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A binary serialisation format inspired by YAML, MessagePack and nvlists.
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Argdata is a binary serialisation format inspired by YAML, MessagePack and nvlists.

It is originally designed and currently used in CloudABI but does not depend on anything specific to CloudABI.

The encoding is optimized for reading:

  • Strings (UTF-8 encoded) and binary data appear verbatim within the encoded data. Strings are followed by a null byte in the encoded data. This way, pointers into the encoded argdata can be used directly, without having to copy or overwrite data.

  • Instead of encoding the number of elements in maps and sequences (the only two structures containing subfields), the length of the subfields (in bytes) is stored before each subfield. This makes it possible to iterate over a map or sequence, without having to recursively iterate depth-first through all the elements. Skipping over a large map containing many other maps is as quick as skipping over a single integer. This means the encoded data can be navigated through quite efficiently, without constructing a data structure next to the encoded data.

So, after the encoded argdata is stored in a buffer, no dynamic memory is needed at all to use any of the values contained within.


Argdata values have one of the following types:

  • null - Nothing.
  • binary - A sequence of bytes.
  • string - An UTF-8 string.
  • bool - Either true or false.
  • int - A signed integer of any size.
  • float - A (64-bit) floating point value.
  • map - A sequence of keys with values. Keys and values can both be any argdata value.
  • seq - A sequence of argdata values.
  • timestamp - A point in time. (In UTC, no timezone information.)
  • fd - A file descriptor number, used on POSIX and CloudABI systems.


This repository contains a C library for handling argdata. See the (well-commented) argdata.h for the details. A C++ interface is available for the same library in argdata.hpp. This repository also provides some example source files.

Binary encoding

The first byte of an encoded argdata value encodes the type of the value, using the tag value in the table below. Unlike many other serialization formats, encoded argdata values do not encode their own length. It is assumed that the length of the full buffer is already known. The length of the value is simply the length of the full buffer, minus the one tag byte encoding the type.

Null values do not have any tag, they don't even take any space at all: They are encoded as , nothing, zero bytes.

Type Tag Value bytes
binary 01 the binary data
bool 02 false: none, true: 01
fd 03 32-bit big-endian integer (e.g. 00 00 00 02 for fd 2)
float 04 64-bit big-endian IEEE754 float
int 05 N-bit signed big-endian integer (see below)
map 06 repeated subfield (see below): key, value, key, value, ...
seq 07 repeated subfield (see below): value, value, value, ...
string 08 null-terminated UTF-8
timestamp 09 N-bit signed big-endian integer (see below), encoding nanoseconds since 1970 (UTC)

For example, the string "123" is encoded as 08 31 32 33 00: The tag byte 08 for the type, followed by the UTF-8 encoding of the string, followed by a terminating null byte.

N-bit signed integers

Integers are signed, and encoded big-endian in as few bytes as needed. The C library doesn't support decoding and encoding integers that don't fit in intmax_t or uintmax_t (usually 64-bit), but the binary format has no restrictions on the size of integers.

Some examples:

Value Bytes
0 none
1 01
127 7F
-128 80
-1 FF
255 00 FF
1000 03 E8
-1000 FC 18
2^32 - 1 00 FF FF FF FF

Since integers are always encoded using the least bytes possible, no integer should be encoded as, for example, 00 or FF FF. (Since the values 0 and -1 can be encoded as an empty sequence and FF, respectively.)


Subfields, inside the map and seq types, are encoded by their length, followed by the encoded value itself. The length is encoded as a variable length unsigned big-endian integer, where the high bit of each byte is not part of the integer, but indicates whether the byte is the last byte of the integer.

For example, the length of a subfield of 6 bytes is encoded as 86: The high bit is on, indicating that this is the last byte of the variable length integer, and the other 7 bits encode the value 6. As another example, a length of 128 is encoded as 01 80: Only the high bit of the second byte is set, indicating that two bytes are used, and the other 14 bits (7 bits of the first, and 7 bits of the second byte) encode the value 128.

Maps and sequences don't encode their own length, or number of elements stored. The subfields simply end when there are no more bytes left.

Maps always have an even number of subfields, as every pair represents a key with its value.

For example, a sequence of values 0, true, and "A" is encoded as: 07 81 05 82 02 01 83 08 41 00:

Bytes Meaning
07 seq type tag
81 length of first subfield: 1
05 first subfield: the integer 0
82 length of second subfield: 2
02 01 second subfield: the boolean value true
83 length of the third subfield: 3
08 41 00 third subfield: the string "A" (with null terminator)

File Descriptors

File descriptor numbers in argdata refer to open file descriptors in the same process or passed along with the message that contained the argdata. This is not useful on every platform, but can for example be used on CloudABI and POSIX systems.

File descriptors are always stored as an 32-bit integer in exactly four bytes, as opposed to integers which use only as many bytes as needed. The reason for this, is to allow substitution of file descriptors in encoded argdata. If the file descriptors were variable length, changing the value could involve resizing and thus re-encoding (part of) the argdata. Being able to substitute these values is useful since in many cases file descriptor numbers cannot be chosen freely.

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