Understanding the Linux Kernel

15.2. Memory Mapping

As already mentioned in Section 7.3 in Chapter 7, a memory region can be associated with a file (or with some portion of it) of a disk-based filesystem. This means that an access to a byte within a page of the memory region is translated by the kernel into an operation on the corresponding byte of the regular file. This technique is called memory mapping.

Two kinds of memory mapping exist:

Shared

Any write operation on the pages of the memory region changes the file on disk; moreover, if a process writes into a page of a shared memory mapping, the changes are visible to all other processes that map the same file.

Private

Meant to be used when the process creates the mapping just to read the file, not to write it. For this purpose, private mapping is more efficient than shared mapping. But any write operation on a privately mapped page will cause it not to map the page in the file any longer. Thus, a write does not change the file on disk, nor is the change visible to any other processes that access the same file.

A process can create a new memory mapping by issuing an mmap( ) system call (see Section 15.2.3 later in this chapter). Programmers must specify either the MAP_SHARED flag or the MAP_PRIVATE flag as a parameter of the system call; as you can probably guess, in the former case the mapping is shared while in the latter it is private. Once the mapping has been created, the process can read the data stored in the file by simply reading from the memory ...