Am 22.03.2016 um 22:53 hat Eric Blake geschrieben: > On 03/22/2016 09:33 AM, Kevin Wolf wrote: > > Some features, like I/O throttling, are implemented outside > > block-backend.c, but still want to keep BlockBackends in a list. In > > order to avoid exposing the whole struct layout in the public header > > file, this patch introduces an embedded public struct where list entry > > structs can be added and a pair of functions to convert between > > BlockBackend and BlockBackendPublic. > > > > Signed-off-by: Kevin Wolf <kw...@redhat.com> > > --- > > block/block-backend.c | 17 +++++++++++++++++ > > include/sysemu/block-backend.h | 9 +++++++++ > > 2 files changed, 26 insertions(+) > > > > diff --git a/block/block-backend.c b/block/block-backend.c > > index df8f717..4394950 100644 > > --- a/block/block-backend.c > > +++ b/block/block-backend.c > > @@ -33,6 +33,7 @@ struct BlockBackend { > > DriveInfo *legacy_dinfo; /* null unless created by drive_new() */ > > QTAILQ_ENTRY(BlockBackend) link; /* for block_backends */ > > QTAILQ_ENTRY(BlockBackend) monitor_link; /* for monitor_block_backends > > */ > > + BlockBackendPublic public; > > Any reason to not put the public struct at offset 0?
No, but also no reason to put it there. :-) > > + > > +/* > > + * Returns a BlockBackend given the associated @public fields. > > + */ > > +BlockBackend *blk_by_public(BlockBackendPublic *public) > > +{ > > + return container_of(public, BlockBackend, public); > > +} > > At least container_of() doesn't care, so I guess it doesn't matter. > > > +/* This struct is embedded in (the private) BlockBackend struct and > > contains > > + * fields that must be public. This is in particular for QLIST_ENTRY() and > > + * friends so that BlockBackends can be kept in lists outside > > block-backend.c */ > > +typedef struct BlockBackendPublic { > > +} BlockBackendPublic; > > No fields? So really all we need this for is so that we can have an > address of a member of the larger struct, so that we can do list > operations based on that address? Well, a list still needs a QLIST_ENTRY, otherwise I could have directly used BlockBackend. I just tried to keep the introduction of .public separate from the addition of a specific list. I think it's strictly speaking invalid C to have an empty struct, but gcc compiles it and so I thought it should be acceptable to have one for a single commit until something is added there. Kevin
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