mirror of
https://git.wownero.com/wownero/wownero.git
synced 2024-12-23 00:48:55 +00:00
fc9b77d855
* More information about array entries (especially nesting) * Varint encoding examples * Expanded string and integer encoding information
199 lines
6.9 KiB
Markdown
199 lines
6.9 KiB
Markdown
# Portable Storage Format
|
|
|
|
## Background
|
|
|
|
Monero makes use of a set of helper classes from a small library named
|
|
[epee](https://github.com/monero-project/monero/tree/master/contrib/epee). Part
|
|
of this library implements a networking protocol called
|
|
[Levin](https://github.com/monero-project/monero/blob/master/contrib/epee/include/net/levin_base.h),
|
|
which internally uses a storage format called [Portable
|
|
Storage](https://github.com/monero-project/monero/tree/master/contrib/epee/include/storages).
|
|
This format (amongst the rest of the
|
|
[epee](https://github.com/monero-project/monero/tree/master/contrib/epee)
|
|
library), is undocumented - or rather relies on the code itself to serve as the
|
|
documentation. Unfortunately, whilst the rest of the library is fairly
|
|
straightforward to decipher, the Portable Storage is less-so. Hence this
|
|
document.
|
|
|
|
## String and Integer Encoding
|
|
|
|
### Integers
|
|
|
|
With few exceptions, integers serialized in epee portable storage format are serialized
|
|
as little-endian.
|
|
|
|
### Varints
|
|
|
|
Varints are used to pack integers in an portable and space optimized way. Varints are stored as little-endian integers, with the lowest 2 bits storing the amount of bytes required, which means the largest value integer that can be packed into 1 byte is 63
|
|
(6 bits).
|
|
|
|
#### Byte Sizes
|
|
|
|
| Lowest 2 bits | Size value | Value range |
|
|
|---------------|---------------|-----------------------------------|
|
|
| b00 | 1 byte | 0 to 63 |
|
|
| b01 | 2 bytes | 64 to 16383 |
|
|
| b10 | 4 bytes | 16384 to 1073741823 |
|
|
| b11 | 8 bytes | 1073741824 to 4611686018427387903 |
|
|
|
|
#### Represenations of Example Values
|
|
| Value | Byte Representation (hex) |
|
|
|----------------------|---------------------------|
|
|
| 0 | 00 |
|
|
| 7 | 1c |
|
|
| 101 | 95 01 |
|
|
| 17,000 | A2 09 01 00 |
|
|
| 7,942,319,744 | 03 BA 98 65 07 00 00 00 |
|
|
|
|
### Strings
|
|
|
|
These are simply length (varint) prefixed char strings without a null
|
|
terminator (though one can always add one if desired). There is no
|
|
specific encoding enforced, and in fact, many times binary blobs are
|
|
stored as these strings. This type should not be confused with the keys
|
|
in sections, as those are restricted to a maximum length of 255 and
|
|
do not use varints to encode the length.
|
|
|
|
"Howdy" => 14 48 6F 77 64 79
|
|
|
|
### Section Keys
|
|
|
|
These are similar to strings except that they are length limited to 255
|
|
bytes, and use a single byte at the front of the string to describe the
|
|
length (as opposed to a varint).
|
|
|
|
"Howdy" => 05 48 6F 77 64 79
|
|
|
|
## Binary Format Specification
|
|
|
|
### Header
|
|
|
|
The format must always start with the following header:
|
|
|
|
| Field | Type | Value |
|
|
|------------------|----------|------------|
|
|
| Signature Part A | UInt32 | 0x01011101 |
|
|
| Signature Part B | UInt32 | 0x01020101 |
|
|
| Version | UInt8 | 0x01 |
|
|
|
|
In total, the 9 byte header will look like this (in hex): `01 11 01 01 01 01 02 01 01`
|
|
|
|
### Section
|
|
|
|
Next we have a root object (or section as the library calls it). This is a map
|
|
of name-value pairs called [entries](#Entry). It starts with a count:
|
|
|
|
| Section | Type |
|
|
|---------------|-----------|
|
|
| Entry count | varint |
|
|
|
|
|
|
Which is followed by the section's name-value [entries](#Entry) sequentially:
|
|
|
|
### Entry
|
|
|
|
| Entry | Type |
|
|
|-------------------|-----------------------|
|
|
| Name | section key |
|
|
| Type | byte |
|
|
| Count<sup>1</sup> | varint |
|
|
| Value(s) | (type dependant data) |
|
|
|
|
<sup>1</sup> Note, this is only present if the entry type has the array flag
|
|
(see below).
|
|
|
|
#### Entry types
|
|
|
|
The types defined are:
|
|
|
|
```cpp
|
|
#define SERIALIZE_TYPE_INT64 1
|
|
#define SERIALIZE_TYPE_INT32 2
|
|
#define SERIALIZE_TYPE_INT16 3
|
|
#define SERIALIZE_TYPE_INT8 4
|
|
#define SERIALIZE_TYPE_UINT64 5
|
|
#define SERIALIZE_TYPE_UINT32 6
|
|
#define SERIALIZE_TYPE_UINT16 7
|
|
#define SERIALIZE_TYPE_UINT8 8
|
|
#define SERIALIZE_TYPE_DOUBLE 9
|
|
#define SERIALIZE_TYPE_STRING 10
|
|
#define SERIALIZE_TYPE_BOOL 11
|
|
#define SERIALIZE_TYPE_OBJECT 12
|
|
#define SERIALIZE_TYPE_ARRAY 13
|
|
```
|
|
|
|
The entry type can be bitwise OR'ed with a flag:
|
|
|
|
```cpp
|
|
#define SERIALIZE_FLAG_ARRAY 0x80
|
|
```
|
|
|
|
This signals there are multiple *values* for the entry. Since only one bit is
|
|
reserved for specifying an array, we can not directly represent nested arrays.
|
|
However, you can place each of the inner arrays inside of a section, and make
|
|
the outer array type `SERIALIZE_TYPE_OBJECT | SERIALIZE_FLAG_ARRAY`. Immediately following the type code byte is a varint specifying the length of the array.
|
|
Finally, the all the elements are serialized in sequence with no padding and
|
|
without any type information. For example:
|
|
|
|
<p style="padding-left:1em; font:italic larger serif">type, count,
|
|
value<sub>1</sub>, value<sub>2</sub>,..., value<sub>n</sub></p>
|
|
|
|
#### Entry values
|
|
|
|
It's important to understand that entry *values* can be encoded any way in which
|
|
an implementation chooses. For example, the integers can be in either big or
|
|
little endian byte order.
|
|
|
|
Entry values which are objects (i.e. `SERIALIZE_TYPE_OBJECT`), are stored as
|
|
[sections](#Section).
|
|
|
|
Note, I have not yet seen the type `SERIALIZE_TYPE_ARRAY` in use. My assumption
|
|
is this would be used for *untyped* arrays and so subsequent entries could be of
|
|
any type.
|
|
|
|
### Overall example
|
|
|
|
Let's put it all together and see what an entire object would look like serialized. To represent our data, let's create a JSON object (since it's a format
|
|
that most will be familiar with):
|
|
|
|
```json
|
|
{
|
|
"short_quote": "Give me liberty or give me death!",
|
|
"long_quote": "Monero is more than just a technology. It's also what the technology stands for.",
|
|
"signed_32bit_int": 20140418,
|
|
"array_of_bools": [true, false, true, true],
|
|
"nested_section": {
|
|
"double": -6.9,
|
|
"unsigned_64bit_int": 11111111111111111111
|
|
}
|
|
}
|
|
```
|
|
|
|
This would translate to:
|
|
|
|
|
|
|
|
## Monero specifics
|
|
|
|
### Entry values
|
|
|
|
#### Hashes, Keys, Blobs
|
|
|
|
These are stored as strings, `SERIALIZE_TYPE_STRING`.
|
|
|
|
#### STL containers (vector, list)
|
|
|
|
These can be arrays of standard integer types, strings or
|
|
`SERIALIZE_TYPE_OBJECT`'s for structs.
|
|
|
|
#### Links to some Monero struct definitions
|
|
|
|
- [Core RPC
|
|
definitions](https://github.com/monero-project/monero/blob/master/src/rpc/core_rpc_server_commands_defs.h)
|
|
- [CryptoNote protocol
|
|
definitions](https://github.com/monero-project/monero/blob/master/src/cryptonote_protocol/cryptonote_protocol_defs.h)
|
|
|
|
|
|
|
|
[//]: # ( vim: set tw=80: )
|