Temporary Memory

In the Macintosh multitasking environment, each application is limited to a particular memory partition (whose size is determined by information in the 'SIZE' resource of that application). The size of your application's partition places certain limits on the size of your application heap and hence on the sizes of the buffers and other data structures that your application uses. In general, you specify an application partition size that is large enough to hold all the buffers, resources, and other data that your application is likely to need during its execution.

If for some reason you need more memory than is currently available in your application heap, you can ask the Operating System to let you use any available memory that is not yet allocated to any other application. This memory, known as temporary memor y, is allocated from the available unused RAM; usually, that memory is not contiguous with the memory in your application's zone. Figure 1-7 shows an application using some temporary memory.

Figure 7 Using temporary memory allocated from unused RAM

In Figure 1-7 , Application 1 has almost exhausted its application heap. As a result, it has requested and received a large block of temporary memory, extending from the top of Application 2's partition to the top of the allocatable space. Application 1 can use the temporary memory in whatever manner it desires.

Your application should use temporary memory only for occasional short-term purposes that could be accomplished in less space, though perhaps less efficiently. For example, if you want to copy a large file, you might try to allocate a fairly large buffer of temporary memory. If you receive the temporary memory, you can copy data from the source file into the destination file using the large buffer. If, however, the request for temporary memory fails, you can instead use a smaller buffer within your application heap. Although using the smaller buffer might prolong the copying operation, the file is nonetheless copied.

One good reason for using temporary memory only occasionally is that you cannot assume that you will always receive the temporary memory you request. For example, in Figure 1-7 , all the available memory is allocated to the two open applications; any further requests by either one for some temporary memory would fail. For complete details on using temporary memory, see the chapter "Memory Manager" in this book.

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