Figure 5-3 shows the relationship between process address space and a mapped file in .NET framework

Create Quick Response Code in .NET framework Figure 5-3 shows the relationship between process address space and a mapped file
Figure 5-3 shows the relationship between process address space and a mapped file
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Figure 5-3 Process Address Space Mapped to a File
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FlushViewOfFile forces the system to write "dirty" (changed) pages to disk Normally, a process accessing a file through mapping and another process accessing it through conventional file I/O will not have coherent views of the file Performing the file I/O without buffering will not help because the mapped memory will not be written to the file immediately Therefore, it is not a good idea to access a mapped file with ReadFile and WriteFile; coherency is not ensured On the other hand, processes that share a file through shared memory will have a coherent view of the file If one process changes a mapped memory location, the other process will obtain that new value when it accesses the corresponding area of the file in its mapped memory This mechanism is illustrated in Figure 5-4, and coherency works because both processes' virtual addresses, although distinct, are in the same physical memory locations The obvious synchronization issues are addressed [4] in s 8-10
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Statements regarding coherency of mapped views do not apply to networked files The files must be local
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Figure 5-4 Shared Memory
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UNIX, at the SVR4 and 43+BSD releases, supports the mmap function, which is similar to MapViewOfFile The parameters specify the same information except that there is no mapping object munmap is the UnmapViewOfFile equivalent There are no equivalents to the CreateFileMapping and OpenFileMapping functions Any normal file can be mapped directly UNIX does not use mapped files to share memory; rather, it has explicit API functions for memory sharing The UNIX functions are shmget, shmctl, shmat, and shmdt
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File Mapping Limitations
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File mapping, as mentioned previously, is a powerful and useful feature The disparity between Windows' 64-bit file system and 32-bit addressing limits these benefits; Win64 does not have these limitations The principal problem is that if the file is large (greater than 23GB in this case), it is not possible to map the entire file into virtual memory space Furthermore, the entire 3GB will not be available because virtual address space will be allocated for other purposes and available contiguous blocks will be much smaller than the theoretical maximum Win64 will largely remove this limitation When you're dealing with large files that cannot be mapped to one view, create code that carefully maps and unmaps file regions as they are needed This technique can be as complex as managing memory
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buffers, although it is not necessary to perform the explicit reads and writes File mapping has two other notable limitations
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A file mapping cannot be expanded You need to know the maximum size when creating the file mapping, and it may be difficult or impossible to determine this size There is no way to allocate memory within a mapped memory region without creating your own memory management functions It would be convenient if there were a way to specify a file mapping and a pointer returned by MapViewOfFile and obtain a heap handle
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Summary: File Mapping
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Here is the standard sequence required by file mapping 1 2 Open the file Be certain that it has GENERIC_READ access If the file is new, set its length either with CreateFileMapping (step 3 below) or by using SetFilePointer followed by SetEndOfFile Map the file with CreateFileMapping or OpenFileMapping Create one or more views with MapViewOfFile Access the file through memory references If necessary, change the mapped regions with UnmapViewOfFile and MapViewOfFile On completion, perform, in order, UnmapViewOfFile, CloseHandle for the mapping handle, and CloseHandle for the file handle
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Example: Sequential File Processing with Mapped Files
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The atou program (Program 2-4) illustrates sequential file processing by converting ASCII files to Unicode, doubling the file length This is an ideal application for memory-mapped files because the most natural way to convert the data is to process it one character at a time without being concerned with file I/O Program 5-3 simply maps the input file and the output filefirst computing the output file length by doubling the input file lengthand converts the characters one at a time This example clearly illustrates the trade-off between the file mapping complexity required to initialize the program and the resulting processing simplicity This complexity may not seem worthwhile given the simplicity of a simple file I/O implementation, but there is a significant performance advantage Appendix C shows that the memory-mapped version can be considerably faster than the file access versions for NTFS files, so the complexity is worthwhile The book's Web site contains additional performance studies; the highlights are summarized here
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Memory-mapping performance improvements apply only to Windows NT and the NTFS Compared with the best sequential file processing techniques, the performance improvements can be 3:1 or greater The performance advantage disappears for larger files In this example, as the input file size approaches about one-third of the physical memory size, normal sequential scanning is preferable The mapping performance degrades at this point since the input file fills one-third of the memory and the output file, which is twice as long, fills the other two-thirds, forcing parts of the output files to be flushed to disk Thus, on a 192MB system, mapping performance degenerates for input files longer than 60MB Most file processing deals with smaller files and can take advantage of file mapping
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Program 5-3 shows only the function Asc2UnMM The main program is the same as for Program 2-4