为什么f1.Read（buf）不将内容读出到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)

}

I create a file, then write in some string. then read out, but there is no content.

The output is :

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

Answer 1

In Go, don't ignore errors. When writing to and reading from a file, keep track of the current file offset.

After the write the offset is at the end-of-file, you need to set the offset to the start-of-file before the read. For example, with diagnostic information,

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

Output:

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]