Array alignment while marshalling data

Simon McVittie simon.mcvittie at
Mon May 23 19:03:43 UTC 2016

On 23/05/16 19:46, marcin at wrote:
> the array length is not aligned to 8 bytes, but to 4 bytes. But type of
> this array is struct, which according to the specification should always
> align to 8 bytes.

The header field array isn't a struct, it's an array (which happens to
contain structs). The array length is a 4-byte integer, so it's aligned
to a multiple of 4 bytes. After the length, there are some number of
structs, and *those* are aligned to a multiple of 8 bytes.

In effect, the beginning of every message looks like this:

bytes 0-3: header start, four 1-byte quantities
4-7: body length
8-11: serial
No padding, 12 is already a multiple of 4
12-15: header field array length
No padding, 16 is already a multiple of 8
16+: first header field struct

Let's say the first header field struct happens to be 13 bytes long.
We'd continue as follows:

16-28: first header field struct
29-31: 3 bytes of padding to reach next multiple of 8
32+: second header field struct

(Yes, this means the marshalling for `a(whatever)`, an array of structs,
is not the same as the marshalling for `(a(whatever))`, a struct
containing a single array of structs. There's no particular reason, it's
just how this stuff was designed 10+ years ago.)

Simon McVittie
Collabora Ltd. <>

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