log
Package log implements a simple logging package.
Contents
Behavior
2026/09/08 17:58:02 starting...
2026/09/08 17:58:02 [httpd] a new signup from 192.168.0.5
2026/09/08 17:58:02 main.go:13: [cwmpd] the user requests to upload the image
2026/09/08 09:58:02.369944 exit...
Design
Since it outputs log messages, it should specify a Writer to print messages.
Besides, the header of each log entry is flexible. It may contain datetime and/or filename and/or line number, and so forth. These rules shall be stored somewhere and can be changed when necessary:
Each time to assemble the log entry, the flags control what is printed in the header. Bits are or’ed together.
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
)
API
A Logger represents an active logging object that generates lines of formatted output to an io.Writer, which writes to the underlying data stream. The formatting rules are encoded in flags that are stored in the logging object.
/*
* New creates a new Logger. Rules are encoded in flag.
*
* func New(out io.Writer, prefix string, flag int) *Logger
*
*/
9 logger1 := log.New(os.Stderr, "[httpd] ", log.LstdFlags|log.Lmsgprefix)
10 logger1.Println("a new signup from 192.168.0.5")
11
12 logger2 := log.New(os.Stderr, "[cwmpd] ", log.LstdFlags|log.Lmsgprefix|log.Lshortfile)
13 logger2.Println("the user requests to upload the image")
14
15 logger := log.New(os.Stderr, "", log.LstdFlags|log.Lmicroseconds|log.LUTC)
16 logger.Println("exit...")
The flags can be changed by SetFlags when necessary.
9 logger := log.New(os.Stderr, "", log.LstdFlags)
10 logger.Println("good night")
11 logger.SetFlags(log.LstdFlags | log.Lshortfile)
12 logger.Println("good night")
Output:
2026/09/27 03:39:53 good night
2026/09/27 03:39:53 main.go:12: good night
The format is very simple. For complex structured and leveled logging, we need to turn to slog.