SGI STL二级空间配置器--内存池源码剖析

template <bool threads, int inst>
class __default_alloc_template {

private:
  // Really we should use static const int x = N
  // instead of enum { x = N }, but few compilers accept the former.
#if ! (defined(__SUNPRO_CC) || defined(__GNUC__))
    enum {_ALIGN = 8};
    enum {_MAX_BYTES = 128};
    enum {_NFREELISTS = 16}; // _MAX_BYTES/_ALIGN
# endif
  static size_t
  _S_round_up(size_t __bytes) 
    { return (((__bytes) + (size_t) _ALIGN-1) & ~((size_t) _ALIGN - 1)); }

__PRIVATE:
  union _Obj {
        union _Obj* _M_free_list_link;
        char _M_client_data[1];    /* The client sees this.        */
  };
private:
# if defined(__SUNPRO_CC) || defined(__GNUC__) || defined(__HP_aCC)
    static _Obj* __STL_VOLATILE _S_free_list[]; 
        // Specifying a size results in duplicate def for 4.1
# else
    static _Obj* __STL_VOLATILE _S_free_list[_NFREELISTS]; 
# endif
  static  size_t _S_freelist_index(size_t __bytes) {
        return (((__bytes) + (size_t)_ALIGN-1)/(size_t)_ALIGN - 1);
  }

  // Returns an object of size __n, and optionally adds to size __n free list.
  static void* _S_refill(size_t __n);
  // Allocates a chunk for nobjs of size size.  nobjs may be reduced
  // if it is inconvenient to allocate the requested number.
  static char* _S_chunk_alloc(size_t __size, int& __nobjs);

  // Chunk allocation state.
  static char* _S_start_free;
  static char* _S_end_free;
  static size_t _S_heap_size;

# ifdef __STL_THREADS
    static _STL_mutex_lock _S_node_allocator_lock;
# endif

    // It would be nice to use _STL_auto_lock here.  But we
    // don't need the NULL check.  And we do need a test whether
    // threads have actually been started.
    class _Lock;
    friend class _Lock;
    class _Lock {
        public:
            _Lock() { __NODE_ALLOCATOR_LOCK; }
            ~_Lock() { __NODE_ALLOCATOR_UNLOCK; }
    };

public:

  /* __n must be > 0      */
  static void* allocate(size_t __n)
  {
    void* __ret = 0;

    if (__n > (size_t) _MAX_BYTES) {
      __ret = malloc_alloc::allocate(__n);
    }
    else {
      _Obj* __STL_VOLATILE* __my_free_list
          = _S_free_list + _S_freelist_index(__n);
      // Acquire the lock here with a constructor call.
      // This ensures that it is released in exit or during stack
      // unwinding.
#     ifndef _NOTHREADS
      /*REFERENCED*/
      _Lock __lock_instance;
#     endif
      _Obj* __RESTRICT __result = *__my_free_list;
      if (__result == 0)
        __ret = _S_refill(_S_round_up(__n));
      else {
        *__my_free_list = __result -> _M_free_list_link;
        __ret = __result;
      }
    }

    return __ret;
  };

  /* __p may not be 0 */
  static void deallocate(void* __p, size_t __n)
  {
    if (__n > (size_t) _MAX_BYTES)
      malloc_alloc::deallocate(__p, __n);
    else {
      _Obj* __STL_VOLATILE*  __my_free_list
          = _S_free_list + _S_freelist_index(__n);
      _Obj* __q = (_Obj*)__p;

      // acquire lock
#       ifndef _NOTHREADS
      /*REFERENCED*/
      _Lock __lock_instance;
#       endif /* _NOTHREADS */
      __q -> _M_free_list_link = *__my_free_list;
      *__my_free_list = __q;
      // lock is released here
    }
  }

  static void* reallocate(void* __p, size_t __old_sz, size_t __new_sz);

} ;