Windows System Programming Third Edition

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CHAPTER 5 MEMORY MANAGEMENT, MEMORY-MAPPED FILES, AND DLLS

File mapping has two other notable limitations.

• 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

and obtain a heap handle.

Summary: File Mapping

Here is the standard sequence required by file mapping.

1. Open the file. Be certain that it has

access.

2. If the file is new, set its length either with

(step 3

below) or by using

followed by .

3. Map the file with

or .

4. Create one or more views with

.

5. Access the file through memory references. If necessary, change the mapped

regions with

and .

6. On completion, perform, in order,

, for the

mapping handle, and

for the file handle.

Example: Sequential File Processing with Mapped Files

The program (Program 2–4) illustrates sequential file processing by convert-

ing 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. Progr

am

5–3 simply maps the input file and the output file—first computing the output file

length by doubling the input file length—and converts the characters one at a

time.

This example clearly illustrates the trade-off between the file mapping com-

plexity required to initialize the program and the resulting processing s

implicity.

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

EXAMPLE: SEQUENTIAL FILE PROCESSING WITH MAPPED FILES

141

access versions for NTFS files, so the complexity is worthwhile. The book’s Web

site contains additional performance studies; the highlights are summarized here.

• 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 gre

ater.

• The performance advantage disappears for larger files. In this example, as the

input fi le size approa ches 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.

Program 5–3 shows only the function

. The main program is the

same as for Program 2–4.

Program 5–3

File Conversion with Memory Mapping