Windows file to memory mapping structure for LuaJIT, which I was going to use for my game but discovered a better solution.

winmap.lua

local mmap = {}
mmap.__index = mmap
local new_map

local ffi = require "ffi"
ffi.cdef[[
	unsigned long GetLastError();
	void* CreateFileA(
		const char* lpFileName,
		unsigned long dwDesiredAccess,
		unsigned long dwShareMode,
		void* lpSecurityAttributes,
		unsigned long dwCreationDisposition,
		unsigned long dwFlagsAndAttributes,
		void* hTemplateFile
	);
	unsigned long GetFileSize(
		void* hFile,
		unsigned long* lpFileSizeHigh
	);
	long WriteFile(
		void* hFile,
		void* lpBuffer,
		unsigned long nNumberOfBytesToWrite,
		unsigned long* lpNumberOfBytesWritten,
		void* lpOverlapped
	);
	unsigned long SetFilePointer(
		void* hFile,
		long lDistanceToMove,
		long* lpDistanceToMoveHigh,
		unsigned long dwMoveMethod
	);
	void* CreateFileMappingA(
		void* hFile,
		void* lpAttributes,
		unsigned long flProtect,
		unsigned long dwMaximumSizeHigh,
		unsigned long dwMaximumSizeLow,
		const char* lpName
	);
	void* MapViewOfFile(
		void* hFileMappingObject,
		unsigned long dwDesiredAccess,
		unsigned long dwFileOffsetHigh,
		unsigned long dwFileOffsetLow,
		size_t dwNumberOfBytesToMap
	);
	long UnmapViewOfFile(void* lpBaseAddress);
	long CloseHandle(void* hObject);
	long DeleteFileA(const char* lpFileName);
]]
local C = ffi.C
local bit = require "bit"

local ERROR_ALREADY_EXISTS = 183
local GENERIC_READ  = 0x80000000
local GENERIC_WRITE = 0x40000000
local OPEN_ALWAYS   = 4
local FILE_ATTRIBUTE_ARCHIVE = 0x20
local FILE_FLAG_RANDOM_ACCESS = 0x10000000
local FILE_BEGIN = 0
local PAGE_READWRITE = 0x4
local FILE_MAP_ALL_ACCESS = 0xf001f

function mmap:__new(filename, newsize)
	local m = ffi.new(self, #filename+1)
	
	-- Open file
	m.filehandle = C.CreateFileA(filename, bit.bor(GENERIC_READ, GENERIC_WRITE), 0, nil,
		OPEN_ALWAYS, bit.bor(FILE_ATTRIBUTE_ARCHIVE, FILE_FLAG_RANDOM_ACCESS), nil)
	if m.filehandle == nil then
		error("Could not create/open file for mmap: "..tostring(C.GetLastError()))
	end
	
	-- Set file size if new
	local exists = C.GetLastError() == ERROR_ALREADY_EXISTS
	if exists then
		local fsize = C.GetFileSize(m.filehandle, nil)
		if fsize == 0 then
			-- Windows will error if mapping a 0-length file, fake a new one
			exists = false
			m.size = newsize
		else
			m.size = fsize
		end
	else
		m.size = newsize
	end
	m.existed = exists
	
	-- Open mapping
	m.maphandle = C.CreateFileMappingA(m.filehandle, nil, PAGE_READWRITE, 0, m.size, nil)
	if m.maphandle == nil then
		error("Could not create file map: "..tostring(C.GetLastError()))
	end
	
	-- Open view
	m.map = C.MapViewOfFile(m.maphandle, FILE_MAP_ALL_ACCESS, 0, 0, 0)
	if m.map == nil then
		error("Could not map: "..tostring(C.GetLastError()))
	end
	
	-- Copy filename (for delete)
	ffi.copy(m.filename, filename)
	
	return m
end

function mmap:getMap()
	return self.map
end

function mmap:__len()
	return self.size
end

function mmap:close(no_ungc)
	if self.map ~= nil then
		C.UnmapViewOfFile(self.map)
		self.map = nil
	end
	if self.maphandle ~= nil then
		C.CloseHandle(self.maphandle)
		self.maphandle = nil
	end
	if self.filehandle ~= nil then
		C.CloseHandle(self.filehandle)
		self.filehandle = nil
	end
	
	if not no_ungc then ffi.gc(self, nil) end
end

function mmap:__gc()
	self:close(true)
end

function mmap:delete()
	self:close()
	C.DeleteFileA(self.filename)
end

new_map = ffi.metatype([[struct {
	short existed;
	void* filehandle;
	void* maphandle;
	void* map;
	int size;
	char filename[?];
}]], mmap)

return new_map