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decon.js: Binary file deconstructor

In tandem with a file giving its structure, decon.js deconstructs binary data into a JSON structure.

For example, deconstructing [a very tiny PNG file] ( using this definition:

PNG: bigendian {
  byte[8] ([137,80,78,71,13,10,26,10]) signature
  Chunk[] chunks

Chunk: {
  uint32 length
  char[4] type
  byte[length] data
  byte[4] crc

produces this output:

[ { length: 13, type: 'IHDR', data: [ 0, 0, 0, 1, 0, 0, 0, 1, 8, 6, 0, 0, 0 ], crc: [ 31, 21, 196, 137 ] },
  { length: 10, type: 'IDAT', data: [ 120, 156, 99, 0, 1, 0, 0, 5, 0, 1 ], crc: [ 13, 10, 45, 180 ] },
  { length: 0, type: 'IEND', data: [], crc: [ 174, 66, 96, 130 ] } ]

Deconstruction definition grammar

A construction file consists of any number of import statements and type and constant definitions. An import statement such as

import "record.con"

instructs decon to parse the named file.

Type statments give a name to a specified type and take the form


where TYPESPEC is either a type name, a structure type specification, an array type specification, or a numeric type specification.

The elemental atomic types are byte, bool, char and null, on which futher atomic types may be based by applying modifiers, either as prefixen or dotted suffixen.

uint: int.unsigned
uword: unsigned size(16) int

Legal modifiers are signed, unsigned, bigendian, littleendian, and size N for some value N, which gives the size in bits of the data type.

An array represents repeating fields of the same type in a file and is specified by giving a length, directly or indirectly, in brackets. Some examples:

Point3D: int[3]
Matrix3D: int[3][3]
Cstring: char[until 0]
Line: char[through '\n']
Tail: char[]

An array of chars is interpreted as a string.

Besides giving an explicit length between the brackets termination conditions can be given too:

  • until X terminates the array when terminating X is consumed but does not include it as part of the value

  • through X does include the terminating value in the parsed result

  • before X terminates the array without consuming the terminator

No terminator (empty braces) specifies an array that is as long as the data allows.

Structures parse sequential records of different types and are specified by listing field specifications, a type and/or value and optional name, between braces, e.g.:

PersonalInfo: {
  "PINF" tag
  String name
  int    age

A literal value, if given is tested against the value read from the file, and deconstruction is aborted if they do not match.

Unions represent a number of alternatives. E.g.:

PetRecord: union {

The first matching type (that is, whose values verify against the data) in the union will become the value of the union. Field names given in a union are ignored. Give "null" as the last type in the union to allow the union to not match at all.

An example of a union:

PlanetData: { PlanetRecord[] facts; }

PlanetRecord = union {
  { "satellite:"; cstring moon }
  { "diameter:"; int32 diameter }

Which would process input



{ facts: [{moon: "Phobos"}, {moon: "Deimos"}, {diameter: 6794}] }


From the command line, usage is

node decon.js [OPTIONS] [DEF.con...] MAIN

where MAIN is the root type, defined in some .con file listed, used to parse a binary file, and write the resulting JSON structure.


  • -p Partial parsing of the input is okay
  • -v Verbose: print extra debugging information
  • -V VAR Produce JavaScript (prepend var VAR =)
  • -i FILE Read from the named file rather than stdin
  • -o FILE Write to the named file rather than stdout
  • -h Produce somewhat more human-readable output

Within node, use, for example:

var decon = require("decon")
decon.parse("int = size(16) littleendian byte");
var bmpcon = decon.import("bmp.con").BitmapFile;
var bmp = bmpcon.deconstructFile("BitmapFile", "dib.bmp");
console.log("Width = " +;