Creating and Using Mappings

mmap(2) establishes a mapping of a named file system object into a process address space. A named file system object can also be partially mapped into a process address space. This basic memory management interface is very simple. Use open(2) to open the file, then use mmap(2) to create the mapping with appropriate access and sharing options. Then, proceed with your application.

The mapping established by mmap(2) replaces any previous mappings for the specified address range.

The flags MAP_SHARED and MAP_PRIVATE specify the type of mapping. You must specify a mapping type. If the MAP_SHARED flag is set, write operations modify the mapped object. No further operations on the object are needed to make the change. If the MAP_PRIVATE flag is set, the first write operation to the mapped area creates a copy of the page. All further write operations reference the copy. Only modified pages are copied.

A mapping type is retained across a fork(2).

After you have established the mapping through mmap(2), the file descriptor used in the call is no longer used. If you close the file, the mapping remains until munmap(2) undoes the mapping. Creating a new mapping replaces an existing mapping.

A mapped file can be shortened by a call to truncate. An attempt to access the area of the file that no longer exists causes a SIGBUS signal.

Mapping /dev/zero gives the calling program a block of zero-filled virtual memory. The size of the block is specified in the call to mmap(2). The following code fragment demonstrates a use of this technique to create a block of zeroed storage in a program. The block's address is chosen by the system.

removed to fr.ch4/pl1.create.mapping.c

Some devices or files are useful only when accessed by mapping. Frame buffer devices used to support bit-mapped displays are an example of this phenomenon. Display management algorithms are much simpler to implement when the algorithms operate directly on the addresses of the display.