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Table of Contents
What is io.Reader?
What is io.Writer?
How do Reader and Writer work together?
Some tips in actual development
Home Backend Development Golang Go io.Reader and io.Writer explained

Go io.Reader and io.Writer explained

Jul 10, 2025 pm 01:13 PM
go io

io.Reader and io.Writer are the core interfaces for handling I/O operations in Go. 1. io.Reader is used to read bytes from data sources, and commonly used implementations include files, buffers and network connections; 2. io.Writer is used to write bytes to targets, such as files, buffers or network connections; 3. Both are often used together, such as data replication through io.Copy; 4. The standard library provides a variety of packaging tools, such as buffered read and write, restricted read size and multiplexing, etc., to improve flexibility and performance. Mastering them can effectively deal with various I/O scenarios.

Go io.Reader and io.Writer explained

It is actually not difficult to understand io.Reader and io.Writer in Go. They are one of the most basic and commonly used interfaces in the Go standard library, and many I/O operations revolve around these two interfaces.

Go io.Reader and io.Writer explained

Simply put:

  • io.Reader is the interface for reading data.
  • io.Writer is the interface for writing data.

Mastering the usage of these two interfaces can make you more comfortable when handling file, network requests, memory operations and other scenarios.

Go io.Reader and io.Writer explained

What is io.Reader?

There is only one method for the io.Reader interface:

 Read(p []byte) (n int, err error)

Its function is to read content from a data source and put the read data into the passed byte slice. The return value includes how many bytes were read and possible errors (such as reading to the end).

Go io.Reader and io.Writer explained

Common types of implementing this interface are:

  • *os.File
  • bytes.Buffer
  • http.Request.Body
  • net.Conn

For example: Read content from a string

 r := strings.NewReader("hello world")
buf := make([]byte, 5)
n, _ := r.Read(buf)
fmt.Println(string(buf[:n])) // Output hello

A few points to note:

  • The Read method does not read all the data at once, but reads it according to the buffer size you provide.
  • You need to loop to call Read until it returns io.EOF to indicate that it has finished reading.
  • Don't repeat the wheel yourself. There are many encapsulated functions in the standard library to help you do this, such as ioutil.ReadAll(r) that can read everything at once.

What is io.Writer?

io.Writer is also very simple, with only one method:

 Write(p []byte) (n int, err error)

It is responsible for writing incoming byte slices to a target, such as a file, network connection, or memory buffer.

Common types that implement this interface include:

  • *os.File
  • bytes.Buffer
  • http.ResponseWriter
  • net.Conn

For example: Write content to memory buffer

 var buf bytes.Buffer
buf.Write([]byte("hello "))
buf.Write([]byte("world"))
fmt.Println(buf.String()) // Output hello world

Note here:

  • The Write method will try to write all data, but it does not guarantee that all of them will be successfully written (although most of the time is no problem).
  • If you want to make sure the write is complete, you can check the return value or use a packaged function such as ioutil.WriteFile to write the file.

How do Reader and Writer work together?

One of Go's design philosophy is combination priority. You will find that many tool functions accept Reader and Writer as parameters.

For example, classic functions for copying data:

 io.Copy(dst Writer, src Reader)

It can be used to:

  • Copy the file contents to another file
  • Write HTTP response body to local file
  • Write string content to memory buffer

Example:

 r := strings.NewReader("copy this data")
var w bytes.Buffer
io.Copy(&w, r)
fmt.Println(w.String()) // Output copy this data

This "plug-and-un" design makes code reuse very convenient.


Some tips in actual development

In actual projects, you will often encounter situations where you need to convert or encapsulate Reader and Writer . Here are a few common practices:

  • Limit read size : Using io.LimitReader(r, n) can prevent too much content from being read.
  • Buffered read and write : use bufio.Reader or bufio.Writer to improve performance.
  • Multiplexing : Use io.MultiWriter to write data to multiple places at the same time (such as log writing to files and consoles).
  • Empty device : Sometimes you just want to discard the output, you can use io.Discard , a special Writer .

These tools are all in the standard library and do not require additional introduction.


Basically that's it. Once you master io.Reader and io.Writer , you will master the core ideas of I/O operations in Go. They seem simple, but when used properly, they can write very flexible and general code.

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