How to read bytewise from stdin?

in, out := bufio.NewReader(os.Stdin), bufio.NewWriter(os.Stdout)
for {
    c, err := in.ReadByte()
    if err == io.EOF {
        break
    }
    out.WriteByte(c)
}

I want to read bytewise from the stdin stream. Unlike the Read methodReadByte doesn't seem to return io.EOF. How can I break if all bytes have been read?

Solution

This is not an issue of the Reader.ReadByte() implementation, nor that of bufio.NewReader().

See this example to prove it:

buf := bytes.NewBufferString("Hello World!\n")
in := bufio.NewReader(buf)
for {
    c, err := in.ReadByte()
    if err == io.EOF {
        break
    }
    fmt.Print(string(c))
}

When running, the above prints

Hello World!

And terminates properly.

Your issue is with os.Stdin. Reading from it is specific to its source. If it is your terminal, reading from it simply blocks and does not report io.EOF. See this example to prove it:

in := bufio.NewReader(os.Stdin)
for {
    fmt.Println("Reading.")
    c, err := in.ReadByte()
    if err == io.EOF {
        break
    }
    fmt.Print(string(c))
}

Its output is:

Reading.

And nothing happens. There is no new iteration, it is blocked. Now if you enter a line and press Enter, e.g. you enter Go!, output will be:

Go!
GReading.
oReading.
!Reading.

Reading.

And again, waits for new input. As you can see, data is fed / available per line. This is what your terminal does: while you enter your line, it is not sent to os.Stdin. Once you press Enter, the whole line is fed and is available from os.Stdin. This is what we see: each letter of the input Go! and a newline character. And we see the Reading. text printed for each iteration. After the input is consumed, in.ReadByte() is blocked again, waiting for new input. It does not report io.EOF.

Now try the following: create a file e.g. a.txt and edit it to have one line: Go! and a newline. Now feed this file as the standard input to your program:

go run play.go < a.txt

Running it we'll see:

Reading.
GReading.
oReading.
!Reading.

Reading.

Reading.

And it terminates so it works! It works because this time the source of os.Stdin is not your console / terminal, but the contents of a file, and once it's consumed, attempting to read from os.Stdin will properly report io.EOF.