The CreateNew methods create a memory-mapped file that is not mapped to an existing file on disk and are suitable for creating shared memory for interprocess communication (IPC).Ī memory-mapped file can be associated with an optional name that allows the memory-mapped file to be shared with other processes. Changes are automatically propagated to disk when the file is unmapped. The CreateFromFile methods create a memory-mapped file from a specified path or a FileStream of an existing file on disk. Memory-mapped files can also be shared across multiple processes. Memory-mapped files enable programmers to work with extremely large files because memory can be managed concurrently, and they allow complete, random access to a file without the need for seeking. Green = CType(Math.Min(Short.MaxValue, (CType(Green, Integer) + value)), Short)īlue = CType(Math.Min(Short.MaxValue, (CType(Blue, Integer) + value)), Short)Īlpha = CType(Math.Min(Short.MaxValue, (CType(Alpha, Integer) + value)), Short)Ī memory-mapped file maps the contents of a file to an application's logical address space. Red = CType(Math.Min(Short.MaxValue, (CType(Red, Integer) + value)), Short) Public Sub Brighten(ByVal value As Short) Using accessor = mmf.CreateViewAccessor(offset, length)ĭim colorSize As Integer = Marshal.SizeOf(GetType(M圜olor))
#MEMORY MAPPED FILE PLUS#
' to the 768th megabyte (the offset plus length).
' Create a random access view, from the 256th megabyte (the offset) Using mmf = MemoryMappedFile.CreateFromFile("c:\ExtremelyLargeImage.data", FileMode.Open, "ImgA") Green = (short)Math.Min(short.MaxValue, (int)Green + value) īlue = (short)Math.Min(short.MaxValue, (int)Blue + value) Īlpha = (short)Math.Min(short.MaxValue, (int)Alpha + value) ĭim offset As Long = &H10000000 ' 256 megabytesĭim length As Long = &H20000000 ' 512 megabytes Red = (short)Math.Min(short.MaxValue, (int)Red + value) Int colorSize = Marshal.SizeOf(typeof(M圜olor)) įor (long i = 0 i < length i += colorSize) Using (var accessor = mmf.CreateViewAccessor(offset, length)) to the 768th megabyte (the offset plus length). Create a random access view, from the 256th megabyte (the offset) Read all posts dedicated to file I/O here.The following example creates a memory-mapped view of a part of an extremely large file and manipulates a portion of it. If I declare ‘bytes’ to have a capacity of 10 elements, then “text” will only be “Here comes”. The above example returns “Here comes some log message.” in the “text” variable. We’ll remove them from the result using the Trim method.
If you remember from the post referenced above then you’ll know that the file was padded with a lot of string termination characters. In the above case ‘bytes’ will be populated with the first 50 byte values of the source files. The ‘bytes’ variable could have any length, it will just be padded with zeroes if it’s larger than the source file. It accepts the position where to start reading the bytes, an array to hold the bytes, and offset and a length.
So instead of WriteArray we call the ReadArray method.
ViewAccessor.ReadArray(0, bytes, 0, bytes.Length) Using (MemoryMappedViewAccessor viewAccessor = memoryMappedFile.CreateViewAccessor()) Using (MemoryMappedFile memoryMappedFile FileMode.OpenOrCreate, "log-map")) We’ll still need the MemoryMappedFile and MemoryMappedViewAccessor objects: The way to read from a file is very similar to writing to it.
#MEMORY MAPPED FILE HOW TO#
Here we’ll quickly look at how to read from the same file by mapping it into memory first. We wrote a short string – “Here comes some log message.” – to a 10KB file. In this post we saw how to write to a memory-mapped file in.
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