布菲奥.阅读器在 bufio 之后不会从文件中读取任何内容.作家写道

Question

代码如下所示：

package main
import (
"bufio"
"fmt"
"io"
"log"
"os"
)
func main() {
file, _ := os.OpenFile("test.txt", os.O_CREATE|os.O_RDWR|os.O_APPEND, 0666)
// write
writer := bufio.NewWriter(file)
for i := 0; i < 10; i++ {
fmt.Fprintln(writer, i)
}
writer.Flush()
// read
reader := bufio.NewReader(file)
for {
line, _, err := reader.ReadLine()
log.Println(string(line))
if err == io.EOF {
break
}
if err != nil {
log.Fatalln("get msg failed.")
}
}
}

我无法读取已写入文件的内容。

文件的偏移量似乎指向文件的末尾。

有人可以解释为什么会发生这种情况吗？

Answer 1

O_APPEND会导致文件描述符的偏移量在每次写入之前前进到文件的末尾。每次写入都会将文件描述符的偏移量提前成功写入的字节数。

在//read行之前添加以下内容：

offset, _ := file.Seek(0, io.SeekCurrent)
fmt.Printf("DEBUG: before: file offset is %dn", offset)
file.Seek(0, 0)
offset, _ = file.Seek(0, io.SeekCurrent)
fmt.Printf("DEBUG: after: file offset is %dn", offset)

您将看到您的程序现在可以正常工作。

<小时 />

参考资料：

从open的手册页：

O_APPEND
The file is opened in append mode.  Before each write(2), the
file offset is positioned at the end of the file, as if with
lseek(2).  The modification of the file offset and the write
operation are performed as a single atomic step.

从write的手册页：

For a seekable file (i.e., one to which lseek(2) may be applied, for
example, a regular file) writing takes place at the file offset, and
the file offset is incremented by the number of bytes actually
written.  If the file was open(2)ed with O_APPEND, the file offset is
first set to the end of the file before writing.  The adjustment of
the file offset and the write operation are performed as an atomic
step.