64 lines
2.2 KiB
Plaintext
64 lines
2.2 KiB
Plaintext
How we use GVariant for serializing D-Bus messages
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--------------------------------------------------
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We stay as close to the original dbus1 framing as possible. dbus1 has
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the following framing:
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1. A fixed header of "yyyyuu"
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2. Additional header fields of "a(yv)"
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3. Padding with NUL bytes to pad up to next 8byte boundary
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4. The body
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Note that the body is not padded at the end, the complete message
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hence might have a non-aligned size. Reading multiple messages at once
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will hence result in possibly unaligned messages in memory.
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The header consists of the following:
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y Endianness, 'l' or 'B'
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y Message Type
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y Flags
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y Protocol version, '1'
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u Length of the body, i.e. the length of part 4 above
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u Serial number
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= 12 bytes
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When using GVariant we keep the basic structure in place, only
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slightly extend the header, and define protocol version '2'. The new
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header:
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y Endianness, 'l' or 'B'
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y Message Type
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y Flags
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y Protocol version, '2'
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u Length of the body, i.e. the length of part 4 above
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u Serial number
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u Length of the additional header fields array
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= 16 bytes
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This has the nice benefit that the beginning of the additional header
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fields array is aligned to an 8 byte boundary. Also, in dbus1
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marshalling arrays start with a length value of 32bit, which means in
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both dbus1 and gvariant marshallings the size of the header fields
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array will be at the same location between bytes 12 and 16. To
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visualize that:
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0 4 8 12 16
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Common: | E | T | F | V | Body Length | Serial | Fields Length |
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dbus1: | ... (as above) ... | Fields array ...
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gvariant: | ... (as above) ... | Fields Length | Fields array ...
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And that's already it.
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Note: on kdbus only native endian messages marshalled in gvariant may
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be sent. If a client receives a message in non-native endianness
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or in dbus1 marshalling it shall ignore the message.
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Note: The GVariant "MAYBE" type is not supported, so that messages can
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be fully converted forth and back between dbus1 and gvariant
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representations.
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