From 607553f9306286fdccf0b356bc3d1087adfe21c4 Mon Sep 17 00:00:00 2001 From: Tom Gundersen Date: Tue, 21 Jan 2014 14:10:18 +0100 Subject: libsystemd: split up into subdirs We still only produce on .so, but let's keep the sources separate to make things a bit less messy. --- src/libsystemd/sd-bus/GVARIANT-SERIALIZATION | 63 ++++++++++++++++++++++++++++ 1 file changed, 63 insertions(+) create mode 100644 src/libsystemd/sd-bus/GVARIANT-SERIALIZATION (limited to 'src/libsystemd/sd-bus/GVARIANT-SERIALIZATION') diff --git a/src/libsystemd/sd-bus/GVARIANT-SERIALIZATION b/src/libsystemd/sd-bus/GVARIANT-SERIALIZATION new file mode 100644 index 0000000000..5dffc25bb3 --- /dev/null +++ b/src/libsystemd/sd-bus/GVARIANT-SERIALIZATION @@ -0,0 +1,63 @@ +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. + +Note: The GVariant "MAYBE" type is not supported, so that messages can + be fully converted forth and back between dbus1 and gvariant + representations. -- cgit v1.2.3-54-g00ecf