Windows® via C/C++, Fifth Edition

Sparsely Committed Memory-Mapped Files

In all the discussion of memory-mapped files so far, we see that the system requires that all storage for the memory-mapped file be committed either in the data file on disk or in the paging file. This means that we can’t use storage as efficiently as we might like. Let’s return to the discussion of the spreadsheet from When to Decommit Physical Storage. Let’s say that you want to share the entire spreadsheet with another process. If we were to use memory-mapped files, we would need to commit the physical storage for the entire spreadsheet:

CELLDATA CellData[200][256];

If a CELLDATA structure is 128 bytes, this array requires 6,553,600 (200 × 256 × 128) bytes of physical storage. As I said in an earlier chapter, ...