On 09/23/2015 04:34 AM, Ahmed S. Darwish wrote:
### Anonymous unions
Used to provide the mempool with polymorphic memory region access
regardless of the type of memory used:
union {
pa_mem mem;
union {
pa_shm shm;
pa_memfd memfd;
pa_privatemem privatemem;
} per_type;
};
This kind of usage looks good.
### Anonymous structs
They are also used in the memfd patches, but they're really used
to minimize code repetitions and make relationships explicit.
Basically, we want to build an inheritance relationship: pa_shm,
pa_memfd, and pa_privatemem has the common parent pa_mem. We also
want to make this relationship explicit since it's at the core of
the anonymous polymorphic union mentioned above.
To do so without introducing extra (compile-time) dereferences,
the following patten is used:
/* Parent anonymous structure representing a plain memory
* area and its size. Different structures inherit from this,
* representing different memory area types (e.g. Posix SHM,
* Linux memfds, or private mallocs).
*
* To inherit from this object, just include PA_MEM_STRUCT as
* the very first element of your structure definition. */
#define PA_MEM_STRUCT struct { \
void *ptr; \
size_t size; \
} PA_GCC_PACKED
typedef struct pa_mem {
PA_MEM_STRUCT;
} pa_mem;
This way, children pa_shm, pa_memfd, and pa_privatemem can be
defined as follows:
typedef struct pa_shm {
PA_MEM_STRUCT;
unsigned id;
bool do_unlink;
} pa_shm;
typedef struct pa_memfd {
PA_MEM_STRUCT;
int fd;
} pa_memfd;
typedef struct pa_privatemem {
PA_MEM_STRUCT;
} pa_privatemem;
I don't like that (on a purely subjective basis).
..
So to summarize, such annotation explicitly shows the inheritance
relationship, minimize code repeition in the struct defintions, and
also saves us an extra compile-time dereference in shm.c, memfd.c,
and privatemem.c code. It also really works in tandem with the
earlier polymorphic union:
union {
pa_mem mem;
union {
pa_shm shm;
pa_memfd memfd;
pa_privatemem privatemem;
} per_type;
};
Hopefully these patterns will be OK from your side :-)
Regards,
--
Alexander E. Patrakov
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