使用 http.Get() 时无效的内存地址或 nil 指针取消引用答案

【问题标题】：invalid memory address or nil pointer dereference when use http.Get()使用 http.Get() 时无效的内存地址或 nil 指针取消引用
【发布时间】：2014-01-29 09:53:31
【问题描述】：

刚开始学习Go lang，写了一个小demo，从txt中读取图片url，将url放入数组，然后将响应保存到文件中。

这是我的代码

package main

import (
"bufio"
"fmt"
"io"
"io/ioutil"
"net/http"
"os"
)

func main() {
fileName := "meinv.txt"
file, _ := os.Open(fileName)

picUrl := make([]string, 2000)
reader := bufio.NewReader(file)
for {
    line, _, err := reader.ReadLine()
    if err != io.EOF {
        fmt.Printf("file load %s \n", line)
        picUrl = append(picUrl, string(line))
    } else {
        file.Close()
        break
    }
}
fmt.Printf("file loaded,read to download \n")
fetchPic(picUrl)

}
func fetchPic(picUrl []string) {

var file string
for key, value := range picUrl {

    fmt.Printf("key is : %d,this line is %s \n\n", key, value)
    httpRequest, err := http.Get(string(value))
    fmt.Print("load ok \n")

    httpRequest.Body.Close()
    result, readErr := ioutil.ReadAll(httpRequest.Body)
    if readErr == nil {
        file = "pics/" + string(key) + ".jpg"
        ioutil.WriteFile(file, result, 0777)
        fmt.Print("Write ok \n")
    }
    len := len(string(result))
    fmt.Printf("length is %d", len)
    if err == nil {
        httpRequest = nil
        //result = nil
    } else {
        fmt.Print("load falt!!!!!!!!! \n")
    }
    defer httpRequest.Body.Close()
}

}

运行它，我得到了

key is : 0,this line is  

load ok 
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x40 pc=0x40123f]

goroutine 1 [running]:
runtime.panic(0x5f8300, 0x877688)
/usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
main.fetchPic(0xc21239d000, 0x38be7, 0x4223c)
/home/lyn/www/goOnDev/fetch.go:40 +0x24f
main.main()
/home/lyn/www/goOnDev/fetch.go:28 +0x1b8
exit status 2

meinv.txt ，每个网址一行有人帮忙吗？THKS

【问题讨论】：

fetch.go:40 is "httpRequest.Body.Close()" fetch.go:28 is "fetchPic(picUrl)" ，在 main() 我调用了函数
您忽略了（可能的）错误，我怀疑这些错误会对您有所帮助。

标签： go

【解决方案1】：

这对我来说似乎很好用

func main() {
    randomURLs := []string{"http://i.imgur.com/I7Rak2y.jpg", "http://i.imgur.com/XuM8GCN.jpg"}
    fetchPic(randomURLs)
}

func fetchPic(picUrl []string) {

    var file string
    for key, value := range picUrl {

        fmt.Printf("key is : %d,this line is %s \n\n", key, value)
        httpRequest, err := http.Get(string(value))
        fmt.Print("load ok \n")

        defer httpRequest.Body.Close()

        result, readErr := ioutil.ReadAll(httpRequest.Body)

        if readErr == nil {
            file = "pics/" + string(key) + ".jpg"
            ioutil.WriteFile(file, result, 0777)
            fmt.Print("Write ok \n")
        }

        len := len(string(result))
        fmt.Printf("length is %d", len)

        if err == nil {
            httpRequest = nil
            //result = nil
        } else {
            fmt.Print("load falt!!!!!!!!! \n")
        }

    }

}

前面的代码输出如下结果：

Running...

key is : 0,this line is http://i.imgur.com/I7Rak2y.jpg 

load ok 
Write ok 
length is 445661key is : 1,this line is http://i.imgur.com/XuM8GCN.jpg 

load ok 
Write ok 
length is 746031
Success: process exited with code 0.

【讨论】：

谢谢你，我已经发布了我所有的代码，我的 main() 函数有什么问题吗？它似乎没问题...
@rainyluo 你的其余代码似乎没问题，但我不知道meinv.txt的内容

【解决方案2】：

在没有检查错误的情况下执行httpRequest, err := http.Get(string(value)) 之后，您正在从 httpRequest.Body 无条件读取：如果http.Get 失败，您将没有有效的httpRequest.Body 可供读取。

经验法则：立即检查每个错误。

【讨论】：

【解决方案3】：

我输入这段代码并测试，我没有遇到任何问题。

package main

import (
  "os"
  "fmt"
  "bufio"
  "net/http"
  "io/ioutil"
)

func read_file(path string) ([]string, error) {
  file, err := os.Open(path)
  if err != nil {
    return nil, err
  }
  defer file.Close()

  var lines []string
  reader := bufio.NewScanner(file)
  for reader.Scan() {
    lines = append(lines, reader.Text())
  }
  return lines, reader.Err()
}

func main(){
  lines, err := read_file("./file.txt")
  if err != nil {
    fmt.Println(err)
    return
  }

  for key, value := range lines {

    fmt.Printf("key is : %d,this line is %s \n\n", key, value)
    httpRequest, err := http.Get(value)
    if err != nil {
      fmt.Println(err)
      return
    }
    defer httpRequest.Body.Close()

    result, readErr := ioutil.ReadAll(httpRequest.Body)
    if readErr != nil {
      fmt.Println(err)
      return
    }
    ioutil.WriteFile("./" + fmt.Sprintf("%d", key) + ".jpg", result, 0777)
  }

}

回答

key is : 0,this line is http://samsam.iteye.com/images/login_icon.png 

key is : 1,this line is http://www.iteye.com/images/logo-small.gif

壳

>>ls
0.jpg  1.jpg  file.txt  test.go

【讨论】：