Go Package io

External

• https://golang.org/pkg/io/
• https://golang.org/pkg/io/ioutil/

Internal

• Standard Library
• File Operations in Go

Overview

The io package consists of a few functions, but mostly interfaces used by other packages.

io.EOF

The error returned by I/O operations that encounter the end of file. It can be tested with:

_, err := ...
if errors.Is(err, io.EOF) {
  // is EOF 
  ...
}

io.Reader

https://golang.org/pkg/io/#Reader

An interface that exposes Read().

Readers can be used to unmarshall JSON into structs as shown here:

Unmarshalling JSON from a Reader into a struct

Files opened with os.Create() implement io.Reader.

Wrapping a []byte into a Reader

A []byte can be wrapped into a Reader with:

import "bytes"

bs := []byte("something")
r := bytes.NewReader(bs)

Getting a Reader from a string

s := "some string"
reader := strings.NewReader(s)

io.Writer

https://golang.org/pkg/io/#Writer

io.Writer is the interface that wraps the basic Write method:

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

Write() writes the given bytes to the underlying data stream.

Files opened with os.Create() implement io.Writer.

Capturing the Content Written into a Writer and Converting to String

bytes.Buffer implements io.Writer so a bytes.Buffer instance can be used to be written into and get the string representation. The following example dumps the content of a net/http.Request into a string, identically with how it will be sent over the wire:

var r = &http.Request{...}

b := &bytes.Buffer{}
if err := r.Write(b); err != nil { ... }
s := b.String()

Closer

https://golang.org/pkg/io/#Closer

Files opened with os.Create() implement io.Closer.

ReadCloser

An interface that exposes basic Read() and Close() methods. The http.Request body is a ReadCloser. See Reader.