Sharing data array between two applications in Delphi

Scratched my head thinking of what a short-but-complete example of sharing memory between two applications might be. The only option is a console application, GUI applications require a minimum of 3 files (DPR + PAS + DFM). So I cooked up a small example where one integers array is shared using a memory mapped file (backed by the page file so I don't need to have a phisical file on disk for this to work). The console application responds to 3 commands:

• EXIT
• SET NUM VALUE Changes the value at index NUM in the array to VALUE
• DUMP NUM displays the value in the array at index NUM
• DUMP ALL displays the whole array

Of course, the command processing code takes up about 80% of the whole application. To test this compile the following console application, find the executable and start it twice. Go to the first window and enter:

SET 1 100SET 2 50

Go to the second console and enter this:

DUMP 1DUMP 2DUMP 3SET 1 150

Go to the first console and enter this:

DUMP 1

There you have it, you've just witnessed sharing memory between two applications.

program Project2;{$APPTYPE CONSOLE}uses  SysUtils, Windows, Classes;type  TSharedArray = array[0..10] of Integer;  PSharedArray = ^TSharedArray;var  hFileMapping: THandle; // Mapping handle obtained using CreateFileMapping  SharedArray: PSharedArray; // Pointer to the shared array  cmd, s: string;  num, value, i: Integer;  L_CMD: TStringList;function ReadNextCommand: string;begin  WriteLn('Please enter command (one of EXIT, SET NUM VALUE, DUMP NUM, DUMP ALL)');  WriteLn;  ReadLn(Result);end;begin  try    hFileMapping := CreateFileMapping(0, nil, PAGE_READWRITE, 0, SizeOf(TSharedArray), '{C616DDE6-23E2-425C-B871-9E0DA54D96DF}');    if hFileMapping = 0 then      RaiseLastOSError    else      try        SharedArray := MapViewOfFile(hFileMapping, FILE_MAP_READ or FILE_MAP_WRITE, 0, 0, SizeOf(TSharedArray));        if SharedArray = nil then          RaiseLastOSError        else          try            WriteLn('Connected to the shared view of the file.');            cmd := ReadNextCommand;            while UpperCase(cmd) <> 'EXIT' do            begin              L_CMD := TStringList.Create;              try                L_CMD.DelimitedText := cmd;                for i:=0 to L_CMD.Count-1 do                  L_CMD[i] := UpperCase(L_CMD[i]);                if (L_CMD.Count = 2) and (L_CMD[0] = 'DUMP') and TryStrToInt(L_CMD[1], num) then                  WriteLn('SharedArray[', num, ']=', SharedArray^[num])                else if (L_CMD.Count = 2) and (L_CMD[0] = 'DUMP') and (L_CMD[1] = 'ALL') then                  begin                    for i:= Low(SharedArray^) to High(SharedArray^) do                      WriteLn('SharedArray[', i, ']=', SharedArray^[i]);                  end                else if (L_CMD.Count = 3) and (L_CMD[0] = 'SET') and TryStrToInt(L_CMD[1], num) and TryStrToInt(L_CMD[2], value) then                  begin                    SharedArray^[num] := Value;                    WriteLn('SharedArray[', num, ']=', SharedArray^[num]);                  end                else                   WriteLn('Error processing command: ' + cmd);              finally L_CMD.Free;              end;              // Requst next command              cmd := ReadNextCommand;            end;          finally UnmapViewOfFile(SharedArray);          end;      finally CloseHandle(hFileMapping);      end;  except    on E: Exception do      Writeln(E.ClassName, ': ', E.Message);  end;end.

A Named File Mapping would be the easiest solution, here is some short example code.In this sample there is a main program that writes some data and reader(s) that only read from it.

Main:

type  TSharedData = record    Handle: THandle;  end;  PSharedData = ^TSharedData;const  BUF_SIZE = 256;var  SharedData: PSharedData;  hFileMapping: THandle;  // Don't forget to close when you're donefunction CreateNamedFileMapping(const Name: String): THandle;begin  Result := CreateFileMapping(INVALID_HANDLE_VALUE, nil, PAGE_READWRITE, 0,    BUF_SIZE, PChar(Name));  Win32Check(Result > 0);  SharedData := MapViewOfFile(Result, FILE_MAP_ALL_ACCESS, 0, 0, BUF_SIZE);  Win32Check(Assigned(SharedData));end;procedure TForm1.Button1Click(Sender: TObject);begin  hFileMapping := CreateNamedFileMapping('MySharedMemory');  Win32Check(hFileMapping > 0);  SharedData^.Handle := CreateHiddenWindow;end;

reader:

var  hMapFile: THandle;   // Don't forget to closefunction GetSharedData: PSharedData;begin  hMapFile := OpenFileMapping(FILE_MAP_ALL_ACCESS, False, 'MySharedMemory');  Win32Check(hMapFile > 0);  Result := MapViewOfFile(hMapFile, FILE_MAP_ALL_ACCESS, 0, 0, BUF_SIZE);  Win32Check(Assigned(Result));end;