Using Pascal/Delphi with R

Contents

• Borland Delphi v. 5

  • Creating a DLL
  • Building a package
  • Calling R
  • Calling conventions
  • Preserving registers
  • Debugging

• Multi-dimensional arrays and memory management in Delphi

Creating a DLL

Create a DLL in the Delphi IDE by using File|New...|DLL. This will create and open a .dpr file containing a "library" module.

Export procedures from your module using the exports clause, e.g.

exports test;

where test is a procedures declared within the library module or within a unit that it uses.

Building a package

Calling R functions

Declare R functions using the external modifier, e.g.

function ismdi:longbool; cdecl; external 'R.dll';
function R_alloc(n,size:integer):pchar; cdecl; external 'R.dll';

Calling conventions

Declare procedures using the "cdecl" modifier, e.g.

procedure test(var a:integer;var b:double;var c:double); cdecl;

To return string values, you must copy them into memory allocated by R. For example, the function GetText below fills a vector with the string "hello":

function Rpchar(s:string):pchar;
begin
  result := R_alloc(1,length(s)+1);
  strpcopy(result,s);
end;

type
  PCharArray = array[1..1000000] of pchar;

procedure GetText(var s:PCharArray;var num:integer); cdecl;
var
  i : integer;
begin
  for i:=1 to num do
    s[i] := RPchar('hello');
end;

Preserving registers

Delphi uses the same register preserving conventions as R, so this is mostly done automatically. The only problem that is likely to arise is with the floating point processor (FPU) registers. Some versions of Delphi change the FPU control word upon entry to a DLL (but this is not true of Delphi 5); all Windows programs are at risk of changing the registers whenever they call a sloppily written DLL, such as the ones that are used in the common dialogs (File Open, etc.).

To protect against messing up R by changing the FPU control word, the following strategy works:

1. On first entry to the DLL, set the control word to the standard R value, then save that as the Delphi default. Put this code in the library module:
   
   procedure Rwin_fpset; cdecl; external 'R.dll';
   
   function Get8087CW:word;
   begin
     asm
       fstcw result
     end;
   end;
   
   begin
     Rwin_fpset();
     Set8087CW(Get8087CW);
   end.
   
   
2. After any operation that may have changed the status word, put this call before returning to R:
   
     Set8087CW(Default8087CW);
   
   
3. The following function may be useful in debugging; it returns false when the control word has been unexpectedly changed:
   
   function Okay8087CW:boolean;
   begin
     result := ((Default8087CW xor Get8087CW) and not $E060) = 0;
   end;
   

Debugging

To allow source level debugging of your code:

1. Under Project|Options...|Linker, change the image base to $15000000 (or some other value that is unlikely to conflict with another DLL).
2. Under Run|Parameters..., enter the path to the R GUI in the "Host Application" box, along with any arguments you might use on the R command line.
3. Set any desired breakpoints in your code.
4. Hit F9 to compile your DLL and run R.
5. Within R, execute code that loads your DLL and calls one of your functions. Delphi should stop at the first breakpoint that gets executed.

Mark Bravington points out the following:

• When you run R in the debugger, Delphi will likely trap several errors in the startup code. As of 1.7.0 the cause of these errors is known (a minor incompatibility between R's memory manager and the MinGW startup code), but the only known fix is to rebuild R using the standard memory manager (by commenting out the line
  
  LEA_MALLOC=YES
  
  in the src/gnuwin32/MkRules file). This is likely to result in a substantially slower executable. Because the error appears to be harmless but difficult to fix, it is not likely to be fixed soon.
• Delphi will start R with the working directory being the one where your DLL was built. If you normally work in a different directory you'll need to do some fiddling on startup; details are given in this message.

Multidimensional arrays in Delphi

Last modified: May 23, 2003, by Duncan Murdoch