On 11/17/2015 11:15 PM, Vladimir Sementsov-Ogievskiy wrote:
You may want to compare to the qcow2 spec, which also lists expected
byte offsets for each field (rather than having to count how many
earlier fields of which widths were specified).
I've compared qed spec with qcow2 spec and like the first one. What the
need of specifying each offset? Creating a c-structure is simpler when
you see types. and than offsetof and sizeof may be used if needed.
Nobody will #define numeric offsets I think.
You'd be surprised (libvirt has some hard-coded numeric offsets:
https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/virstoragefile.c;h=2aa1d90;hb=5ed7afa9d#l159)
Also, listing offsets makes it obvious that you are NOT relying on
compiler padding, and makes it obvious whether you have been careful
that all 64-bit quantities are 8-byte-aligned without wasting space.
The original cow format (not qcow or qcow2) has the awful distinction of
NOT having specified offsets, and had different layouts on 32-bit
platforms than it did on 64-bit platforms; hence, we retired it in
commit 550830f9 as unsupportable.