logfmt: github.com/kr/logfmt Index | Examples | Files

package logfmt

import "github.com/kr/logfmt"

Package implements the decoding of logfmt key-value pairs.

Example logfmt message:

foo=bar a=14 baz="hello kitty" cool%story=bro f %^asdf

Example result in JSON:

{ "foo": "bar", "a": 14, "baz": "hello kitty", "cool%story": "bro", "f": true, "%^asdf": true }


ident_byte = any byte greater than ' ', excluding '=' and '"'
string_byte = any byte excluding '"' and '\'
garbage = !ident_byte
ident = ident_byte, { ident byte }
key = ident
value = ident | '"', { string_byte | '\', '"' }, '"'
pair = key, '=', value | key, '=' | key
message = { garbage, pair }, garbage


package main

import (

type Measurement struct {
    Key  string
    Val  float64
    Unit string // (e.g. ms, MB, etc)

type Measurements []*Measurement

var measurePrefix = []byte("measure.")

func (mm *Measurements) HandleLogfmt(key, val []byte) error {
    if !bytes.HasPrefix(key, measurePrefix) {
        return nil
    i := bytes.LastIndexFunc(val, isDigit)
    v, err := strconv.ParseFloat(string(val[:i+1]), 10)
    if err != nil {
        return err
    m := &Measurement{
        Key:  string(key[len(measurePrefix):]),
        Val:  v,
        Unit: string(val[i+1:]),
    *mm = append(*mm, m)
    return nil

// return true if r is an ASCII digit only, as opposed to unicode.IsDigit.
func isDigit(r rune) bool {
    return '0' <= r && r <= '9'

func main() {
    var data = []byte("measure.a=1ms measure.b=10 measure.c=100MB measure.d=1s garbage")

    mm := make(Measurements, 0)
    if err := logfmt.Unmarshal(data, &mm); err != nil {
        log.Fatalf("err=%q", err)
    for _, m := range mm {
        fmt.Printf("%v\n", *m)



Package Files

decode.go scanner.go unquote.go


var ErrUnterminatedString = errors.New("logfmt: unterminated string")

func Unmarshal Uses

func Unmarshal(data []byte, v interface{}) (err error)

Unmarshal parses the logfmt encoding data and stores the result in the value pointed to by v. If v is an Handler, HandleLogfmt will be called for each key-value pair.

If v is not a Handler, it will pass v to NewStructHandler and use the returned StructHandler for decoding.

type Handler Uses

type Handler interface {
    HandleLogfmt(key, val []byte) error

Handler is the interface implemented by objects that accept logfmt key-value pairs. HandleLogfmt must copy the logfmt data if it wishes to retain the data after returning.

func NewStructHandler Uses

func NewStructHandler(v interface{}) (Handler, error)

type HandlerFunc Uses

type HandlerFunc func(key, val []byte) error

The HandlerFunc type is an adapter to allow the use of ordinary functions as logfmt handlers. If f is a function with the appropriate signature, HandlerFunc(f) is a Handler object that calls f.

func (HandlerFunc) HandleLogfmt Uses

func (f HandlerFunc) HandleLogfmt(key, val []byte) error

type InvalidUnmarshalError Uses

type InvalidUnmarshalError struct {
    Type reflect.Type

An InvalidUnmarshalError describes an invalid argument passed to Unmarshal. (The argument to Unmarshal must be a non-nil pointer.)

func (*InvalidUnmarshalError) Error Uses

func (e *InvalidUnmarshalError) Error() string

type StructHandler Uses

type StructHandler struct {
    // contains filtered or unexported fields

StructHandler unmarshals logfmt into a struct. It matches incoming keys to the the struct's fields (either the struct field name or its tag, preferring an exact match but also accepting a case-insensitive match.

Field types supported by StructHandler are:

all numeric types (e.g. float32, int, etc.)
bool - true if key is present, false otherwise (the value is ignored).
time.Duration - uses time.ParseDuration

If a field is a pointer to an above type, and a matching key is not present in the logfmt data, the pointer will be untouched.

If v is not a pointer to an Handler or struct, Unmarshal will return an error.

func (*StructHandler) HandleLogfmt Uses

func (h *StructHandler) HandleLogfmt(key, val []byte) error

type UnmarshalTypeError Uses

type UnmarshalTypeError struct {
    Value string       // the logfmt value
    Type  reflect.Type // type of Go value it could not be assigned to

An UnmarshalTypeError describes a logfmt value that was not appropriate for a value of a specific Go type.

func (*UnmarshalTypeError) Error Uses

func (e *UnmarshalTypeError) Error() string

Package logfmt imports 9 packages (graph) and is imported by 38 packages. Updated 2021-01-22. Refresh now. Tools for package owners.