Local Memory: 2.2 Structure of a Local Memory Heap: Chunks and Chunk Handles

Just as blocks on the local heap are accessed with handles, chunks are accessed via chunk handles. Each chunk handle is an offset into the block containing the local memory heap; thus, the segment address of the locked heap, combined with the chunk handle, make up a pointer to a location within the local memory heap's chunk handle table. That location contains another offset which, when combined with the segment address of the block, composes a far-pointer to the actual chunk.

Chunks are movable within the local heap; whenever a chunk is created or resized, the local memory manager may move any chunks in that heap. There is no locking mechanism for chunks; thus, creating or resizing a chunk can potentially invalidate pointers to all the other chunks, forcing the application to again dereference the handles for these chunks. Be warned that many message handlers can cause heap compaction. As a general rule, you should not save chunk addresses around message sends; instead, dereference the chunk handles to get the current address.

Chunks are aligned along dwords. This speeds up chunk moves and similar operations. This means that when you request a chunk, its size may be slightly larger than you request.

Objects are special kinds of LMem chunks. An optr is simply the global memory handle of an LMem heap followed by the chunk handle of an object. For this reason, many LMem routines come in two formats: one which is passed an optr, and one which is passed the global and chunk handles. There is also a macro, ConstructOptr() , which is passed a memory handle and a chunk handle and returns an optr constructed from the two.

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