projects / thirdparty / systemd.git / blame
| Commit | Line | Data |
|---|---|---|
| c1b9d935 LP |
1 | How we use GVariant for serializing D-Bus messages |
| 2 | -------------------------------------------------- | |
| 3 | ||
| 4 | We stay as close to the original dbus1 framing as possible. dbus1 has | |
| 5 | the following framing: | |
| 6 | ||
| 7 | 1. A fixed header of "yyyyuu" | |
| 8 | 2. Additional header fields of "a(yv)" | |
| 9 | 3. Padding with NUL bytes to pad up to next 8byte boundary | |
| 10 | 4. The body | |
| 11 | ||
| 12 | Note that the body is not padded at the end, the complete message | |
| 13 | hence might have a non-aligned size. Reading multiple messages at once | |
| 14 | will hence result in possibly unaligned messages in memory. | |
| 15 | ||
| 16 | The header consists of the following: | |
| 17 | ||
| 18 | y Endianness, 'l' or 'B' | |
| 19 | y Message Type | |
| 20 | y Flags | |
| 21 | y Protocol version, '1' | |
| 22 | u Length of the body, i.e. the length of part 4 above | |
| 23 | u Serial number | |
| 24 | ||
| 25 | = 12 bytes | |
| 26 | ||
| 27 | When using GVariant we keep the basic structure in place, only | |
| 28 | slightly extend the header, and define protocol version '2'. The new | |
| 29 | header: | |
| 30 | ||
| 31 | y Endianness, 'l' or 'B' | |
| 32 | y Message Type | |
| 33 | y Flags | |
| 34 | y Protocol version, '2' | |
| 35 | u Length of the body, i.e. the length of part 4 above | |
| 36 | u Serial number | |
| 37 | u Length of the additional header fields array | |
| 38 | ||
| 39 | = 16 bytes | |
| 40 | ||
| 41 | This has the nice benefit that the beginning of the additional header | |
| 42 | fields array is aligned to an 8 byte boundary. Also, in dbus1 | |
| 43 | marshalling arrays start with a length value of 32bit, which means in | |
| 44 | both dbus1 and gvariant marshallings the size of the header fields | |
| 45 | array will be at the same location between bytes 12 and 16. To | |
| 46 | visualize that: | |
| 47 | ||
| 48 | 0 4 8 12 16 | |
| 49 | Common: | E | T | F | V | Body Length | Serial | Fields Length | | |
| 50 | ||
| 51 | dbus1: | ... (as above) ... | Fields array ... | |
| 52 | ||
| 53 | gvariant: | ... (as above) ... | Fields Length | Fields array ... | |
| 54 | ||
| 55 | And that's already it. | |
| 56 | ||
| 57 | Note: on kdbus only native endian messages marshalled in gvariant may | |
| 58 | be sent. If a client receives a message in non-native endianness | |
| 59 | or in dbus1 marshalling it shall ignore the message. |