Question

I want to share array data between two applications. In my mind, first program create the array and the second program can read the array from already allocated memory area. The array is not a dynamic array.

I found a way to share pointer using OpenFileMapping and MapViewOfFile. I have no luck to implement array sharing and I think i don’t want to use IPC method yet.

Is it possible to plan a scheme like this (sharing array)? My purpose is to minimize memory usage and reading data quickly.

Answer 1

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 1 100
SET 2 50

Go to the second console and enter this:

DUMP 1
DUMP 2
DUMP 3
SET 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.