
86///
87/// If you see errors like
88/// "cannot move out of `x`, a captured variable in an `FnMut` closure",
89/// try replacing `x` with `&mut *x`, or, if that doesn't work, try moving a
90/// `let` into the closure body. See `error_retry_closure` and
91/// `error_retry_indirect_closure` in examples/buffer_errors.rs.
92///
93/// If you see errors like
94/// "captured variable cannot escape `FnMut` closure body",
95/// use an explicit loop instead of `retry_on_intr`, assuming you're using
96/// that. See `error_retry_closure_uninit` in examples/buffer_errors.rs.
97pub trait Buffer<T>: private::Sealed<T> {}
98
99// Implement `Buffer` for all the types that implement `Sealed`.
100impl<T> Buffer<T> for &mut [T] {}
101impl<T, const N: usize> Buffer<T> for &mut [T; N] {}
102#[cfg(feature = "alloc")]
103impl<T> Buffer<T> for &mut Vec<T> {}
104impl<T> Buffer<T> for &mut [MaybeUninit<T>] {}
105impl<T, const N: usize> Buffer<T> for &mut [MaybeUninit<T>; N] {}
106#[cfg(feature = "alloc")]
107impl<T> Buffer<T> for &mut Vec<MaybeUninit<T>> {}
108#[cfg(feature = "alloc")]
109impl<'a, T> Buffer<T> for SpareCapacity<'a, T> {}
110
111impl<T> private::Sealed<T> for &mut [T] {
112    type Output = usize;
113
114    #[inline]
115    fn parts_mut(&mut self) -> (*mut T, usize) {
116        (self.as_mut_ptr(), self.len())
117    }
118
119    #[inline]
120    unsafe fn assume_init(self, len: usize) -> Self::Output {
121        len
122    }
123}
124
125impl<T, const N: usize> private::Sealed<T> for &mut [T; N] {
126    type Output = usize;
127
128    #[inline]
129    fn parts_mut(&mut self) -> (*mut T, usize) {
130        (self.as_mut_ptr(), N)
131    }
132
133    #[inline]
134    unsafe fn assume_init(self, len: usize) -> Self::Output {
135        len
136    }
137}
138
139// `Vec` implements `DerefMut` to `&mut [T]`, however it doesn't get
140// auto-derefed in a `impl Buffer<u8>`, so we add this `impl` so that our users
141// don't have to add an extra `*` in these situations.
142#[cfg(feature = "alloc")]
143impl<T> private::Sealed<T> for &mut Vec<T> {
144    type Output = usize;
145
146    #[inline]
147    fn parts_mut(&mut self) -> (*mut T, usize) {
148        (self.as_mut_ptr(), self.len())
149    }
150
151    #[inline]
152    unsafe fn assume_init(self, len: usize) -> Self::Output {
153        len
154    }
155}
156
157impl<'a, T> private::Sealed<T> for &'a mut [MaybeUninit<T>] {
158    type Output = (&'a mut [T], &'a mut [MaybeUninit<T>]);
159
160    #[inline]
161    fn parts_mut(&mut self) -> (*mut T, usize) {
162        (self.as_mut_ptr().cast(), self.len())
163    }
164
165    #[inline]
166    unsafe fn assume_init(self, len: usize) -> Self::Output {
167        let (init, uninit) = self.split_at_mut(len);
168
169        // SAFETY: The user asserts that the slice is now initialized.
170        let init = slice::from_raw_parts_mut(init.as_mut_ptr().cast::<T>(), init.len());
171
172        (init, uninit)
173    }
174}
175
176impl<'a, T, const N: usize> private::Sealed<T> for &'a mut [MaybeUninit<T>; N] {
177    type Output = (&'a mut [T], &'a mut [MaybeUninit<T>]);
178
179    #[inline]
180    fn parts_mut(&mut self) -> (*mut T, usize) {
181        (self.as_mut_ptr().cast(), self.len())
182    }
183
184    #[inline]
185    unsafe fn assume_init(self, len: usize) -> Self::Output {
186        let (init, uninit) = self.split_at_mut(len);
187
188        // SAFETY: The user asserts that the slice is now initialized.
189        let init = slice::from_raw_parts_mut(init.as_mut_ptr().cast::<T>(), init.len());
190
191        (init, uninit)
192    }
193}
194
195#[cfg(feature = "alloc")]
196impl<'a, T> private::Sealed<T> for &'a mut Vec<MaybeUninit<T>> {
197    type Output = (&'a mut [T], &'a mut [MaybeUninit<T>]);
198
199    #[inline]
200    fn parts_mut(&mut self) -> (*mut T, usize) {
201        (self.as_mut_ptr().cast(), self.len())
202    }
203
204    #[inline]
205    unsafe fn assume_init(self, len: usize) -> Self::Output {
206        let (init, uninit) = self.split_at_mut(len);
207
208        // SAFETY: The user asserts that the slice is now initialized.
209        let init = slice::from_raw_parts_mut(init.as_mut_ptr().cast::<T>(), init.len());
210
211        (init, uninit)
212    }
213}
214
215/// A type that implements [`Buffer`] by appending to a `Vec`, up to its
216/// capacity.
217///
218/// To use this, use the [`spare_capacity`] function.
219///
220/// Because this uses the capacity, and never reallocates, the `Vec` should
221/// have some non-empty spare capacity.
222#[cfg(feature = "alloc")]
223#[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
224pub struct SpareCapacity<'a, T>(&'a mut Vec<T>);
225
226/// Construct an [`SpareCapacity`], which implements [`Buffer`].
227///
228/// This wraps a `Vec` and uses the spare capacity of the `Vec` as the buffer
229/// to receive data in, automatically calling `set_len` on the `Vec` to set the
230/// length to include the received elements.
231///
232/// This uses the existing capacity, and never allocates, so the `Vec` should
233/// have some non-empty spare capacity!
234///
235/// # Examples
236///
237/// ```
238/// # fn test(input: rustix::fd::BorrowedFd) -> rustix::io::Result<()> {
239/// use rustix::buffer::spare_capacity;
240/// use rustix::io::{read, Errno};
241///
242/// let mut buf = Vec::with_capacity(1024);
243/// match read(input, spare_capacity(&mut buf)) {
244///     Ok(0) => { /* end of stream */ }
245///     Ok(n) => { /* `buf` is now `n` bytes longer */ }
246///     Err(Errno::INTR) => { /* `buf` is unmodified */ }
247///     Err(e) => {
248///         return Err(e);
249///     }
250/// }
251///
252/// # Ok(())
253/// # }
254/// ```
255#[cfg(feature = "alloc")]
256#[cfg_attr(docsrs, doc(cfg(feature = "alloc")))]
257pub fn spare_capacity<'a, T>(v: &'a mut Vec<T>) -> SpareCapacity<'a, T> {
258    debug_assert_ne!(
259        v.capacity(),
260        0,
261        "`extend` uses spare capacity, and never allocates new memory, so the `Vec` passed to it \
262         should have some spare capacity."
263    );
264
265    SpareCapacity(v)
266}
267
268#[cfg(feature = "alloc")]
269impl<'a, T> private::Sealed<T> for SpareCapacity<'a, T> {
270    /// The mutated `Vec` reflects the number of bytes read. We also return
271    /// this number, and a value of 0 indicates the end of the stream has
272    /// been reached.
273    type Output = usize;
274
275    #[inline]
276    fn parts_mut(&mut self) -> (*mut T, usize) {
277        let spare = self.0.spare_capacity_mut();
278        (spare.as_mut_ptr().cast(), spare.len())
279    }
280
281    #[inline]
282    unsafe fn assume_init(self, len: usize) -> Self::Output {
283        // We initialized `len` elements; extend the `Vec` to include them.
284        self.0.set_len(self.0.len() + len);
285        len
286    }
287}
288
289mod private {
290    pub trait Sealed<T> {
291        /// The result of the process operation.
292        type Output;
293
294        /// Return a pointer and length for this buffer.
295        ///
296        /// The length is the number of elements of type `T`, not a number of
297        /// bytes.
298        ///
299        /// It's tempting to have this return `&mut [MaybeUninit<T>]` instead,
300        /// however that would require this function to be `unsafe`, because
301        /// callers could use the `&mut [MaybeUninit<T>]` slice to set elements
302        /// to `MaybeUninit::<T>::uninit()`, which would be a problem if `Self`
303        /// is `&mut [T]` or similar.
304        fn parts_mut(&mut self) -> (*mut T, usize);
305
306        /// Convert a finished buffer pointer into its result.
307        ///
308        /// # Safety
309        ///
310        /// At least `len` elements of the buffer must now be initialized.
311        #[must_use]
312        unsafe fn assume_init(self, len: usize) -> Self::Output;
313    }
314}
315
316#[cfg(test)]
317mod tests {
318    #[allow(unused_imports)]
319    use super::*;
320
321    #[cfg(not(windows))]
322    #[test]
323    fn test_compilation() {
324        use crate::io::read;
325        use core::mem::MaybeUninit;
326
327        // We need to obtain input stream, so open our own source file.
328        let input = std::fs::File::open("src/buffer.rs").unwrap();
329
330        let mut buf = vec![0_u8; 3];
331        buf.reserve(32);
332        let _x: usize = read(&input, spare_capacity(&mut buf)).unwrap();
333        let _x: (&mut [u8], &mut [MaybeUninit<u8>]) =
334            read(&input, buf.spare_capacity_mut()).unwrap();
335        let _x: usize = read(&input, &mut buf).unwrap();
336        let _x: usize = read(&input, &mut *buf).unwrap();
337        let _x: usize = read(&input, &mut buf[..]).unwrap();
338        let _x: usize = read(&input, &mut (*buf)[..]).unwrap();
339
340        let mut buf = [0, 0, 0];
341        let _x: usize = read(&input, &mut buf).unwrap();
342        let _x: usize = read(&input, &mut buf[..]).unwrap();
343
344        let mut buf = [
345            MaybeUninit::uninit(),
346            MaybeUninit::uninit(),
347            MaybeUninit::uninit(),
348        ];
349        let _x: (&mut [u8], &mut [MaybeUninit<u8>]) = read(&input, &mut buf).unwrap();
350        let _x: (&mut [u8], &mut [MaybeUninit<u8>]) = read(&input, &mut buf[..]).unwrap();
351
352        let mut buf = vec![
353            MaybeUninit::uninit(),
354            MaybeUninit::uninit(),
355            MaybeUninit::uninit(),
356        ];
357        let _x: (&mut [u8], &mut [MaybeUninit<u8>]) = read(&input, &mut buf).unwrap();
358        let _x: (&mut [u8], &mut [MaybeUninit<u8>]) = read(&input, &mut buf[..]).unwrap();
359    }
360
361    #[cfg(not(windows))]
362    #[test]
363    fn test_slice() {
364        use crate::io::read;
365        use std::io::{Seek, SeekFrom};
366
367        // We need to obtain input stream with contents that we can compare
368        // against, so open our own source file.
369        let mut input = std::fs::File::open("src/buffer.rs").unwrap();
370
371        let mut buf = [0_u8; 64];
372        let nread = read(&input, &mut buf).unwrap();
373        assert_eq!(nread, buf.len());
374        assert_eq!(
375            &buf[..58],
376            b"//! Utilities for functions that return data via buffers.\n"
377        );
378        input.seek(SeekFrom::End(-1)).unwrap();
379        let nread = read(&input, &mut buf).unwrap();
380        assert_eq!(nread, 1);
381        input.seek(SeekFrom::End(0)).unwrap();
382        let nread = read(&input, &mut buf).unwrap();
383        assert_eq!(nread, 0);
384    }
385
386    #[cfg(not(windows))]
387    #[test]
388    fn test_slice_uninit() {
389        use crate::io::read;
390        use core::mem::MaybeUninit;
391        use std::io::{Seek, SeekFrom};
392
393        // We need to obtain input stream with contents that we can compare
394        // against, so open our own source file.
395        let mut input = std::fs::File::open("src/buffer.rs").unwrap();
396
397        let mut buf = [MaybeUninit::<u8>::uninit(); 64];
398        let (init, uninit) = read(&input, &mut buf).unwrap();
399        assert_eq!(uninit.len(), 0);
400        assert_eq!(
401            &init[..58],
402            b"//! Utilities for functions that return data via buffers.\n"
403        );
404        assert_eq!(init.len(), buf.len());
405        assert_eq!(
406            unsafe { core::mem::transmute::<&mut [MaybeUninit<u8>], &mut [u8]>(&mut buf[..58]) },
407            b"//! Utilities for functions that return data via buffers.\n"
408        );
409        input.seek(SeekFrom::End(-1)).unwrap();
410        let (init, uninit) = read(&input, &mut buf).unwrap();
411        assert_eq!(init.len(), 1);
412        assert_eq!(uninit.len(), buf.len() - 1);
413        input.seek(SeekFrom::End(0)).unwrap();
414        let (init, uninit) = read(&input, &mut buf).unwrap();
415        assert_eq!(init.len(), 0);
416        assert_eq!(uninit.len(), buf.len());
417    }
418
419    #[cfg(not(windows))]
420    #[test]
421    fn test_spare_capacity() {
422        use crate::io::read;
423        use std::io::{Seek, SeekFrom};
424
425        // We need to obtain input stream with contents that we can compare
426        // against, so open our own source file.
427        let mut input = std::fs::File::open("src/buffer.rs").unwrap();
428
429        let mut buf = Vec::with_capacity(64);
430        let nread = read(&input, spare_capacity(&mut buf)).unwrap();
431        assert_eq!(nread, buf.capacity());
432        assert_eq!(nread, buf.len());
433        assert_eq!(
434            &buf[..58],
435            b"//! Utilities for functions that return data via buffers.\n"
436        );
437        buf.clear();
438        input.seek(SeekFrom::End(-1)).unwrap();
439        let nread = read(&input, spare_capacity(&mut buf)).unwrap();
440        assert_eq!(nread, 1);
441        assert_eq!(buf.len(), 1);
442        buf.clear();
443        input.seek(SeekFrom::End(0)).unwrap();
444        let nread = read(&input, spare_capacity(&mut buf)).unwrap();
445        assert_eq!(nread, 0);
446        assert!(buf.is_empty());
447    }
448}
rustix/buffer.rs

rustix/
buffer.rs
