In include/qemu/compiler.h, we have this gem:
> #ifndef container_of
> #define container_of(ptr, type, member) ({ \
> const typeof(((type *) 0)->member) *__mptr = (ptr); \
> (type *) ((char *) __mptr - offsetof(type, member));})
> #endif
>
> /* Convert from a base type to a parent type, with compile time checking. */
> #ifdef __GNUC__
> #define DO_UPCAST(type, field, dev) ( __extension__ ( { \
> char __attribute__((unused)) offset_must_be_zero[ \
> -offsetof(type, field)]; \
> container_of(dev, type, field);}))
> #else
> #define DO_UPCAST(type, field, dev) container_of(dev, type, field)
> #endifThat comment is horrible. In object-oriented programming, there are two common sets of terminology: base and derived classes (derived adds on to base) parent and child classes (child inherits from parent) Going from "base type" to "parent type" makes NO SENSE, since those two terms are synonyms between the two terminologies. Furthermore, according to these references: https://programmers.stackexchange.com/questions/148615/what-is-upcasting-downcasting http://en.wikipedia.org/wiki/Downcasting the act of going from a base class to a derived class (or from a parent type to a child type) is considered downcasting. Yet our definition of DO_UPCAST is used to go from a base class to the derived class. Example usage in blockdev.c is to declare a derived class: > /* internal snapshot private data */ > typedef struct InternalSnapshotState { > BlkTransactionState common; ... > }; then to take a generic parent pointer and convert it into the derived type: > static void internal_snapshot_prepare(BlkTransactionState *common, > Error **errp) > { ... > InternalSnapshotState *state; ... > state = DO_UPCAST(InternalSnapshotState, common, common); I much prefer the name container_of() (which is a bit more obvious that it is finding the container or derived type that embeds the parent type), but if we have to keep the ugly name, could we at least clean up the comment to make sense, and fix the name to be DO_DOWNCAST to match what it is actually doing? -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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