package log

import "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.

Index

• Constants
• func Fatal(v ...any)
• func Fatalf(format string, v ...any)
• func Fatalln(v ...any)
• func Flags() int
• func Output(calldepth int, s string) error
• func Panic(v ...any)
• func Panicf(format string, v ...any)
• func Panicln(v ...any)
• func Prefix() string
• func Print(v ...any)
• func Printf(format string, v ...any)
• func Println(v ...any)
• func SetFlags(flag int)
• func SetOutput(w io.Writer)
• func SetPrefix(prefix string)
• func Writer() io.Writer
• type Logger
  • func Default() *Logger
  • func New(out io.Writer, prefix string, flag int) *Logger
  • func (l *Logger) Fatal(v ...any)
  • func (l *Logger) Fatalf(format string, v ...any)
  • func (l *Logger) Fatalln(v ...any)
  • func (l *Logger) Flags() int
  • func (l *Logger) Output(calldepth int, s string) error
  • func (l *Logger) Panic(v ...any)
  • func (l *Logger) Panicf(format string, v ...any)
  • func (l *Logger) Panicln(v ...any)
  • func (l *Logger) Prefix() string
  • func (l *Logger) Print(v ...any)
  • func (l *Logger) Printf(format string, v ...any)
  • func (l *Logger) Println(v ...any)
  • func (l *Logger) SetFlags(flag int)
  • func (l *Logger) SetOutput(w io.Writer)
  • func (l *Logger) SetPrefix(prefix string)
  • func (l *Logger) Writer() io.Writer

Examples

• Logger
• Logger.Output

Constants

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
)

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

Functions

func Fatal

func Fatal(v ...any)

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

func Fatalf

func Fatalf(format string, v ...any)

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

func Fatalln

func Fatalln(v ...any)

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

func Flags

func Flags() int

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

func Output

func Output(calldepth int, s string) error

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 Panic

func Panic(v ...any)

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

func Panicf

func Panicf(format string, v ...any)

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

func Panicln

func Panicln(v ...any)

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

func Prefix

func Prefix() string

Prefix returns the output prefix for the standard logger.

func Print

func Print(v ...any)

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

func Printf

func Printf(format string, v ...any)

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

func Println

func Println(v ...any)

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

func SetFlags

func SetFlags(flag int)

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

func SetOutput

func SetOutput(w io.Writer)

SetOutput sets the output destination for the standard logger.

func SetPrefix

func SetPrefix(prefix string)

SetPrefix sets the output prefix for the standard logger.

func Writer

func Writer() io.Writer

Writer returns the output destination for the standard logger.

Types

type Logger

type Logger struct {
	// contains filtered or unexported fields
}

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.

package main

import (
	"bytes"
	"fmt"
	"log"
)

func main() {
	var (
		buf    bytes.Buffer
		logger = log.New(&buf, "logger: ", log.Lshortfile)
	)

	logger.Print("Hello, log file!")

	fmt.Print(&buf)
}

logger: example_test.go:19: Hello, log file!

func Default

func Default() *Logger

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

func New

func New(out io.Writer, prefix string, flag int) *Logger

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 (*Logger) Fatal

func (l *Logger) Fatal(v ...any)

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

func (*Logger) Fatalf

func (l *Logger) Fatalf(format string, v ...any)

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

func (*Logger) Fatalln

func (l *Logger) Fatalln(v ...any)

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

func (*Logger) Flags

func (l *Logger) Flags() int

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

func (*Logger) Output

func (l *Logger) Output(calldepth int, s string) error

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.

package main

import (
	"bytes"
	"fmt"
	"log"
)

func main() {
	var (
		buf    bytes.Buffer
		logger = log.New(&buf, "INFO: ", log.Lshortfile)

		infof = func(info string) {
			logger.Output(2, info)
		}
	)

	infof("Hello world")

	fmt.Print(&buf)
}

INFO: example_test.go:36: Hello world

func (*Logger) Panic

func (l *Logger) Panic(v ...any)

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

func (*Logger) Panicf

func (l *Logger) Panicf(format string, v ...any)

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

func (*Logger) Panicln

func (l *Logger) Panicln(v ...any)

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

func (*Logger) Prefix

func (l *Logger) Prefix() string

Prefix returns the output prefix for the logger.

func (*Logger) Print

func (l *Logger) Print(v ...any)

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

func (*Logger) Printf

func (l *Logger) Printf(format string, v ...any)

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

func (*Logger) Println

func (l *Logger) Println(v ...any)

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

func (*Logger) SetFlags

func (l *Logger) SetFlags(flag int)

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

func (*Logger) SetOutput

func (l *Logger) SetOutput(w io.Writer)

SetOutput sets the output destination for the logger.

func (*Logger) SetPrefix

func (l *Logger) SetPrefix(prefix string)

SetPrefix sets the output prefix for the logger.

func (*Logger) Writer

func (l *Logger) Writer() io.Writer

Writer returns the output destination for the logger.

Directories