分配slab对象的kmem_cache_alloc函数

stuman

1. void * kmem_cache_alloc(kmem_cache_t *cachep, int flags)
   
2. {
   
3. unsigned long save_flags;
   
4. void *objp;
   
5. struct array_cache *ac;
   
6. local_irq_save(save_flags);
   
7. ac = cache_p->array[smp_processor_id()];
   
8. if (ac->avail) {
   
9. ac->touched = 1;
   
10. objp = ((void **)(ac+1))[--ac->avail];
    
11. } else
    
12. objp = cache_alloc_refill(cachep, flags);
    
13. local_irq_restore(save_flags);
    
14. return objp;
    
15. }

复制代码

以上代码用于分配slab对象，我对代码objp = ((void **)(ac+1))[--ac->avail];很不理解，请问其中ac+1与--ac->avail分别是什么意思？

firkraag

The function tries first to retrieve a free object from the local cache. If there are free objects, the avail field contains the index in the local cache of the entry that points to the last freed object. Because the local cache array is stored right after the ac descriptor, ((void**)(ac+1))[--ac->avail] gets the address of that free object and decreases the value of ac->avail.

aweii

复习一下C语言的指针