io.Copy

Learn to transfer data with io.Copy.

io.Copy

io.Copy(dst, src) streams data from a Reader to a Writer: it works in 32 KB chunks without loading everything into memory. Even gigabyte files are copied with constant memory.

src, _ := os.Open("buyuk.iso")
defer src.Close()
dst, _ := os.Create("kopya.iso")
defer dst.Close()
n, err := io.Copy(dst, src)
fmt.Println(n, "bayt kopyalandı")

Stream chains

Combining Readers and Writers performs complex work in a single pass. For example, compressing a file while computing its hash at the same time:

h := sha256.New()
gz := gzip.NewWriter(io.MultiWriter(cikis, h))
io.Copy(gz, giris)
gz.Close()
fmt.Printf("%x\n", h.Sum(nil))

io.TeeReader(r, w) writes what it reads to a Writer at the same time.

Exercise Copy and hash

Copy the contents of strings.NewReader("gopher") into a bytes.Buffer with io.Copy while also writing to a sha256.New() hash through io.TeeReader. Print the number of bytes copied and the first 4 bytes of the hash with %x.

Show example solution

package main

import (
    "bytes"
    "crypto/sha256"
    "fmt"
    "io"
    "strings"
)

func main() {
    h := sha256.New()
    var buf bytes.Buffer
    n, err := io.Copy(&buf, io.TeeReader(strings.NewReader("gopher"), h))
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Printf("%d %x\n", n, h.Sum(nil)[:4])
}

Test yourself

Which uses the least memory to send a large file in an HTTP response?
- os.ReadFile then Write
- Converting the file to a string
- io.Copy(w, f)
- b, _ := io.ReadAll(f); w.Write(b)
Show answer

io.Copy(w, f) — io.Copy streams piece by piece; ReadAll and ReadFile load the whole file into memory.

Copy by streaming

n, err := io.____(os.Stdout, strings.NewReader("merhaba"))

Show answer

n, err := io.Copy(os.Stdout, strings.NewReader("merhaba"))

Helper and role

io.Copy

Streaming copy
io.TeeReader

Duplicate while reading
io.MultiWriter

Duplicate while writing
io.LimitReader

Byte limit

Next lesson: Summary - io package

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies