Memory Mapped Files
Writing code nowadays can be considered as a "cheerful experience", we have a complete set of tools and development environments that make us more productive, helping us in the delivery of high-quality software. 15 years ago it was a hard task to find IDEs with Intellisense support, we used to store our application settings in .INI files and we didn't even have any virtualization environment in order to perform tests before sending out our solution to the customers, that's why it's a new adventure each time we got engaged on a new development project and coding hasn't ever been as enjoyable as it is today. A few years ago I read a book written by Jeffrey Richter called "Advanced Windows" and I learned heaps from this book, as a matter of fact, last year I bought his latest book called "Windows via C/C++" which is a classic and a must have in every Windows developer toolbox, same thing occurs with Charles Petzold's book, however I learned a lot about Windows internal mechanisms by reading Richter's, for instance, how can processes exchange information between them? What's the difference between a Mutex and a Semaphore for threads synchronization? just to mention a few. Windows is a product that has evolved and grown throughout the years, despite of new APIs and functionality that have been included into it we will always need access to the PC's memory, for example we can mention Inter-Process Communication (IPC) and virtual memory access, this post is about it pretty much, how can I get access to the virtual memory so I can address and use a given amount of bytes? I think you know already what I'm talking about. Memory Mapped Files can be defined as "a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on-disk, but can also be a device, shared memory object or other resource that the operating system can reference through a file descriptor". They have three main purposes which are:
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Load and execute .exe files and DLLs
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Access files on-disk very quickly without using buffers. Windows handles this for us
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Allow many processes to share information in memory
The following diagram depicts some implementations for memory mapped files
Once the file is mapped in memory, a view based on that file needs to be created, I mean, a region or space within the file which allows a process to read and write onto it, when creating the view we specify bytes to be mapped, file offset and desired access, this process is shown in the image below
One of the new features bundled in .NET Framework 4.0 besides DLR (Dynamic Language Runtime), parallel processing and support for memory mapped files. The code that accompanies this post demonstrates how to do it using the new MemoryMappedFile class plus my own implementation with .NET 3.5, C# and a little of Interop.
Further reading about this topic can be found at Virtual Memory and Paging
Regards,
Angel
Format: swf
Duration: 22 min