How to get data from a file without reading it into memory #167

Question

Is there a way to point a pointer to a text data file on a hard drive without reading into memory. Here is the problem. I have a third-party DLL that requires a pointer to a large char string 10000 + chars. If I were to read into memory and then call the DLL it could cause problems.

You can use Mapped Files. A mapped file is a region in memory that is mapped to a file on disk. After you map a file to memory you get a pointer to the memory region and use it like any other pointer - Windows will load and unload pages from the file to memory as needed. Here is a very simple implementation of a mapped file. It is used only to read data from the file so you might want to change it to also allow writing. After you create an instance, the Content property is a pointer to the file content.

{ ... }
type
  TMappedFile=class
  private
    FMapping: THandle;
    FContent: PChar;
    FSize: Integer;
    procedure MapFile(const FileName: string);
  public
    constructor Create(const FileName: string);
    destructor Destroy; override;
    property Content: PChar read FContent;
    property Size: Integer read FSize;
  end;

implementation

uses
  sysutils;

{ TMappedFile }

constructor TMappedFile.Create(const FileName: string);
begin
  inherited Create;
  MapFile(FileName);
end;

destructor TMappedFile.Destroy;
begin
  UnmapViewOfFile(FContent);
  CloseHandle(FMapping);
  inherited;
end;

procedure TMappedFile.MapFile(const FileName: string);
var
  FileHandle: THandle;
begin
  FileHandle := FileOpen(FileName, fmOpenRead or fmShareDenyWrite);
  Win32Check(FileHandle <> 0);
  try
    FSize := GetFileSize(FileHandle, nil);
    FMapping := CreateFileMapping(FileHandle, nil, PAGE_READONLY, 0, 0, nil);
    Win32Check(FMapping <> 0);
  finally
    FileClose(FileHandle);
  end;
  FContent := MapViewOfFile(FMapping, FILE_MAP_READ, 0, 0, 0);
  Win32Check(FContent <> nil);
end;

Original resource:	The Delphi Pool
Author:	Eyal Post
Added:	2010/06/02
Last updated:	2010/06/02