Go Academy · Lesson

io Utility Functions

Copy, TeeReader, MultiWriter.

io Utility Functions is a free Go Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

The io Toolbox

The io package ships small, powerful helpers that combine readers and writers: Copy, TeeReader, MultiWriter, and more.

io.Copy

io.Copy(dst, src) streams all bytes from a reader to a writer and returns the count.

package main

import (
	"fmt"
	"io"
	"os"
	"strings"
)

func main() {
	n, _ := io.Copy(os.Stdout, strings.NewReader("copied\n"))
	fmt.Println("bytes:", n)
}

io.CopyN

io.CopyN(dst, src, n) copies exactly n bytes — useful for reading a known-length prefix.

package main

import (
	"io"
	"os"
	"strings"
)

func main() {
	io.CopyN(os.Stdout, strings.NewReader("abcdef"), 3)
	os.Stdout.WriteString("\n")
}

io.MultiWriter

io.MultiWriter fans one write out to several writers at once — like the Unix tee command.

package main

import (
	"bytes"
	"fmt"
	"io"
	"os"
)

func main() {
	var buf bytes.Buffer
	w := io.MultiWriter(os.Stdout, &buf)
	io.WriteString(w, "to both\n")
	fmt.Println("buffer copy:", buf.String())
}

io.TeeReader

io.TeeReader(r, w) returns a reader that copies everything it reads into w — perfect for logging or hashing data as it flows.

package main

import (
	"bytes"
	"fmt"
	"io"
	"strings"
)

func main() {
	var spy bytes.Buffer
	tee := io.TeeReader(strings.NewReader("stream"), &spy)
	io.ReadAll(tee)
	fmt.Println("observed:", spy.String())
}

io.MultiReader

io.MultiReader concatenates several readers into one continuous stream.

package main

import (
	"fmt"
	"io"
	"strings"
)

func main() {
	r := io.MultiReader(strings.NewReader("Hello, "), strings.NewReader("World"))
	data, _ := io.ReadAll(r)
	fmt.Println(string(data))
}

io.Discard

io.Discard is a writer that throws everything away — useful to drain a reader you do not need.

package main

import (
	"fmt"
	"io"
	"strings"
)

func main() {
	n, _ := io.Copy(io.Discard, strings.NewReader("ignored"))
	fmt.Println("discarded:", n, "bytes")
}

Hashing with TeeReader

A classic combo: read a stream for its real purpose while a TeeReader feeds the same bytes into a hash, so you compute a checksum in one pass.

io.Pipe

io.Pipe connects a writer to a reader in memory, often across goroutines, for streaming producer/consumer patterns.

Choosing the Right Helper

Move data: Copy / CopyN
Duplicate output: MultiWriter
Observe input: TeeReader
Join inputs: MultiReader
Drain input: Discard

Composability

These helpers all speak io.Reader/io.Writer, so they nest and chain freely to build sophisticated pipelines from tiny parts.

Quick Check

Test your io utilities knowledge.

Recap

You learned the io utility helpers.

Copy / CopyN stream data
MultiWriter duplicates output
TeeReader observes input
MultiReader joins, Discard drains
All compose via the Reader/Writer interfaces

Frequently asked questions

Is the “io Utility Functions” lesson free?

Yes — the full text of “io Utility Functions” is free to read here on the web, and the Go Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Go Academy course, upgrade to CoddyKit PRO.

What will I learn in “io Utility Functions”?

Copy, TeeReader, MultiWriter. You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Go Academy?

No prior experience is required. Go Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “io Utility Functions” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Go Academy lesson?

Yes. Every Go Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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