读取接口参数（p []字节）读取后将字节长度变为 512

Question

下面是示例代码：人员的定义type Person struct {   Data string   done bool}实现写入func (person *Person) Write(p []byte) (int,error) {    person.Data = string(p)    return len(p), nil}实现读取func (person *Person) Read(p []byte) (int,error) {    if person.done {        return 0, io.EOF    }    for index, b := range []byte(person.Data) {        p[index] = b    }    // len(p) = 512 <- length p []byte    person.done = true    return len(person.Data), nil}写入一些数据func writeTo(out io.Writer) error {    if _, err := out.Write([]byte("test")); err != nil {        return err    }    fmt.Println("Write done")    return nil}主要func main() {    p := &Person{}        writeTo(p)    s, err := ioutil.ReadAll(p)    if err != nil {        panic(err)    }        fmt.Println("result:",string(s),len(s))}如所注意到的，p []字节长度为512。我想知道为什么它变成512长度。

Answer 1

您的代码调用 ：ReadAll

s, err := ioutil.ReadAll(p)

ReadAll基本上像这样工作：

package main

import "fmt"

func main() {

   // first, make an empty slice, with a reserve space of 512

   b := make([]byte, 0, 512)

   // then, write data into the reserve space

   n := copy(b[:cap(b)], "north")

   // then, increase the slice to how much data was just written

   b = b[:n]

   // 5 [110 111 114 116 104]

   fmt.Println(n, b)

}

https://github.com/golang/go/blob/go1.16.3/src/io/io.go#L626