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:

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.

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"