About File Mapping
File mapping is the association of a file's contents with a portion of the virtual address
space of a process. The operating system creates a
file-mapping object to maintain this association. A
file view is the portion of virtual address space that the process uses to access the
file's contents. Processes read from and write to the file view using pointers,
just as they would with dynamically allocated memory. Processes can also
manipulate the file view with the virtual memory function
VirtualProtect.
The file-mapping functions in the Microsoft® Win32® application programming interface (API) allow a process to create
file-mapping objects and file views to easily access and share data. The following
illustration shows the relationship between the file on disk, a file-mapping object,
and a file view.
The file on disk can be any file that you want to map into memory, or it can
be the system pagefile.
The file-mapping object can consist of all or only part of the file. It is
backed by the file on disk. This means that when the system swaps out pages of the
file-mapping object, any changes made to the file-mapping object are written
to the file. When the pages of the file-mapping object are swapped back in, they
are restored from the file.
A file view can consist of all or only part of the file-mapping object. A
process manipulates the file through the file views. A process can create multiple
views for a file-mapping object.
Windows NT: The file views created by each process reside in the virtual address space of
that process.
Windows 95: All file views reside in the shared address space. The shared address space
exists in the range between 2 and 3 gigabytes in the virtual address space for
each process. It contains the 16-bit heap and shared system DLLs, as well as
file views.
When multiple processes use the same file-mapping object to create views for a
local file, the data is coherent. That is, the views contain identical copies
of the file on disk. The file cannot reside on a remote computer if you want to
share memory between multiple processes.
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