# Trait std::io::Read

1.0.0 · source ·

    pub trait Read {
        // Required method
        fn read(&mut self, buf: &mut [u8]) -> Result<usize>;

        // Provided methods
        fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> { ... }
        fn is_read_vectored(&self) -> bool { ... }
        fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> { ... }
        fn read_to_string(&mut self, buf: &mut String) -> Result<usize> { ... }
        fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> { ... }
        fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()> { ... }
        fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()> { ... }
        fn by_ref(&mut self) -> &mut Self
           where Self: Sized { ... }
        fn bytes(self) -> Bytes<Self> ⓘ
           where Self: Sized { ... }
        fn chain<R: Read>(self, next: R) -> Chain<Self, R> ⓘ
           where Self: Sized { ... }
        fn take(self, limit: u64) -> Take<Self> ⓘ
           where Self: Sized { ... }
    }

Expand description

The `Read` trait allows for reading bytes from a source.

Implementors of the `Read` trait are called ‘readers’.

Readers are defined by one required method, `read()`. Each call to `read()` will attempt to pull bytes from this source into a provided buffer. A number of other methods are implemented in terms of `read()`, giving implementors a number of ways to read bytes while only needing to implement a single method.

Readers are intended to be composable with one another. Many implementors throughout `std::io` take and provide types which implement the `Read` trait.

Please note that each call to `read()` may involve a system call, and therefore, using something that implements `BufRead`, such as `BufReader`, will be more efficient.

## Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let mut f = File::open("foo.txt")?;
        let mut buffer = [0; 10];

        // read up to 10 bytes
        f.read(&mut buffer)?;

        let mut buffer = Vec::new();
        // read the whole file
        f.read_to_end(&mut buffer)?;

        // read into a String, so that you don't need to do the conversion.
        let mut buffer = String::new();
        f.read_to_string(&mut buffer)?;

        // and more! See the other methods for more details.
        Ok(())
    }

Run

Read from `&str` because `&[u8]` implements `Read`:

    use std::io::prelude::*;

    fn main() -> io::Result<()> {
        let mut b = "This string will be read".as_bytes();
        let mut buffer = [0; 10];

        // read up to 10 bytes
        b.read(&mut buffer)?;

        // etc... it works exactly as a File does!
        Ok(())
    }

Run

## Required Methods§

source

#### fn read(&mut self, buf: &mut [u8]) -> Result<usize>

Pull some bytes from this source into the specified buffer, returning how many bytes were read.

This function does not provide any guarantees about whether it blocks waiting for data, but if an object needs to block for a read and cannot, it will typically signal this via an `Err` return value.

If the return value of this method is `Ok(n)`, then implementations must guarantee that `0 <= n <= buf.len()`. A nonzero `n` value indicates that the buffer `buf` has been filled in with `n` bytes of data from this source. If `n` is `0`, then it can indicate one of two scenarios:

1. This reader has reached its “end of file” and will likely no longer be able to produce bytes. Note that this does not mean that the reader will always no longer be able to produce bytes. As an example, on Linux, this method will call the `recv` syscall for a `TcpStream`, where returning zero indicates the connection was shut down correctly. While for `File`, it is possible to reach the end of file and get zero as result, but if more data is appended to the file, future calls to `read` will return more data.
2. The buffer specified was 0 bytes in length.

It is not an error if the returned value `n` is smaller than the buffer size, even when the reader is not at the end of the stream yet. This may happen for example because fewer bytes are actually available right now (e. g. being close to end-of-file) or because read() was interrupted by a signal.

As this trait is safe to implement, callers in unsafe code cannot rely on `n <= buf.len()` for safety. Extra care needs to be taken when `unsafe` functions are used to access the read bytes. Callers have to ensure that no unchecked out-of-bounds accesses are possible even if `n > buf.len()`.

No guarantees are provided about the contents of `buf` when this function is called, so implementations cannot rely on any property of the contents of `buf` being true. It is recommended that implementations only write data to `buf` instead of reading its contents.

Correspondingly, however, callers of this method in unsafe code must not assume any guarantees about how the implementation uses `buf`. The trait is safe to implement, so it is possible that the code that’s supposed to write to the buffer might also read from it. It is your responsibility to make sure that `buf` is initialized before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one obtains via `MaybeUninit<T>`) is not safe, and can lead to undefined behavior.

##### Errors

If this function encounters any form of I/O or other error, an error variant will be returned. If an error is returned then it must be guaranteed that no bytes were read.

An error of the `ErrorKind::Interrupted` kind is non-fatal and the read operation should be retried if there is nothing else to do.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let mut f = File::open("foo.txt")?;
        let mut buffer = [0; 10];

        // read up to 10 bytes
        let n = f.read(&mut buffer[..])?;

        println!("The bytes: {:?}", &buffer[..n]);
        Ok(())
    }

Run

## Provided Methods§

1.36.0 · source

#### fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like `read`, except that it reads into a slice of buffers.

Data is copied to fill each buffer in order, with the final buffer written to possibly being only partially filled. This method must behave equivalently to a single call to `read` with concatenated buffers.

The default implementation calls `read` with either the first nonempty buffer provided, or an empty one if none exists.

source

#### fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)

Determines if this `Read`er has an efficient `read_vectored` implementation.

If a `Read`er does not override the default `read_vectored` implementation, code using it may want to avoid the method all together and coalesce writes into a single buffer for higher performance.

The default implementation returns `false`.

source

#### fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Read all bytes until EOF in this source, placing them into `buf`.

All bytes read from this source will be appended to the specified buffer `buf`. This function will continuously call `read()` to append more data to `buf` until `read()` returns either `Ok(0)` or an error of non-`ErrorKind::Interrupted` kind.

If successful, this function will return the total number of bytes read.

##### Errors

If this function encounters an error of the kind `ErrorKind::Interrupted` then the error is ignored and the operation will continue.

If any other read error is encountered then this function immediately returns. Any bytes which have already been read will be appended to `buf`.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let mut f = File::open("foo.txt")?;
        let mut buffer = Vec::new();

        // read the whole file
        f.read_to_end(&mut buffer)?;
        Ok(())
    }

Run

(See also the `std::fs::read` convenience function for reading from a file.)

source

#### fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Read all bytes until EOF in this source, appending them to `buf`.

If successful, this function returns the number of bytes which were read and appended to `buf`.

##### Errors

If the data in this stream is not valid UTF-8 then an error is returned and `buf` is unchanged.

See `read_to_end` for other error semantics.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let mut f = File::open("foo.txt")?;
        let mut buffer = String::new();

        f.read_to_string(&mut buffer)?;
        Ok(())
    }

Run

(See also the `std::fs::read_to_string` convenience function for reading from a file.)

1.6.0 · source

#### fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Read the exact number of bytes required to fill `buf`.

This function reads as many bytes as necessary to completely fill the specified buffer `buf`.

No guarantees are provided about the contents of `buf` when this function is called, so implementations cannot rely on any property of the contents of `buf` being true. It is recommended that implementations only write data to `buf` instead of reading its contents. The documentation on `read` has a more detailed explanation on this subject.

##### Errors

If this function encounters an error of the kind `ErrorKind::Interrupted` then the error is ignored and the operation will continue.

If this function encounters an “end of file” before completely filling the buffer, it returns an error of the kind `ErrorKind::UnexpectedEof`. The contents of `buf` are unspecified in this case.

If any other read error is encountered then this function immediately returns. The contents of `buf` are unspecified in this case.

If this function returns an error, it is unspecified how many bytes it has read, but it will never read more than would be necessary to completely fill the buffer.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let mut f = File::open("foo.txt")?;
        let mut buffer = [0; 10];

        // read exactly 10 bytes
        f.read_exact(&mut buffer)?;
        Ok(())
    }

Run
source

#### fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (`read_buf` #78485)

Pull some bytes from this source into the specified buffer.

This is equivalent to the `read` method, except that it is passed a `BorrowedCursor` rather than `[u8]` to allow use with uninitialized buffers. The new data will be appended to any existing contents of `buf`.

The default implementation delegates to `read`.

source

#### fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (`read_buf` #78485)

Read the exact number of bytes required to fill `cursor`.

This is similar to the `read_exact` method, except that it is passed a `BorrowedCursor` rather than `[u8]` to allow use with uninitialized buffers.

##### Errors

If this function encounters an error of the kind `ErrorKind::Interrupted` then the error is ignored and the operation will continue.

If this function encounters an “end of file” before completely filling the buffer, it returns an error of the kind `ErrorKind::UnexpectedEof`.

If any other read error is encountered then this function immediately returns.

If this function returns an error, all bytes read will be appended to `cursor`.

source

#### fn by_ref(&mut self) -> &mut Self where Self: Sized,

Creates a “by reference” adaptor for this instance of `Read`.

The returned adapter also implements `Read` and will simply borrow this current reader.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::Read;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let mut f = File::open("foo.txt")?;
        let mut buffer = Vec::new();
        let mut other_buffer = Vec::new();

        {
            let reference = f.by_ref();

            // read at most 5 bytes
            reference.take(5).read_to_end(&mut buffer)?;

        } // drop our &mut reference so we can use f again

        // original file still usable, read the rest
        f.read_to_end(&mut other_buffer)?;
        Ok(())
    }

Run
source

#### fn bytes(self) -> Bytes<Self> ⓘwhere Self: Sized,

Transforms this `Read` instance to an `Iterator` over its bytes.

The returned type implements `Iterator` where the `Item` is `Result<u8, io::Error>`. The yielded item is `Ok` if a byte was successfully read and `Err` otherwise. EOF is mapped to returning `None` from this iterator.

The default implementation calls `read` for each byte, which can be very inefficient for data that’s not in memory, such as `File`. Consider using a `BufReader` in such cases.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::io::BufReader;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let f = BufReader::new(File::open("foo.txt")?);

        for byte in f.bytes() {
            println!("{}", byte.unwrap());
        }
        Ok(())
    }

Run
source

#### fn chain<R: Read>(self, next: R) -> Chain<Self, R> ⓘwhere Self: Sized,

Creates an adapter which will chain this stream with another.

The returned `Read` instance will first read all bytes from this object until EOF is encountered. Afterwards the output is equivalent to the output of `next`.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let f1 = File::open("foo.txt")?;
        let f2 = File::open("bar.txt")?;

        let mut handle = f1.chain(f2);
        let mut buffer = String::new();

        // read the value into a String. We could use any Read method here,
        // this is just one example.
        handle.read_to_string(&mut buffer)?;
        Ok(())
    }

Run
source

#### fn take(self, limit: u64) -> Take<Self> ⓘwhere Self: Sized,

Creates an adapter which will read at most `limit` bytes from it.

This function returns a new instance of `Read` which will read at most `limit` bytes, after which it will always return EOF (`Ok(0)`). Any read errors will not count towards the number of bytes read and future calls to `read()` may succeed.

##### Examples

`File`s implement `Read`:

    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let f = File::open("foo.txt")?;
        let mut buffer = [0; 5];

        // read at most five bytes
        let mut handle = f.take(5);

        handle.read(&mut buffer)?;
        Ok(())
    }

Run

## Implementors§

source§

### impl Read for &File

source§

### impl Read for &TcpStream

source§

### impl Read for &[u8]

Read is implemented for `&[u8]` by copying from the slice.

Note that reading updates the slice to point to the yet unread part. The slice will be empty when EOF is reached.

source§

### impl Read for File

source§

### impl Read for TcpStream

1.10.0 · source§

### impl Read for UnixStream

Available on Unix only.
source§

### impl Read for ChildStderr

source§

### impl Read for ChildStdout

1.73.0 · source§

### impl Read for Arc<File>

source§

### impl Read for Empty

source§

### impl Read for Repeat

source§

### impl Read for Stdin

source§

### impl Read for StdinLock<'_>

1.10.0 · source§

### impl<'a> Read for &'a UnixStream

Available on Unix only.
1.63.0 · source§

### impl<A: Allocator> Read for VecDeque<u8, A>

Read is implemented for `VecDeque<u8>` by consuming bytes from the front of the `VecDeque`.

source§

### impl<R: Read + ?Sized> Read for &mut R

source§

### impl<R: Read + ?Sized> Read for Box<R>

source§

### impl<R: ?Sized + Read> Read for BufReader<R>

source§

### impl<T> Read for Cursor<T>where T: AsRef<[u8]>,

source§

### impl<T: Read> Read for Take<T>

source§

### impl<T: Read, U: Read> Read for Chain<T, U>