How we use GVariant for serializing D-Bus messages
--------------------------------------------------
-We stay close to the original dbus1 framing as possible, but make
-certain changes to adapt for GVariant. dbus1 has the following
-framing:
+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)"
y Flags
y Protocol version, '1'
u Length of the body, i.e. the length of part 4 above
- u 32bit Serial number
+ u Serial number
= 12 bytes
-This header is then followed by the the fields array, whose first
-value is a 32bit array size.
-
When using GVariant we keep the basic structure in place, only
-slightly alter the header, and define protocol version '2'. The new
+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 Reserved, must be 0
- t 64bit Cookie
+ 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 is then followed by the GVariant fields array ("a{tv}"), and
-finally the actual body as variant (v). Putting this altogether a
-packet on dbus2 hence qualifies as a fully compliant GVariant
-structure of (yyyyuta{tv}v).
-
-For details on gvariant, see:
-
-https://people.gnome.org/~desrt/gvariant-serialisation.pdf
-
-Regarding the framing of dbus2, also see:
-
-https://wiki.gnome.org/Projects/GLib/GDBus/Version2
-
-The first four bytes of the header are defined the same way for dbus1
-and dbus2. The first bytes contain the endianess field and the
-protocol version, so that the remainder of the message can be safely
-made sense of just by looking at the first 32bit.
-
-Note that the length of the body is no longer included in the header
-on dbus2! In fact, the message size must be known in advance, from the
-underlying transport in order to parse dbus2 messages, while it is
-directly included in dbus1 message headers. This change of semantics
-is an effect of GVariant's basic design.
-
-The serial number has been renamed cookie and has been extended from
-32bit to 64bit. It is recommended to avoid the higher 32bit of the
-cookie field though, to simplify compatibility with dbus1 peers. Note
-that not only the cookie/serial field in the fixed header, but also
-the reply_cookie/reply_serial additional header field has been
-increased from 32bit to 64bit, too!
-
-The header field identifiers have been extended from 8bit to
-64bit. This has been done to simplify things (as kdbus otherwise uses
-exclusively 64bit types, unless there is a strong reason not to), and
-has no effect on the serialization size, as due to alignment for each
-8bit header field identifier 56 bits of padding had to be added.
-
-Note that the header size changed, due to these changes. However,
-consider that on dbus1 the beginning of the fields array contains the
-32bit array size (since that is how arrays are encoded on dbus1),
-thus, if one considers that size part of the header, instead of the
-array, the size of the header on dbus1 and dbus2 stays identical, at
-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 | ...
+ Common: | E | T | F | V | Body Length | Serial | Fields Length |
- dbus1: | (as above) | Body Length | Serial | Fields Length | Fields array ...
+ dbus1: | ... (as above) ... | Fields array ...
- gvariant: | (as above) | Reserved | Cookie | Fields array ...
+ gvariant: | ... (as above) ... | Fields Length | Fields array ...
And that's already it.
-Note: to simplify parsing, valid kdbus/dbus2 messages must include the
-entire fixed header and additional header fields in a single non-memfd
-message part. Also, the signature string of the body variant all the
-way to the end of the message must be in a single non-memfd part
-too. The parts for this extended header and footer can be the same
-one, and can also continue any amount of additional body bytes.
-
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.
~ianmdlvl / elogind.git / blobdiff