go - 为什么 f1.Read(buf) 没有读出内容到 buf？

Question

package main

import (
    "fmt"
    "os"
    "io"
)

func main() {

    f1,_ := os.Create("f1")

    io.WriteString(f1, "some content")

    buf := make([]byte, 8)

    f1.Read(buf)

    fmt.Println(buf)

}

我创建一个文件，然后写入一些字符串。然后读出来，但是没有内容。

输出是:

go run test.go
[0 0 0 0 0 0 0 0]

Answer 1

在 Go 中，不要忽略错误。写入和读取文件时，请跟踪当前文件偏移量。

写入后偏移量位于文件末尾，您需要在读取前将偏移量设置为文件开头。例如，带有诊断信息，

package main

import (
    "fmt"
    "io"
    "os"
)

func main() {
    f1, err := os.Create("f1")
    if err != nil {
        fmt.Println(err)
        return
    }
    defer f1.Close()
    // The file offset is its current location.
    s, err := f1.Seek(0, io.SeekCurrent)
    if err != nil {
        fmt.Println(s, err)
        return
    }
    fmt.Println("offset:", s)
    // writing takes place at the file offset, and
    // the file offset is incremented by the number of bytes actually
    // written.
    n, err := f1.WriteString("some content")
    fmt.Println("write: ", n, err)
    if err != nil {
        fmt.Println(n, err)
        return
    }
    // The file offset is its current location
    s, err = f1.Seek(0, io.SeekCurrent)
    if err != nil {
        fmt.Println(s, err)
        return
    }
    fmt.Println("offset:", s)

    buf := make([]byte, 8)

    // the read operation commences at the
    // file offset, and the file offset is incremented by the number of
    // bytes read.  If the file offset is at or past the end of file, no
    // bytes are read, and read() returns zero.
    n, err = f1.Read(buf[:cap(buf)])
    fmt.Println("read:  ", n, err)
    buf = buf[:n]
    fmt.Println("buffer:", len(buf), buf)
    if err != nil {
        if err != io.EOF {
            fmt.Println(n, err)
            return
        }
    }

    // The file offset is set to the start-of-file.
    s, err = f1.Seek(0, io.SeekStart)
    if err != nil {
        fmt.Println(s, err)
        return
    }
    fmt.Println("offset:", s)
    // the read operation commences at the
    // file offset, and the file offset is incremented by the number of
    // bytes read.  If the file offset is at or past the end of file, no
    // bytes are read, and read() returns zero.
    n, err = f1.Read(buf[:cap(buf)])
    fmt.Println("read:  ", n, err)
    buf = buf[:n]
    fmt.Println("buffer:", len(buf), buf)
    if err != nil {
        if err != io.EOF {
            fmt.Println(n, err)
            return
        }
    }
}

Playground :https://play.golang.org/p/hPUn1ltKP2t

输出:

offset: 0
write:  12 <nil>
offset: 12
read:   0 EOF
buffer: 0 []
offset: 0
read:   8 <nil>
buffer: 8 [115 111 109 101 32 99 111 110]