jie

log.Printf("hello log %s", "!!")

# Printf
func (format string, v ...interface{}) {
	std.Output(2, fmt.Sprintf(format, v...))
}

# std 启用的是系统标准错误流
var std = New(os.Stderr, "", LstdFlags)

func New(out io.Writer, prefix string, flag int) *Logger {
	return &Logger{out: out, prefix: prefix, flag: flag}
}

# Logger 
type Logger struct {
	mu     sync.Mutex 
	prefix string     // prefix to write at beginning of each line
	flag   int        // properties
	out    io.Writer  // destination for output
	buf    []byte     // for accumulating text to write
}

// Output writes the output for a logging event. The string s contains
// the text to print after the prefix specified by the flags of the
// Logger. A newline is appended if the last character of s is not
// already a newline. Calldepth is used to recover the PC and is
// provided for generality, although at the moment on all pre-defined
// paths it will be 2.
func (l *Logger) Output(calldepth int, s string) error {
	now := time.Now() // get this early.
	var file string
	var line int
	l.mu.Lock()
	defer l.mu.Unlock()
	if l.flag&(Lshortfile|Llongfile) != 0 {
		// Release lock while getting caller info - it's expensive.
		l.mu.Unlock()
		var ok bool
		_, file, line, ok = runtime.Caller(calldepth)
		if !ok {
			file = "???"
			line = 0
		}
		l.mu.Lock()
	}
	l.buf = l.buf[:0]
	l.formatHeader(&l.buf, now, file, line)
	l.buf = append(l.buf, s...)
	if len(s) == 0 || s[len(s)-1] != 'n' {
		l.buf = append(l.buf, 'n')
	}
	_, err := l.out.Write(l.buf)
	return err
}