# Trait std::io::Read [−] [src]

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

        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 by_ref(&mut self) -> &mut Self where Self: Sized { ... }
        fn bytes(self) -> Bytes<Self> where Self: Sized { ... }
        fn chars(self) -> Chars<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 { ... }
        fn tee<W: Write>(self, out: W) -> Tee<Self, W> where Self: Sized { ... }
    }

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

Implementors of the `Read` trait are sometimes 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`:

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); let mut buffer = [0; 10]; // read up to 10 bytes try!(f.read(&mut buffer)); let mut buffer = vec![0; 10]; // read the whole file try!(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(); try!(f.read_to_string(&mut buffer)); // and more! See the other methods for more details. Ok(()) } }

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

    let mut f = try!(File::open("foo.txt"));
    let mut buffer = [0; 10];

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

    let mut buffer = vec![0; 10];
    // read the whole file
    try!(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();
    try!(f.read_to_string(&mut buffer));

    // and more! See the other methods for more details.

## 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 but cannot it will typically signal this via an `Err` return value.

If the return value of this method is `Ok(n)`, then it must be guaranteed 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.
2. The buffer specified was 0 bytes in length.

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.

# 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.

# Examples

`File`s implement `Read`:

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); let mut buffer = [0; 10]; // read 10 bytes try!(f.read(&mut buffer[..])); Ok(()) } }

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

    let mut f = try!(File::open("foo.txt"));
    let mut buffer = [0; 10];

    // read 10 bytes
    try!(f.read(&mut buffer[..]));

## Provided Methods

### `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`:

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); let mut buffer = Vec::new(); // read the whole file try!(f.read_to_end(&mut buffer)); Ok(()) } }

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

    let mut f = try!(File::open("foo.txt"));
    let mut buffer = Vec::new();

    // read the whole file
    try!(f.read_to_end(&mut buffer));

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

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

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); let mut buffer = String::new(); try!(f.read_to_string(&mut buffer)); Ok(()) } }

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

    let mut f = try!(File::open("foo.txt"));
    let mut buffer = String::new();

    try!(f.read_to_string(&mut buffer));

### `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.

# 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`:

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); let mut buffer = [0; 10]; // read exactly 10 bytes try!(f.read_exact(&mut buffer)); Ok(()) } }

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

    let mut f = try!(File::open("foo.txt"));
    let mut buffer = [0; 10];

    // read exactly 10 bytes
    try!(f.read_exact(&mut buffer));

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

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

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

# Examples

`File`s implement `Read`:

fn main() { use std::io; use std::io::Read; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(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 try!(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 try!(f.read_to_end(&mut other_buffer)); Ok(()) } }

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

    let mut f = try!(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
        try!(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
    try!(f.read_to_end(&mut other_buffer));

### `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, R::Err>`. The yielded item is `Ok` if a byte was successfully read and `Err` otherwise for I/O errors. EOF is mapped to returning `None` from this iterator.

# Examples

`File`s implement `Read`:

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); for byte in f.bytes() { println!("{}", byte.unwrap()); } Ok(()) } }

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

    let mut f = try!(File::open("foo.txt"));

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

### `fn chars(self) -> Chars<Self> where Self: Sized`

Unstable (`io` #27802)

: the semantics of a partial read/write of where errors happen is currently unclear and may change

Transforms this `Read` instance to an `Iterator` over `char`s.

This adaptor will attempt to interpret this reader as a UTF-8 encoded sequence of characters. The returned iterator will return `None` once EOF is reached for this reader. Otherwise each element yielded will be a `Result<char, E>` where `E` may contain information about what I/O error occurred or where decoding failed.

Currently this adaptor will discard intermediate data read, and should be avoided if this is not desired.

# Examples

`File`s implement `Read`:

#![feature(io)] fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); for c in f.chars() { println!("{}", c.unwrap()); } Ok(()) } }

    #![feature(io)]
    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    let mut f = try!(File::open("foo.txt"));

    for c in f.chars() {
        println!("{}", c.unwrap());
    }

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

Creates an adaptor 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`:

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f1 = try!(File::open("foo.txt")); let mut f2 = try!(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. try!(handle.read_to_string(&mut buffer)); Ok(()) } }

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

    let mut f1 = try!(File::open("foo.txt"));
    let mut f2 = try!(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.
    try!(handle.read_to_string(&mut buffer));

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

Creates an adaptor 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`:

fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); let mut buffer = [0; 5]; // read at most five bytes let mut handle = f.take(5); try!(handle.read(&mut buffer)); Ok(()) } }

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

    let mut f = try!(File::open("foo.txt"));
    let mut buffer = [0; 5];

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

    try!(handle.read(&mut buffer));

### `fn tee<W: Write>(self, out: W) -> Tee<Self, W> where Self: Sized`

Deprecated since 1.6.0

: error handling semantics unclear and don't seem to have an ergonomic resolution

Creates a reader adaptor which will write all read data into the given output stream.

Whenever the returned `Read` instance is read it will write the read data to `out`. The current semantics of this implementation imply that a `write` error will not report how much data was initially read.

# Examples

`File`s implement `Read`:

#![feature(io)] fn main() { use std::io; use std::io::prelude::*; use std::fs::File; fn foo() -> io::Result<()> { let mut f = try!(File::open("foo.txt")); let mut buffer1 = Vec::with_capacity(10); let mut buffer2 = Vec::with_capacity(10); // write the output to buffer1 as we read let mut handle = f.tee(&mut buffer1); try!(handle.read(&mut buffer2)); Ok(()) } }

    #![feature(io)]
    use std::io;
    use std::io::prelude::*;
    use std::fs::File;

    let mut f = try!(File::open("foo.txt"));
    let mut buffer1 = Vec::with_capacity(10);
    let mut buffer2 = Vec::with_capacity(10);

    // write the output to buffer1 as we read
    let mut handle = f.tee(&mut buffer1);

    try!(handle.read(&mut buffer2));

## Implementors

- `impl Read for File`
- `impl<'a> Read for &'a File`
- `impl<R: Read> Read for BufReader<R>`
- `impl<T> Read for Cursor<T> where T: AsRef<[u8]>`
- `impl<'a, R: Read + ?Sized> Read for &'a mut R`
- `impl<R: Read + ?Sized> Read for Box<R>`
- `impl<'a> Read for &'a [u8]`
- `impl Read for Empty`
- `impl Read for Repeat`
- `impl Read for Stdin`
- `impl<'a> Read for StdinLock<'a>`
- `impl<T: Read, U: Read> Read for Chain<T, U>`
- `impl<T: Read> Read for Take<T>`
- `impl<R: Read, W: Write> Read for Tee<R, W>`
- `impl Read for TcpStream`
- `impl<'a> Read for &'a TcpStream`
- `impl Read for ChildStdout`
- `impl Read for ChildStderr`

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