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Read and write memory mapped files.
Memory mapped files are a mechanism in operating systems that allows file access through virtual memory. After opening a file with MmFile, the contents can be read from or written to with standard slice / pointer operations. Changes to the memory are automatically reflected in the underlying file.
Memory mapping can increase I/O performance of large files, compared to buffered read / write operations from std.file and std.stdio. However, I/O errors are not handled as safely: when for example the disk that the file is on gets removed, reading from it may result in a segfault.
Walter Bright, Matthew Wilson

Source std/mmfile.d

class MmFile;
MmFile objects control the memory mapped file resource.
Read an existing file
import std.file;
std.file.write(deleteme, "hello"); // deleteme is a temporary filename
scope(exit) remove(deleteme);

// Use a scope class so the file will be closed at the end of this function
scope mmfile = new MmFile(deleteme);

writeln(mmfile.length); // "hello".length

// Access file contents with the slice operator
// This is typed as `void[]`, so cast to `char[]` or `ubyte[]` to use it
const data = cast(const(char)[]) mmfile[];

// At this point, the file content may not have been read yet.
// In that case, the following memory access will intentionally
// trigger a page fault, causing the kernel to load the file contents
writeln(data[0 .. 5]); // "hello"
Write a new file
import std.file;
scope(exit) remove(deleteme);

scope mmfile = new MmFile(deleteme, MmFile.Mode.readWriteNew, 5, null);
writeln(mmfile.length); // 5

auto data = cast(ubyte[]) mmfile[];

// This write to memory will be reflected in the file contents
data[] = '\n';


writeln(; // "\n\n\n\n\n"
enum Mode: int;
The mode the memory mapped file is opened with.
Read existing file
Delete existing file, write new file
Read/Write existing file, create if not existing
Read/Write existing file, copy on write
scope this(string filename);
Open memory mapped file filename for reading. File is closed when the object instance is deleted.
scope this(string filename, Mode mode, ulong size, void* address, size_t window = 0);
Open memory mapped file filename in mode. File is closed when the object instance is deleted.
string filename name of the file. If null, an anonymous file mapping is created.
Mode mode access mode defined above.
ulong size the size of the file. If 0, it is taken to be the size of the existing file.
void* address the preferred address to map the file to, although the system is not required to honor it. If null, the system selects the most convenient address.
size_t window preferred block size of the amount of data to map at one time with 0 meaning map the entire file. The window size must be a multiple of the memory allocation page size.
const @property ulong length();
Gives size in bytes of the memory mapped file.
alias opDollar = length;
Forwards length.
Mode mode();
Read-only property returning the file mode.
void[] opSlice();
Returns entire file contents as an array.
void[] opSlice(ulong i1, ulong i2);
Returns slice of file contents as an array.
ubyte opIndex(ulong i);
Returns byte at index i in file.
ubyte opIndexAssign(ubyte value, ulong i);
Sets and returns byte at index i in file to value.