Re: A few questions about memory allocation

It was thus said that the Great 'Sewbacca' via lua-l once stated:
> I was wondering the other day how Lua handles memory allocations for a 
> few reasons.
> 
> The next best thing would be to preallocate an arena and just hand 
> userdatas with pointers.
> If I allocate a userdata object of a small size, say the size of a 
> pointer, does Lua allocate it on the heap or does it manage an arena of 
> some kind?
> I suspect it allocates the data on the heap, so it's probably best to 
> allocate the C structure directly in the memory provided by Lua, rather 
> than calling malloc myself and just storing just a pointer right?
> Of course could manage my own arena and hand out light userdata objects, 
> but this means I cannot attach different metatables to different pointers.
> 
> Any comments and or suggestions would be appreciated!

  Read up on lua_Alloc: <https://www.lua.org/manual/5.5/manual.html#lua_Alloc>

  It states:

	typedef void * (*lua_Alloc) (void *ud,
	                             void *ptr,
	                             size_t osize,
	                             size_t nsize);
	
	The type of the memory-allocator function used by Lua states. The
	allocator function must provide a functionality similar to realloc,
	but not exactly the same. Its arguments are ud, an opaque pointer
	passed to lua_newstate; ptr, a pointer to the block being
	allocated/reallocated/freed; osize, the original size of the block
	or some code about what is being allocated; and nsize, the new size
	of the block.

	When ptr is not NULL, osize is the size of the block pointed by ptr,
	that is, the size given when it was allocated or reallocated.

	When ptr is NULL, osize encodes the kind of object that Lua is
	allocating.  osize is any of LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION,
	LUA_TUSERDATA, or LUA_TTHREAD when (and only when) Lua is creating a
	new object of that type.  When osize is some other value, Lua is
	allocating memory for something else.


So just provide your own allocation function (via lua_newstate()).  I think
something like:

	void *my_alloc(void *ud,void *ptr,size_t osize,size_t nsize)
	{
	  if (ptr == NULL) /* grabbing some memory */
	  {
	    if ((osize == LUA_TUSERDATA) && (nsize == sizeof(arenea_object))
	      return next_arenaobject();
	    else
	      return realloc(ptr,nsize);
	  }
	  else
	  {
	    if (nsize == 0) /* freeing memory */
	    {
	      if (is_arenaobject(ptr))
	        mark_areanaobject_free(ptr);
	      else
	        free(ptr);
	    }
	    else
	      return realloc(ptr,nsize);
	  }
	}

  -spc

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