Handle Packed Binary Data

The struct module provides routines for interpreting byte strings as packed binary data.

Functions can be individually imported and directly accessed using the named import syntax:

import { pack, unpack } from 'struct';

let buffer = pack('bhl', -13, 1234, 444555666);
let values = unpack('bhl', buffer);

Alternatively, the module namespace can be imported using a wildcard import statement:

import * as struct from 'struct';

let buffer = struct.pack('bhl', -13, 1234, 444555666);
let values = struct.unpack('bhl', buffer);

Additionally, the struct module namespace may also be imported by invoking the ucode interpreter with the -lstruct switch.

Format Strings

Format strings describe the data layout when packing and unpacking data. They are built up from format-characters, which specify the type of data being packed/unpacked. In addition, special characters control the byte order, size and alignment.

Each format string consists of an optional prefix character which describes the overall properties of the data and one or more format characters which describe the actual data values and padding.

Byte Order, Size, and Alignment

By default, C types are represented in the machine's native format and byte order, and properly aligned by skipping pad bytes if necessary (according to the rules used by the C compiler).

This behavior is chosen so that the bytes of a packed struct correspond exactly to the memory layout of the corresponding C struct.

Whether to use native byte ordering and padding or standard formats depends on the application.

Alternatively, the first character of the format string can be used to indicate the byte order, size and alignment of the packed data, according to the following table:

Character	Byte order	Size	Alignment
`@`	native	native	native
`=`	native	standard	none
`<`	little-endian	standard	none
`>`	big-endian	standard	none
`!`	network (= big-endian)	standard	none

If the first character is not one of these, '@' is assumed.

Native byte order is big-endian or little-endian, depending on the host system. For example, Intel x86, AMD64 (x86-64), and Apple M1 are little-endian; IBM z and many legacy architectures are big-endian.

Native size and alignment are determined using the C compiler's sizeof expression. This is always combined with native byte order.

Standard size depends only on the format character; see the table in the format-characters section.

Note the difference between '@' and '=': both use native byte order, but the size and alignment of the latter is standardized.

The form '!' represents the network byte order which is always big-endian as defined in IETF RFC 1700.

There is no way to indicate non-native byte order (force byte-swapping); use the appropriate choice of '<' or '>'.

Notes:

(1) Padding is only automatically added between successive structure members. No padding is added at the beginning or the end of the encoded struct.

(2) No padding is added when using non-native size and alignment, e.g. with '<', '>', '=', and '!'.

(3) To align the end of a structure to the alignment requirement of a particular type, end the format with the code for that type with a repeat count of zero.

Format Characters

Format characters have the following meaning; the conversion between C and ucode values should be obvious given their types. The 'Standard size' column refers to the size of the packed value in bytes when using standard size; that is, when the format string starts with one of '<', '>', '!' or '='. When using native size, the size of the packed value is platform dependent.

Format	C Type	Ucode type	Standard size	Notes
`x`	pad byte	no value		(7)
`c`	`char`	string	1
`b`	`signed char`	int	1	(1), (2)
`B`	`unsigned char`	int	1	(2)
`?`	`_Bool`	bool	1	(1)
`h`	`short`	int	2	(2)
`H`	`unsigned short`	int	2	(2)
`i`	`int`	int	4	(2)
`I`	`unsigned int`	int	4	(2)
`l`	`long`	int	4	(2)
`L`	`unsigned long`	int	4	(2)
`q`	`long long`	int	8	(2)
`Q`	`unsigned long long`	int	8	(2)
`n`	`ssize_t`	int		(3)
`N`	`size_t`	int		(3)
`e`	(6)	double	2	(4)
`f`	`float`	double	4	(4)
`d`	`double`	double	8	(4)
`s`	`char[]`	double		(9)
`p`	`char[]`	double		(8)
`P`	`void *`	int		(5)
`*`	`char[]`	string		(10)

Notes:

(1) The '?' conversion code corresponds to the _Bool type defined by C99. If this type is not available, it is simulated using a char. In standard mode, it is always represented by one byte.
(2) When attempting to pack a non-integer using any of the integer conversion codes, this module attempts to convert the given value into an integer. If the value is not convertible, a type error exception is thrown.
(3) The 'n' and 'N' conversion codes are only available for the native size (selected as the default or with the '@' byte order character). For the standard size, you can use whichever of the other integer formats fits your application.
(4) For the 'f', 'd' and 'e' conversion codes, the packed representation uses the IEEE 754 binary32, binary64 or binary16 format (for 'f', 'd' or 'e' respectively), regardless of the floating-point format used by the platform.
(5) The 'P' format character is only available for the native byte ordering (selected as the default or with the '@' byte order character). The byte order character '=' chooses to use little- or big-endian ordering based on the host system. The struct module does not interpret this as native ordering, so the 'P' format is not available.
(6) The IEEE 754 binary16 "half precision" type was introduced in the 2008 revision of the IEEE 754 standard. It has a sign bit, a 5-bit exponent and 11-bit precision (with 10 bits explicitly stored), and can represent numbers between approximately 6.1e-05 and 6.5e+04 at full precision. This type is not widely supported by C compilers: on a typical machine, an unsigned short can be used for storage, but not for math operations. See the Wikipedia page on the half-precision floating-point format for more information.
(7) When packing, 'x' inserts one NUL byte.
(8) The 'p' format character encodes a "Pascal string", meaning a short variable-length string stored in a fixed number of bytes, given by the count. The first byte stored is the length of the string, or 255, whichever is smaller. The bytes of the string follow. If the string passed in to pack() is too long (longer than the count minus 1), only the leading count-1 bytes of the string are stored. If the string is shorter than count-1, it is padded with null bytes so that exactly count bytes in all are used. Note that for unpack(), the 'p' format character consumes count bytes, but that the string returned can never contain more than 255 bytes.
(9) For the 's' format character, the count is interpreted as the length of the bytes, not a repeat count like for the other format characters; for example, '10s' means a single 10-byte string mapping to or from a single ucode byte string, while '10c' means 10 separate one byte character elements (e.g., cccccccccc) mapping to or from ten different ucode byte strings. If a count is not given, it defaults to 1. For packing, the string is truncated or padded with null bytes as appropriate to make it fit. For unpacking, the resulting bytes object always has exactly the specified number of bytes. As a special case, '0s' means a single, empty string (while '0c' means 0 characters).
(10) The * format character serves as wildcard. For pack() it will append the corresponding byte argument string as-is, not applying any padding or zero filling. When a repeat count is given, that many bytes of the input byte string argument will be appended at most on pack(), effectively truncating longer input strings. For unpack(), the wildcard format will yield a byte string containing the entire remaining input data bytes, or - when a repeat count is given - that many bytes of input data at most.

A format character may be preceded by an integral repeat count. For example, the format string '4h' means exactly the same as 'hhhh'.

Whitespace characters between formats are ignored; a count and its format must not contain whitespace though.

When packing a value x using one of the integer formats ('b', 'B', 'h', 'H', 'i', 'I', 'l', 'L', 'q', 'Q'), if x is outside the valid range for that format, a type error exception is raised.

For the '?' format character, the return value is either true or false. When packing, the truish result value of the argument is used. Either 0 or 1 in the native or standard bool representation will be packed, and any non-zero value will be true when unpacking.

Examples

Note: Native byte order examples (designated by the '@' format prefix or lack of any prefix character) may not match what the reader's machine produces as that depends on the platform and compiler.

Pack and unpack integers of three different sizes, using big endian ordering:

import { pack, unpack } from 'struct';

pack(">bhl", 1, 2, 3);  // "\x01\x00\x02\x00\x00\x00\x03"
unpack(">bhl", "\x01\x00\x02\x00\x00\x00\x03");  // [ 1, 2, 3 ]

Attempt to pack an integer which is too large for the defined field:

$ ucode -lstruct -p 'struct.pack(">h", 99999)'
Type error: Format 'h' requires numeric argument between -32768 and 32767
In [-p argument], line 1, byte 24:

 `struct.pack(">h", 99999)`
  Near here -------------^

Demonstrate the difference between 's' and 'c' format characters:

import { pack } from 'struct';

pack("@ccc", "1", "2", "3");  // "123"
pack("@3s", "123");           // "123"

The ordering of format characters may have an impact on size in native mode since padding is implicit. In standard mode, the user is responsible for inserting any desired padding.

Note in the first pack() call below that three NUL bytes were added after the packed '#' to align the following integer on a four-byte boundary. In this example, the output was produced on a little endian machine:

import { pack } from 'struct';

pack("@ci", "#", 0x12131415);  // "#\x00\x00\x00\x15\x14\x13\x12"
pack("@ic", 0x12131415, "#");  // "\x15\x14\x13\x12#"

The following format 'ih0i' results in two pad bytes being added at the end, assuming the platform's ints are aligned on 4-byte boundaries:

import { pack } from 'struct';

pack("ih0i", 0x01010101, 0x0202);  // "\x01\x01\x01\x01\x02\x02\x00\x00"

Use the wildcard format to extract the remainder of the input data:

import { unpack } from 'struct';

unpack("ccc*", "foobarbaz");   // [ "f", "o", "o", "barbaz" ]
unpack("ccc3*", "foobarbaz");  // [ "f", "o", "o", "bar" ]

Use the wildcard format to pack binary stings as-is into the result data:

import { pack } from 'struct';

pack("h*h", 0x0101, "\x02\x00\x03", 0x0404);  // "\x01\x01\x02\x00\x03\x04\x04"
pack("c3*c", "a", "foobar", "c");  // "afooc"

Source: lib/struct.c, line 63

Classes

struct.buffer

Represents a struct buffer instance created by buffer().

struct.instance

Represents a struct instance created by new().

Methods

buffer(initialDataopt) → {buffer}

Creates a new struct buffer instance.

The buffer() function creates a new struct buffer object that can be used for incremental packing and unpacking of binary data. If an initial data string is provided, the buffer is initialized with this content.

Note that even when initial data is provided, the buffer position is always set to zero. This design assumes that the primary intent when initializing a buffer with data is to read (unpack) from the beginning. If you want to append data to a pre-initialized buffer, you need to explicitly move the position to the end, either by calling end() or by setting the position manually with pos().

Returns a new struct buffer instance.

Parameters:

Name	Type	Description
`initialData`	string	(optional) Optional initial data to populate the buffer with.

Returns: buffer

Example

// Create an empty buffer
const emptyBuf = struct.buffer();

// Create a buffer with initial data
const dataBuf = struct.buffer("\x01\x02\x03\x04");

// Read from the beginning of the initialized buffer
const value = dataBuf.get('I');

// Append data to the initialized buffer
dataBuf.end().put('I', 5678);

// Alternative chained syntax for initializing and appending
const buf = struct.buffer("\x01\x02\x03\x04").end().put('I', 5678);

Source: lib/struct.c, line 3027

new(format) → {instance}

Precompile format string.

The new() function precompiles the given format string argument and returns a struct object instance useful for packing and unpacking multiple items without having to recompute the internal format each time.

Returns an precompiled struct format instance.

Raises a runtime exception if the format string is invalid.

Parameters:

Name	Type	Description
`format`	string	The format string.

Returns: instance

Example

// Create a format of three consecutive unsigned int values in network byte order.
const fmt = struct.new('!III');
const buf = fmt.pack(1, 2, 3);  // "\x00\x00\x00\x01…"
print(fmt.unpack(buf), "\n");   // [ 1, 2, 3 ]

Source: lib/struct.c, line 2869

pack(format, …values) → {string}

Pack given values according to specified format.

The pack() function creates a byte string containing the argument values packed according to the given format string.

Returns the packed string.

Raises a runtime exception if a given argument value does not match the required type of the corresponding format string directive or if and invalid format string is provided.

Parameters:

Name	Type	Description
`format`	string	The format string.
`values`	*	(repeatable) Variable number of values to pack.

Returns: string

Example

// Pack the values 1, 2, 3 as three consecutive unsigned int values
// in network byte order.
const data = pack('!III', 1, 2, 3);

Source: lib/struct.c, line 2726

unpack(format, input, offsetopt) → {array}

Unpack given byte string according to specified format.

The unpack() function interpretes a byte string according to the given format string and returns the resulting values. If the optional offset argument is given, unpacking starts from this byte position within the input. If not specified, the start offset defaults to 0, the start of the given input string.

Returns an array of unpacked values.

Raises a runtime exception if the format string is invalid or if an invalid input string or offset value is given.

Parameters:

Name	Type	Description
`format`	string	The format string.
`input`	string	The input string to unpack.
`offset`	number	(optional, default: `0`) The offset within the input string to start unpacking from.

Returns: array

Example

// Unpack three consecutive unsigned int values in network byte order.
const numbers =
  unpack('!III', '\x00\x00\x00\x01\x00\x00\x00\x02\x00\x00\x00\x03');
print(numbers, "\n"); // [ 1, 2, 3 ]

Source: lib/struct.c, line 2782