Golang将自定义os.File添加到os.Stdout

Question

I'm trying to write output to a file whenever I print something to the console. There didn't seem to be any nice examples out there using a continuous stream, but rather reading a single value so I came up with the following code:

package main

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

type ahhh struct {
    *os.File
    __writer io.Writer
}

func (me *ahhh) Write(b []byte) (n int, err error) {
    return me.__writer.Write(b)
}

func write_print_to_file(file_name string) {
    file, _ := os.OpenFile(file_name, os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)

    new_stdout := &ahhh{file, io.MultiWriter(file, os.Stdout)}

    os.Stdout = new_stdout
}

func main() {
    write_print_to_file("output.log")
    fmt.Println("Hello world!")
}

For some reason os.Stdout = new_stdout doesn't want to work. I'm fairly certain this should be an allowed assignment as I use something of similar structure in my FUSE file system, so I'm curious why os.Stdout doesn't want to accept my inherited os.File.

Alternatively if this really isn't possible any good suggestions for making a continuous data stream to a file?

Answer 1

$ go doc os.stdout
var (
  Stdin  = NewFile(uintptr(syscall.Stdin), "/dev/stdin")
  Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")
  Stderr = NewFile(uintptr(syscall.Stderr), "/dev/stderr")
)
    Stdin, Stdout, and Stderr are open Files pointing to the standard input,
    standard output, and standard error file descriptors.

    Note that the Go runtime writes to standard error for panics and crashes;
    closing Stderr may cause those messages to go elsewhere, perhaps to a file
    opened later.

---

$ go doc os.newfile
func NewFile(fd uintptr, name string) *File
    NewFile returns a new File with the given file descriptor and name. The
    returned value will be nil if fd is not a valid file descriptor. On Unix
    systems, if the file descriptor is in non-blocking mode, NewFile will
    attempt to return a pollable File (one for which the SetDeadline methods
    work).

---

$ go doc -u os.file
type File struct {
  *file // os specific
}
    File represents an open file descriptor.

func Create(name string) (*File, error)
func NewFile(fd uintptr, name string) *File
func Open(name string) (*File, error)
func OpenFile(name string, flag int, perm FileMode) (*File, error)
func newFile(fd uintptr, name string, kind newFileKind) *File
func openFdAt(fd int, path string) (*File, error)
func openFileNolog(name string, flag int, perm FileMode) (*File, error)
func (f *File) Chdir() error
func (f *File) Chmod(mode FileMode) error
func (f *File) Chown(uid, gid int) error
func (f *File) Close() error
func (f *File) Fd() uintptr
func (f *File) Name() string
func (f *File) Read(b []byte) (n int, err error)
func (f *File) ReadAt(b []byte, off int64) (n int, err error)
func (f *File) Readdir(n int) ([]FileInfo, error)
func (f *File) Readdirnames(n int) (names []string, err error)
func (f *File) Seek(offset int64, whence int) (ret int64, err error)
func (f *File) SetDeadline(t time.Time) error
func (f *File) SetReadDeadline(t time.Time) error
func (f *File) SetWriteDeadline(t time.Time) error
func (f *File) Stat() (FileInfo, error)
func (f *File) Sync() error
func (f *File) SyscallConn() (syscall.RawConn, error)
func (f *File) Truncate(size int64) error
func (f *File) Write(b []byte) (n int, err error)
func (f *File) WriteAt(b []byte, off int64) (n int, err error)
func (f *File) WriteString(s string) (n int, err error)
func (f *File) checkValid(op string) error
func (f *File) chmod(mode FileMode) error
func (file File) close() error
func (f *File) pread(b []byte, off int64) (n int, err error)
func (f *File) pwrite(b []byte, off int64) (n int, err error)
func (f *File) read(b []byte) (n int, err error)
func (f *File) readdir(n int) (fi []FileInfo, err error)
func (f *File) readdirnames(n int) (names []string, err error)
func (f *File) seek(offset int64, whence int) (ret int64, err error)
func (f *File) setDeadline(t time.Time) error
func (f *File) setReadDeadline(t time.Time) error
func (f *File) setWriteDeadline(t time.Time) error
func (f *File) wrapErr(op string, err error) error
func (f *File) write(b []byte) (n int, err error)
type file struct {
  pfd         poll.FD
  name        string
  dirinfo     *dirInfo // nil unless directory being read
  nonblock    bool     // whether we set nonblocking mode
  stdoutOrErr bool     // whether this is stdout or stderr
}
    file is the real representation of *File. The extra level of indirection
    ensures that no clients of os can overwrite this data, which could cause the
    finalizer to close the wrong file descriptor.

func (file *file) close() error

---

I'm fairly certain this should be an allowed assignment.

new_stdout := &ahhh{file, io.MultiWriter(file, os.Stdout)}
os.Stdout = new_stdout

Is it an *os.File? No. Why should it be allowed?