io.Reader
- Go course
- Intermediate
- Lesson 35/80
- 4 min
Learn the Reader interface and the Read method.
io.Reader
io.Reader is one of Go's most important interfaces; it has a single method:
type Reader interface {
Read(p []byte) (n int, err error)
}Read writes at most len(p) bytes into the given slice and returns how many it wrote. When data runs out it returns io.EOF. Files, network connections, HTTP bodies, strings.Reader and bytes.Buffer are all Readers, so the same code works on all of them.
io.ReadAll and wrappers
To read small data entirely, io.ReadAll(r) is enough. Readers can wrap each other: io.LimitReader(r, n) reads at most n bytes, io.MultiReader(a, b) reads one after another, and gzip.NewReader(r) decompresses data.
r := io.LimitReader(strings.NewReader("merhaba dünya"), 7)
b, _ := io.ReadAll(r)
fmt.Println(string(b)) // merhabaLimiting an incoming body with LimitReader prevents memory attacks.
Practice Read in chunks
Read from strings.NewReader("gopher") in a loop with a 4-byte buffer. Print the piece read each time; leave the loop on io.EOF.
Starter code
package main
import (
"fmt"
"io"
"strings"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"fmt"
"io"
"strings"
)
func main() {
r := strings.NewReader("gopher")
buf := make([]byte, 4)
for {
n, err := r.Read(buf)
if n > 0 {
fmt.Println(string(buf[:n]))
}
if err == io.EOF {
break
}
}
}Expected output
goph er
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 If <code>Read</code> returns both <code>n > 0</code> and <code>io.EOF</code>, what should you do?
- Panic as an error
- Read again
- Process the n bytes first, then stop reading Correct answer
- Discard the bytes
Explanation: By contract, bytes read are always processed first; EOF only says no more data remains.
2 Data ended
n, err := r.Read(buf)
if err == io.____ {
break
}n, err := r.Read(buf)
if err == io.EOF {
break
}Explanation: io.EOF is the special error reporting that no data is left to read.
3 Reader type
| Term | Meaning |
|---|---|
os.File |
File |
strings.Reader |
Text |
http.Response.Body |
HTTP body |
io.LimitReader |
Byte limit |