Chapter 17 -- Memory-Mapped Files

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Chapter 17

Working with files is something almost every application must do, and it's always a hassle. Should your application open the file, read it, and close the file, or should it open the file and use a buffering algorithm to read from and write to different portions of the file? Microsoft Windows offers the best of both worlds: memory-mapped files.

Like virtual memory, memory-mapped files allow you to reserve a region of address space and commit physical storage to the region. The difference is that the physical storage comes from a file that is already on the disk instead of the system's paging file. Once the file has been mapped, you can access it as if the whole file were loaded in memory.

Memory-mapped files are used for three different purposes:

• The system uses memory-mapped files to load and execute .exe and DLL files. This greatly conserves both paging file space and the time required for an application to begin executing.
• You can use memory-mapped files to access a data file on disk. This shelters you from performing file I/O operations on the file and from buffering the file's contents.
• You can use memory-mapped files to allow multiple processes running on the same machine to share data with each other. Windows does offer other methods for communicating data among processes—but these other methods are implemented using memory-mapped files, making memory-mapped files the most efficient way for multiple processes on a single machine to communicate with one another.

In this chapter, we will examine each of these uses for memory-mapped files.