How to use http Response method Read in go?

Question

It's been a while since I dealt with go and I seem to have forgotten how to read the docs.

If I do a get request like this...

resp, err := http.Get("https://example.com")

I then see that the response has a Body that is of an io.ReadCloser type (https://golang.org/pkg/io/#ReadCloser). I see the ReadCloser is an interface to Reader and Closer.

When I look at the Reader interface I see it has a Read method that reads bytes into p (https://golang.org/pkg/io/#Reader).

To recap, the http Response has a body which is a io.ReadCloser Interface which itself contains a Reader interface which has a Read method.

When I try this I would expect to Read the response bytes into HTML, but I see nothing...

var html []byte
num, err := resp.Body.Read(html)
fmt.Println("\n\thtml: ", html)
fmt.Printf("\n\tnum: %v, err: %v\n", num, err)

Output:

html:  []
num: 0, err: <nil>

What am I missing here?

You need to allocate space to read into. You're only providing a nil slice, so Read can't read anything. — Mr_Pink
– Mr_Pink, Commented Jan 16, 2017 at 3:05
Read reads up to len(html) bytes into html , here it is zero — Sarath Sadasivan Pillai
– Sarath Sadasivan Pillai, Commented Jan 16, 2017 at 3:15

Community · Accepted Answer · 2025-04-29 16:22:38Z

short answer: You are passing a slice html where its len would be '0' and Read reads up to len(html) bytes into html

Here is snippet from https://golang.org/pkg/io/#Reader

Read reads up to len(p) bytes into p. It returns the number of bytes read (0 <= n <= len(p)) and any error encountered. Even if Read returns n < len(p), it may use all of p as scratch space during the call. If some data is available but not len(p) bytes, Read conventionally returns what is available instead of waiting for more.

Now what you may do use io and update the code as

body, err := io.ReadAll(resp.Body)

Code

package main

import (
    "fmt"
    "io"
    "net/http"
)

func main() {
    resp, err := http.Get("http://httpbin.org/ip")
    if err != nil {
        fmt.Println("Error making request", err.Error())
        return
    }
    defer resp.Body.Close()
    body, err := io.ReadAll(resp.Body)
    if err != nil {
        fmt.Println("Error reading response", err.Error())
        return
    }
    fmt.Println("Response\n", string(body))
}

Here is play link, in go playground HTTP requests won't work it is a security precaution so you may test this in your machine

Alex Pliutau · Accepted Answer · 2017-01-16 02:52:11Z

1

Try to use "io/ioutil" package:

import "io/ioutil"
...
body, err := ioutil.ReadAll(resp.Body)
resp.Body.Close()
fmt.Println(string(body)

answered Jan 16, 2017 at 2:52

Alex Pliutau

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Comments

NWRichmond · Accepted Answer · 2024-04-23 15:31:15Z

0

You're missing a good helper and friend ReadAll()

Your code should looks like this:

package main

import (
    "fmt"
    "io" // with go v1.15 and below, use "io/ioutil" instead
    "net/http"
)

func main() {
    resp, err := http.Get("https://example.com")
    // check err

    defer resp.Body.Close() // we have to close Body if we want to reuse the connection

    html, err := io.ReadAll(resp.Body) // with go v1.15 and below, use `ioutil.ReadAll`
    // check err

    fmt.Println(string(html))
}

edited Apr 23, 2024 at 15:31

NWRichmond

3233 silver badges9 bronze badges

answered Jan 16, 2017 at 2:53

Yandry Pozo

5,1333 gold badges27 silver badges28 bronze badges

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