Reading specific number of bytes from a buffered reader in golang

I am aware of the specific function in golang from the bufio package.

func (b *Reader) Peek(n int) ([]byte, error)

> Peek returns the next n bytes without advancing the reader. The bytes
> stop being valid at the next read call. If Peek returns fewer than n
> bytes, it also returns an error explaining why the read is short. The
> error is ErrBufferFull if n is larger than b's buffer size.

I need to be able to read a specific number of bytes from a Reader that will advance the reader. Basically, identical to the function above, but it advances the reader. Does anybody know how to accomplish this?

Note that the bufio.Read method calls the underlying io.Read at most once, meaning that it can return n < len(p), without reaching EOF. If you want to read exactly len(p) bytes or fail with an error, you can use io.ReadFull like this:

n, err := io.ReadFull(reader, p)

This works even if the reader is buffered.

func (b *Reader) Read(p []byte) (n int, err error)

http://golang.org/pkg/bufio/#Reader.Read

The number of bytes read will be limited to len(p)

TLDR:

my42bytes, err := ioutil.ReadAll(io.LimitReader(myReader, 42))

Full answer:

@monicuta mentioned io.ReadFull which works great. Here I provide another method. It works by chaining ioutil.ReadAll and io.LimitReader together. Let's read the doc first:

> $ go doc ioutil.ReadAll
> func ReadAll(r io.Reader) ([]byte, error)
> ReadAll reads from r until an error or EOF and returns the data it read. A
> successful call returns err == nil, not err == EOF. Because ReadAll is
> defined to read from src until EOF, it does not treat an EOF from Read as an
> error to be reported.
>
> $ go doc io.LimitReader
> func LimitReader(r Reader, n int64) Reader
> LimitReader returns a Reader that reads from r but stops with EOF after n
> bytes. The underlying implementation is a *LimitedReader.

So if you want to get 42 bytes from myReader, you do this

import (
        "io"
        "io/ioutil"
)

func main() {
        // myReader := ...
        my42bytes, err := ioutil.ReadAll(io.LimitReader(myReader, 42))
        if err != nil {
                panic(err)
        }
        //...
}

Here is the equivalent code with io.ReadFull
> $ go doc io.ReadFull
> func ReadFull(r Reader, buf []byte) (n int, err error)
> ReadFull reads exactly len(buf) bytes from r into buf. It returns the number
> of bytes copied and an error if fewer bytes were read. The error is EOF only
> if no bytes were read. If an EOF happens after reading some but not all the
> bytes, ReadFull returns ErrUnexpectedEOF. On return, n == len(buf) if and
> only if err == nil. If r returns an error having read at least len(buf)
> bytes, the error is dropped.

import (
        "io"
)

func main() {
        // myReader := ...
        buf := make([]byte, 42)
        _, err := io.ReadFull(myReader, buf)
        if err != nil {
                panic(err)
        }
        //...
}

Compared to io.ReadFull, an advantage is that you don't need to manually make a buf, where len(buf) is the number of bytes you want to read, then pass buf as an argument when you Read

Instead you simply tell io.LimitReader you want at most 42 bytes from myReader, and call ioutil.ReadAll to read them all, returning the result as a slice of bytes. If successful, the returned slice is guaranteed to be of length 42.

I am prefering Read() especially if you are going to read any type of files and it could be also useful in sending data in chunks, below is an example to show how it is used

fs, err := os.Open("fileName"); 

if err != nil{
	fmt.Println("error reading file")
	return
}

defer fs.Close()

reader := bufio.NewReader(fs)

buf := make([]byte, 1024)

for{
	v, _ := reader.Read(buf) //ReadString and ReadLine() also applicable or alternative
	
	if v == 0{
		return
	}
	//in case it is a string file, you could check its content here...
	fmt.Print(string(buf))
}

Pass a n-bytes sized buffer to the reader.