Source: log.go in package log

Package log implements a simple logging package. It defines a type, Logger, with methods for formatting output. It also has a predefined 'standard' Logger accessible through helper functions Print[f|ln], Fatal[f|ln], and Panic[f|ln], which are easier to use than creating a Logger manually. That logger writes to standard error and prints the date and time of each logged message. Every log message is output on a separate line: if the message being printed does not end in a newline, the logger will add one. The Fatal functions call os.Exit(1) after writing the log message. The Panic functions call panic after writing the log message.

package log

import (
	"fmt"
	"io"
	"os"
	"runtime"
	"sync"
	"time"
)

These flags define which text to prefix to each log entry generated by the Logger. Bits are or'ed together to control what's printed. With the exception of the Lmsgprefix flag, there is no control over the order they appear (the order listed here) or the format they present (as described in the comments). The prefix is followed by a colon only when Llongfile or Lshortfile is specified. For example, flags Ldate | Ltime (or LstdFlags) produce, 2009/01/23 01:23:23 message while flags Ldate | Ltime | Lmicroseconds | Llongfile produce, 2009/01/23 01:23:23.123123 /a/b/c/d.go:23: message

const (
	Ldate         = 1 << iota     // the date in the local time zone: 2009/01/23
	Ltime                         // the time in the local time zone: 01:23:23
	Lmicroseconds                 // microsecond resolution: 01:23:23.123123.  assumes Ltime.
	Llongfile                     // full file name and line number: /a/b/c/d.go:23
	Lshortfile                    // final file name element and line number: d.go:23. overrides Llongfile
	LUTC                          // if Ldate or Ltime is set, use UTC rather than the local time zone
	Lmsgprefix                    // move the "prefix" from the beginning of the line to before the message
	LstdFlags     = Ldate | Ltime // initial values for the standard logger
)

A Logger represents an active logging object that generates lines of output to an io.Writer. Each logging operation makes a single call to the Writer's Write method. A Logger can be used simultaneously from multiple goroutines; it guarantees to serialize access to the Writer.

type Logger struct {
	mu     sync.Mutex // ensures atomic writes; protects the following fields
	prefix string     // prefix on each line to identify the logger (but see Lmsgprefix)
	flag   int        // properties
	out    io.Writer  // destination for output
	buf    []byte     // for accumulating text to write
}

New creates a new Logger. The out variable sets the destination to which log data will be written. The prefix appears at the beginning of each generated log line, or after the log header if the Lmsgprefix flag is provided. The flag argument defines the logging properties.

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

SetOutput sets the output destination for the logger.

func (l *Logger) SetOutput(w io.Writer) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.out = w
}

var std = New(os.Stderr, "", LstdFlags)

Default returns the standard logger used by the package-level output functions.

func Default() *Logger { return std }

Cheap integer to fixed-width decimal ASCII. Give a negative width to avoid zero-padding.

Assemble decimal in reverse order.

	var b [20]byte
	bp := len(b) - 1
	for i >= 10 || wid > 1 {
		wid--
		q := i / 10
		b[bp] = byte('0' + i - q*10)
		bp--
		i = q

i < 10

	b[bp] = byte('0' + i)
	*buf = append(*buf, b[bp:]...)
}

formatHeader writes log header to buf in following order: * l.prefix (if it's not blank and Lmsgprefix is unset), * date and/or time (if corresponding flags are provided), * file and line number (if corresponding flags are provided), * l.prefix (if it's not blank and Lmsgprefix is set).

func (l *Logger) formatHeader(buf *[]byte, t time.Time, file string, line int) {
	if l.flag&Lmsgprefix == 0 {
		*buf = append(*buf, l.prefix...)
	}
	if l.flag&(Ldate|Ltime|Lmicroseconds) != 0 {
		if l.flag&LUTC != 0 {
			t = t.UTC()
		}
		if l.flag&Ldate != 0 {
			year, month, day := t.Date()
			itoa(buf, year, 4)
			*buf = append(*buf, '/')
			itoa(buf, int(month), 2)
			*buf = append(*buf, '/')
			itoa(buf, day, 2)
			*buf = append(*buf, ' ')
		}
		if l.flag&(Ltime|Lmicroseconds) != 0 {
			hour, min, sec := t.Clock()
			itoa(buf, hour, 2)
			*buf = append(*buf, ':')
			itoa(buf, min, 2)
			*buf = append(*buf, ':')
			itoa(buf, sec, 2)
			if l.flag&Lmicroseconds != 0 {
				*buf = append(*buf, '.')
				itoa(buf, t.Nanosecond()/1e3, 6)
			}
			*buf = append(*buf, ' ')
		}
	}
	if l.flag&(Lshortfile|Llongfile) != 0 {
		if l.flag&Lshortfile != 0 {
			short := file
			for i := len(file) - 1; i > 0; i-- {
				if file[i] == '/' {
					short = file[i+1:]
					break
				}
			}
			file = short
		}
		*buf = append(*buf, file...)
		*buf = append(*buf, ':')
		itoa(buf, line, -1)
		*buf = append(*buf, ": "...)
	}
	if l.flag&Lmsgprefix != 0 {
		*buf = append(*buf, l.prefix...)
	}
}

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()

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
}

Printf calls l.Output to print to the logger. Arguments are handled in the manner of fmt.Printf.

func (l *Logger) Printf(format string, v ...interface{}) {
	l.Output(2, fmt.Sprintf(format, v...))
}

Print calls l.Output to print to the logger. Arguments are handled in the manner of fmt.Print.

func (l *Logger) Print(v ...interface{}) { l.Output(2, fmt.Sprint(v...)) }

Println calls l.Output to print to the logger. Arguments are handled in the manner of fmt.Println.

func (l *Logger) Println(v ...interface{}) { l.Output(2, fmt.Sprintln(v...)) }

Fatal is equivalent to l.Print() followed by a call to os.Exit(1).

func (l *Logger) Fatal(v ...interface{}) {
	l.Output(2, fmt.Sprint(v...))
	os.Exit(1)
}

Fatalf is equivalent to l.Printf() followed by a call to os.Exit(1).

func (l *Logger) Fatalf(format string, v ...interface{}) {
	l.Output(2, fmt.Sprintf(format, v...))
	os.Exit(1)
}

Fatalln is equivalent to l.Println() followed by a call to os.Exit(1).

func (l *Logger) Fatalln(v ...interface{}) {
	l.Output(2, fmt.Sprintln(v...))
	os.Exit(1)
}

Panic is equivalent to l.Print() followed by a call to panic().

func (l *Logger) Panic(v ...interface{}) {
	s := fmt.Sprint(v...)
	l.Output(2, s)
	panic(s)
}

Panicf is equivalent to l.Printf() followed by a call to panic().

func (l *Logger) Panicf(format string, v ...interface{}) {
	s := fmt.Sprintf(format, v...)
	l.Output(2, s)
	panic(s)
}

Panicln is equivalent to l.Println() followed by a call to panic().

func (l *Logger) Panicln(v ...interface{}) {
	s := fmt.Sprintln(v...)
	l.Output(2, s)
	panic(s)
}

Flags returns the output flags for the logger. The flag bits are Ldate, Ltime, and so on.

func (l *Logger) Flags() int {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.flag
}

SetFlags sets the output flags for the logger. The flag bits are Ldate, Ltime, and so on.

func (l *Logger) SetFlags(flag int) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.flag = flag
}

Prefix returns the output prefix for the logger.

func (l *Logger) Prefix() string {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.prefix
}

SetPrefix sets the output prefix for the logger.

func (l *Logger) SetPrefix(prefix string) {
	l.mu.Lock()
	defer l.mu.Unlock()
	l.prefix = prefix
}

Writer returns the output destination for the logger.

func (l *Logger) Writer() io.Writer {
	l.mu.Lock()
	defer l.mu.Unlock()
	return l.out
}

SetOutput sets the output destination for the standard logger.

func SetOutput(w io.Writer) {
	std.mu.Lock()
	defer std.mu.Unlock()
	std.out = w
}

Flags returns the output flags for the standard logger. The flag bits are Ldate, Ltime, and so on.

func Flags() int {
	return std.Flags()
}

SetFlags sets the output flags for the standard logger. The flag bits are Ldate, Ltime, and so on.

func SetFlags(flag int) {
	std.SetFlags(flag)
}

Prefix returns the output prefix for the standard logger.

func Prefix() string {
	return std.Prefix()
}

SetPrefix sets the output prefix for the standard logger.

func SetPrefix(prefix string) {
	std.SetPrefix(prefix)
}

Writer returns the output destination for the standard logger.

func Writer() io.Writer {
	return std.Writer()
}

These functions write to the standard logger.

Print calls Output to print to the standard logger. Arguments are handled in the manner of fmt.Print.

func Print(v ...interface{}) {
	std.Output(2, fmt.Sprint(v...))
}

Printf calls Output to print to the standard logger. Arguments are handled in the manner of fmt.Printf.

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

Println calls Output to print to the standard logger. Arguments are handled in the manner of fmt.Println.

func Println(v ...interface{}) {
	std.Output(2, fmt.Sprintln(v...))
}

Fatal is equivalent to Print() followed by a call to os.Exit(1).

func Fatal(v ...interface{}) {
	std.Output(2, fmt.Sprint(v...))
	os.Exit(1)
}

Fatalf is equivalent to Printf() followed by a call to os.Exit(1).

func Fatalf(format string, v ...interface{}) {
	std.Output(2, fmt.Sprintf(format, v...))
	os.Exit(1)
}

Fatalln is equivalent to Println() followed by a call to os.Exit(1).

func Fatalln(v ...interface{}) {
	std.Output(2, fmt.Sprintln(v...))
	os.Exit(1)
}

Panic is equivalent to Print() followed by a call to panic().

func Panic(v ...interface{}) {
	s := fmt.Sprint(v...)
	std.Output(2, s)
	panic(s)
}

Panicf is equivalent to Printf() followed by a call to panic().

func Panicf(format string, v ...interface{}) {
	s := fmt.Sprintf(format, v...)
	std.Output(2, s)
	panic(s)
}

Panicln is equivalent to Println() followed by a call to panic().

func Panicln(v ...interface{}) {
	s := fmt.Sprintln(v...)
	std.Output(2, s)
	panic(s)
}

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 the count of the number of frames to skip when computing the file name and line number if Llongfile or Lshortfile is set; a value of 1 will print the details for the caller of Output.

func Output(calldepth int, s string) error {
	return std.Output(calldepth+1, s) // +1 for this frame.