Palm OS Programming, 2nd Edition

Handling Large Amounts of Data

With all the different limitations on allocating memory, including the size of the dynamic heap, and the maximum size of an allocated chunk, it is not surprising that techniques to get around these problems would be useful. Since helpful is what we aim to be, we provide a couple of ways around these limitations.

Relocatable Chunks in the Storage Area

If you need to allocate a 50K chunk of memory (for a buffer, perhaps), you won’t be able to use the dynamic heap. At least not if you’d like to be able to run on all device/OS releases. What you can do, however, is allocate a chunk of memory from the storage heap instead. The Data Manager call DmNewHandle will allocate a relocatable chunk of memory in the storage heap that is not attached to any database. Like all chunks in the storage heap, it’s directly addressable in memory. Thus, you can read it just as if it were in the dynamic heap. That is, after locking it, of course. To write to the chunk, you’d need to use one of the Data Manager calls DmWrite, DmSet, or DmStrCopy (see Chapter 9). Just like MemHandleNew, DmNewHandle restricts allocations to slightly less than 64 KB.

When you are through with the chunk, you can free the handle with MemHandleFree. There is a real potential for mischief in this strategy, however. If you fail to free handles allocated with DmNewHandle, they will not be automatically freed when your application quits (unlike in the dynamic heap). Fortunately, at least on a soft reset, ...