[pulseaudio-discuss] Allowing anonymous structs and unions

[Alexander E. Patrakov]

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