structlayout

structlayout

The structlayout utility prints the layout of a struct – that is the byte offset and size of each field, respecting alignment/padding.

The information is printed in human-readable form by default, but can be emitted as JSON with the -json flag. This makes it easy to consume this information in other tools.

A utility called structlayout-pretty takes this JSON and prints an ASCII graphic representing the memory layout.

structlayout-optimize is another tool. Inspired by maligned, it reads structlayout JSON on stdin and reorders fields to minimize the amount of padding. The tool can itself emit JSON and feed into e.g. structlayout-pretty.

structlayout-svg is a third-party tool that, similarly to structlayout-pretty, visualises struct layouts. It does so by generating a fancy-looking SVG graphic. You can install it via

go get github.com/ajstarks/svgo/structlayout-svg

Installation

See the main README for installation instructions.

Examples

$ structlayout bufio Reader
Reader.buf []byte: 0-24 (24 bytes)
Reader.rd io.Reader: 24-40 (16 bytes)
Reader.r int: 40-48 (8 bytes)
Reader.w int: 48-56 (8 bytes)
Reader.err error: 56-72 (16 bytes)
Reader.lastByte int: 72-80 (8 bytes)
Reader.lastRuneSize int: 80-88 (8 bytes)

$ structlayout -json bufio Reader | jq .
[
  {
    "name": "Reader.buf",
    "type": "[]byte",
    "start": 0,
    "end": 24,
    "size": 24,
    "is_padding": false
  },
  {
    "name": "Reader.rd",
    "type": "io.Reader",
    "start": 24,
    "end": 40,
    "size": 16,
    "is_padding": false
  },
  {
    "name": "Reader.r",
    "type": "int",
    "start": 40,
    "end": 48,
    "size": 8,
    "is_padding": false
  },
...

$ structlayout -json bufio Reader | structlayout-pretty 
    +--------+
  0 |        | <- Reader.buf []byte
    +--------+
    -........-
    +--------+
 23 |        |
    +--------+
 24 |        | <- Reader.rd io.Reader
    +--------+
    -........-
    +--------+
 39 |        |
    +--------+
 40 |        | <- Reader.r int
    +--------+
    -........-
    +--------+
 47 |        |
    +--------+
 48 |        | <- Reader.w int
    +--------+
    -........-
    +--------+
 55 |        |
    +--------+
 56 |        | <- Reader.err error
    +--------+
    -........-
    +--------+
 71 |        |
    +--------+
 72 |        | <- Reader.lastByte int
    +--------+
    -........-
    +--------+
 79 |        |
    +--------+
 80 |        | <- Reader.lastRuneSize int
    +--------+
    -........-
    +--------+
 87 |        |
    +--------+

$ structlayout -json bytes Buffer | structlayout-svg -t "bytes.Buffer" > /tmp/struct.svg