Golang中append()与for循环的使用问题求助

问题点 #

• 循环结束后，err和bytesReturned变量未定义，无法在循环外使用
• 执行dataFile = append(dataFile, buff)时提示类型不匹配，无法合并切片

原代码 #

import (
  "fmt"
    "net"
    "log"
    "bufio"
    "os"
  "io"
)

func readData(conn net.Conn){
  fmt.Println("Reading data...\n")

  // Create a new reader from conn
  reader := bufio.NewReader(conn)

  // Make a slice to hold all data being transmitted
  var dataFile []byte

  // Create a buffer to store information as it comes across conn
  buff := make([]byte, 4096)

  for {
  bytesReturned, err := reader.Read(buff)
  dataFile = append(dataFile, buff)
  if err != nil {
    if err != io.EOF{
      log.Fatalln(err)
    }
    break //Break the loop when err returns EOF
  }
}
  
  err = os.WriteFile("data.txt", dataFile, 0666)

  if err != nil {
    log.Fatal(err)
  }
    

  fmt.Println("Read", bytesReturned,"bytes.")
}



// Start of main --------------------------------
func main(){

 //Listen on local port 80
 listener, err := net.Listen("tcp", ":5555")
 if err != nil {
   log.Fatalln("Unable to bind port")
 }

 fmt.Println("Port 5555 binded...\n")

 //handle multiple connections
 for {
   conn, err := listener.Accept()
   if err != nil {
     log.Fatalln("Unable to accept connection...")
   }
   go readData(conn)
 }
}

错误原因与修复 #

1. 循环外变量未定义 #

bytesReturned和err是在for循环内部用:=声明的局部变量，作用域仅限于循环内部，循环外部无法访问。

修复方式：在循环外提前声明变量，循环内用=赋值：

var bytesReturned int
var err error
for {
  bytesReturned, err = reader.Read(buff)
  // ... 其余代码
}

2. 切片append错误 #

append(dataFile, buff)的写法错误，因为append接收单个元素或展开的切片。正确做法是用...展开有效数据切片，并且只追加Read返回的有效部分（buff[:bytesReturned]），避免混入缓冲区旧数据：

dataFile = append(dataFile, buff[:bytesReturned]...)

修复后的完整代码 #

import (
	"fmt"
	"io"
	"log"
	"net"
	"bufio"
	"os"
)

func readData(conn net.Conn) {
	fmt.Println("Reading data...\n")

	reader := bufio.NewReader(conn)
	var dataFile []byte
	buff := make([]byte, 4096)
	var bytesReturned int
	var err error

	for {
		bytesReturned, err = reader.Read(buff)
		if bytesReturned > 0 {
			dataFile = append(dataFile, buff[:bytesReturned]...)
		}
		if err != nil {
			if err != io.EOF {
				log.Fatalln(err)
			}
			break
		}
	}

	err = os.WriteFile("data.txt", dataFile, 0666)
	if err != nil {
		log.Fatal(err)
	}

	fmt.Println("Read", len(dataFile), "bytes.")
}

func main() {
	listener, err := net.Listen("tcp", ":5555")
	if err != nil {
		log.Fatalln("Unable to bind port")
	}

	fmt.Println("Port 5555 binded...\n")

	for {
		conn, err := listener.Accept()
		if err != nil {
			log.Fatalln("Unable to accept connection...")
		}
		go readData(conn)
	}
}

内容的提问来源于stack exchange，提问作者MFC-4