Memory-Mapped Files

Suppose that your program is accessing a file (either reading its content or adding content to it.) Each time your program accesses this file, the OS will need to access a location on disk, which takes considerable time than accessing main memory.

To shorten these access times, the OS can memory-map the file: it will copy the content of the file (or a part of it) to memory, so the program will access the file as a regular variable in memory (specifically, an array of characters.)

When a process writes (= makes changes) to a memory-mapped file, these changes won't be written to disk right away.

• To force the OS to back up all the recent changes to disk, the program needs to call the sync() system call.
• Changes to a memory-mapped file are flushed to disk also when the program closes the file via the close() system call. This is why it is so important to include in your programs a function call to close a file after it was used.

Some systems (e.g., Linux) provide special system calls to memory map files and use direct disk access otherwise.