Trait Read 

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

    // Provided methods
    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> { ... }
    fn by_ref(&mut self) -> &mut Self
       where Self: Sized { ... }
}

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

Files 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(())
}

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(())
}

Required Methods

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 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, 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 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

Files 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(())
}

Provided Methods

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, 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

Files 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(())
}

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

Files 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(())
}

Implementations on Foreign Types

impl Read for &[u8]

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

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

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

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

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

Implementors