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Package fixedwidth provides encoding and decoding for fixed-width formatted Data.

go get


Struct Tags

The struct tag schema schema used by fixedwidth is: fixed:"{startPos},{endPos},[{alignment},[{padChar}]]"1.

The startPos and endPos arguments control the position within a line. startPos and endPos must both be positive integers greater than 0. Positions start at 1. The interval is inclusive.

The alignment argument controls the alignment of the value within it's interval. The valid options are default2, right, left, and none. The alignment is optional and can be omitted.

The padChar argument controls the character that will be used to pad any empty characters in the interval after writing the value. The default padding character is a space. The padChar is optional and can be omitted.

Fields without tags are ignored.


// define some data to encode
people := []struct {
    ID        int     `fixed:"1,5"`
    FirstName string  `fixed:"6,15"`
    LastName  string  `fixed:"16,25"`
    Grade     float64 `fixed:"26,30"`
    Age       uint    `fixed:"31,33"`
    Alive     bool    `fixed:"34,39"`
    {1, "Ian", "Lopshire", 99.5, 20, true},

data, err := Marshal(people)
if err != nil {
fmt.Printf("%s", data)
// Output:
// 1    Ian       Lopshire  99.5020 true


// define the format
var people []struct {
    ID        int     `fixed:"1,5"`
    FirstName string  `fixed:"6,15"`
    LastName  string  `fixed:"16,25"`
    Grade     float64 `fixed:"26,30"`
    Age       uint    `fixed:"31,33"`
    Alive     bool    `fixed:"34,39"`
    Github    bool    `fixed:"40,41"`

// define some fixed-with data to parse
data := []byte("" +
    "1    Ian       Lopshire  99.50 20 false f" + "\n" +
    "2    John      Doe       89.50 21 true t" + "\n" +
    "3    Jane      Doe       79.50 22 false F" + "\n" +
    "4    Ann       Carraway  79.59 23 false T" + "\n")

err := Unmarshal(data, &people)
if err != nil {

fmt.Printf("%+v\n", people[0])
fmt.Printf("%+v\n", people[1])
fmt.Printf("%+v\n", people[2])
fmt.Printf("%+v\n", people[3])
// Output:
//{ID:1 FirstName:Ian LastName:Lopshire Grade:99.5 Age:20 Alive:false Github:false}
//{ID:2 FirstName:John LastName:Doe Grade:89.5 Age:21 Alive:true Github:true}
//{ID:3 FirstName:Jane LastName:Doe Grade:79.5 Age:22 Alive:false Github:false}
//{ID:4 FirstName:Ann LastName:Carraway Grade:79.59 Age:23 Alive:false Github:true}

It is also possible to read data incrementally

decoder := fixedwidth.NewDecoder(bytes.NewReader(data))
for {
    var element myStruct
    err := decoder.Decode(&element)
    if err == io.EOF {
    if err != nil {

UTF-8, Codepoints, and Multibyte Characters

fixedwidth supports encoding and decoding fixed-width data where indices are expressed in unicode codepoints and not raw bytes. The data must be UTF-8 encoded.

decoder := fixedwidth.NewDecoder(strings.NewReader(data))
// Decode as usual now
buff := new(bytes.Buffer)
encoder := fixedwidth.NewEncoder(buff)
// Encode as usual now

Alignment Behavior

Alignment Encoding Decoding
default Field is left aligned The padding character is trimmed from both right and left of value
left Field is left aligned The padding character is trimmed from right of value
right Field is right aligned The padding character is trimmed from left of value
none Field is left aligned The padding character is not trimmed from value. Useful for nested structs.


  1. {} indicates an argument. [] indicates and optional segment ^
  2. The default alignment is similar to left but has slightly different behavior required to maintain backwards compatibility ^