The main advantage of this method is that data doesn’t need to be duplicated and sent to another process - it’s just shared (so you’re actually saving some memory and cpu cycles). Basically, a memory mapped file is a space allocated on the user-mode portion of memory which is then made ‘public’ by the kernel, so other. A Memory Mapped File is a low level feature provided by the operating system. It allows to map a file to virtual memory, so any operation over that memory will be reflected to the file. Pip install download. The file is created by a process and can (but not necessarily) persists after the process ends. MMF is a common technique used for Inter Process Communication. Mar 30, 2017 Memory-Mapped Files.; 9 minutes to read +8; In this article. A memory-mapped file contains the contents of a file in virtual memory. This mapping between a file and memory space enables an application, including multiple processes, to modify the file by reading and writing directly to the memory.

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I wrote a code for writing the content to the mapped buffer which mapped by using the mmap() system call.After I did some the changes in the mapped buffer,then I called the msync().It should update to the file on disk.

https://gsmtree.amebaownd.com/posts/19105065. But,It doesn't made any changes to the file on disk.

Trying

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satsat

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If you want your changes to be reflected in the on-disk file, you must map the file as MAP_SHARED, not MAP_PRIVATE.

Additionally, you cannot extend the file simply by writing beyond the end of the mapping. You must use ftruncate() to extend the file to the new size, then change the mapping to include the new portion of the file. The portable way to change the mapping is to unmap the mapping then recreate it with the new size; on Linux you can instead use mremap().

Your len and len_file variables should be of type size_t, and you should use memcpy() rather than strcat(), since you know exactly the length of the string, exactly where you want to copy it, and you don't want to copy the null-terminator.

C# Memory Mapped File Example

The following modification of your code works on Linux (using mremap()) :

cafcaf

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Note that you've provided a mapping for the file that is exactly the size of the file. If you create the file in your call to open(2), it will have a length of 0, and I wouldn't be surprised if the kernel doesn't bother setting up any kind of memory mapping from a 0 length mapping. (Maybe it does? I've never tried..)

I would suggest using ftruncate(2) to extend the length of your file before performing the mapping. (Note that extending files using ftruncate(2) isn't very portable; not all platforms provide extending functionality and not all filesystem drivers support the extending functionality. See your system's manpage for details.)

You must use the MAP_SHARED mapping for your file modifications to be saved to disk.

Your use of perror(3) isn't quite correct; perror(3) will not terminate your program, so it will continue executing with incorrect assumptions:

Should read:

(Or exit(EXIT_FAILURE) if you want to be more portable -- I find that a little harder on the eyes but I live in Linux-land.)

strcat(3) expects to find an ASCII NUL character (byte value 0x00, C representation '0') -- the usual C end-of-string marker -- at the end of the dest string. Your file will not contain an ASCII NUL if you create it in this program -- its length is zero, after all -- and I don't know the consequences of trying to read a zero-byte file via mmap(2). If the file already exists and has data in it, it probably doesn't have an ASCII NUL encoded in the file. strcat(3) is almost certainly the wrong tool to write into your file. (No one wants ASCII NULs in their files anyway.) Try memcpy(3) instead.

sarnoldsarnold

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