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authorLennart Poettering <lennart@poettering.net>2013-12-05 02:46:54 +0100
committerLennart Poettering <lennart@poettering.net>2013-12-05 02:47:36 +0100
commitc1b9d935725103e95901f347b8981647ce4dd546 (patch)
tree4f5474c2022f7812d7e261b29896e89de14cf490 /src/libsystemd-bus/GVARIANT-SERIALIZATION
parent65dae17a2f890b12a07dd4901b3db02b1031c463 (diff)
bus: add support for serializing to gvariant
(deserialization is still missing, hence this is not hooked up to kdbus)
Diffstat (limited to 'src/libsystemd-bus/GVARIANT-SERIALIZATION')
-rw-r--r--src/libsystemd-bus/GVARIANT-SERIALIZATION59
1 files changed, 59 insertions, 0 deletions
diff --git a/src/libsystemd-bus/GVARIANT-SERIALIZATION b/src/libsystemd-bus/GVARIANT-SERIALIZATION
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+How we use GVariant for serializing D-Bus messages
+--------------------------------------------------
+
+We stay as close to the original dbus1 framing as possible. dbus1 has
+the following framing:
+
+ 1. A fixed header of "yyyyuu"
+ 2. Additional header fields of "a(yv)"
+ 3. Padding with NUL bytes to pad up to next 8byte boundary
+ 4. The body
+
+Note that the body is not padded at the end, the complete message
+hence might have a non-aligned size. Reading multiple messages at once
+will hence result in possibly unaligned messages in memory.
+
+The header consists of the following:
+
+ y Endianness, 'l' or 'B'
+ y Message Type
+ y Flags
+ y Protocol version, '1'
+ u Length of the body, i.e. the length of part 4 above
+ u Serial number
+
+ = 12 bytes
+
+When using GVariant we keep the basic structure in place, only
+slightly extend the header, and define protocol version '2'. The new
+header:
+
+ y Endianness, 'l' or 'B'
+ y Message Type
+ y Flags
+ y Protocol version, '2'
+ u Length of the body, i.e. the length of part 4 above
+ u Serial number
+ u Length of the additional header fields array
+
+ = 16 bytes
+
+This has the nice benefit that the beginning of the additional header
+fields array is aligned to an 8 byte boundary. Also, in dbus1
+marshalling arrays start with a length value of 32bit, which means in
+both dbus1 and gvariant marshallings the size of the header fields
+array will be at the same location between bytes 12 and 16. To
+visualize that:
+
+ 0 4 8 12 16
+ Common: | E | T | F | V | Body Length | Serial | Fields Length |
+
+ dbus1: | ... (as above) ... | Fields array ...
+
+ gvariant: | ... (as above) ... | Fields Length | Fields array ...
+
+And that's already it.
+
+Note: on kdbus only native endian messages marshalled in gvariant may
+ be sent. If a client receives a message in non-native endianness
+ or in dbus1 marshalling it shall ignore the message.