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.

Repeated calls to the reader use the same cursor, so for example calling read_to_end twice on a File will only return the file’s contents once. It’s recommended to first call rewind() in that case.

§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(())
}

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§

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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:

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.
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

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(())
}

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 Reader has an efficient read_vectored implementation.

If a Reader 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.

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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

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 = 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.)

§Implementing `read_to_end`

When implementing the io::Read trait, it is recommended to allocate memory using Vec::try_reserve. However, this behavior is not guaranteed by all implementations, and read_to_end may not handle out-of-memory situations gracefully.

fn read_to_end(&mut self, dest_vec: &mut Vec<u8>) -> io::Result<usize> {
    let initial_vec_len = dest_vec.len();
    loop {
        let src_buf = self.example_datasource.fill_buf()?;
        if src_buf.is_empty() {
            break;
        }
        dest_vec.try_reserve(src_buf.len()).map_err(|_| io::ErrorKind::OutOfMemory)?;
        dest_vec.extend_from_slice(src_buf);

        // Any irreversible side effects should happen after `try_reserve` succeeds,
        // to avoid losing data on allocation error.
        let read = src_buf.len();
        self.example_datasource.consume(read);
    }
    Ok(dest_vec.len() - initial_vec_len)
}

Run

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

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 = 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

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(())
}

Run

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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.

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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.

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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(())
}

Run

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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

Files 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

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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

Files 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

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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

Files 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(())
}