How can a function with 'varargs' retrieve the contents of the stack?

Question

Normally, in Delphi one would declare a function with a variable number of arguments using the 'array of const' method. However, for compatibility with code written in C, there's an much-unknown 'varargs' directive that can be added to a function declaration (I learned this while reading Rudy's excellent 'Pitfalls of convering' document).

As an example, one could have a function in C, declared like this :

void printf(const char *fmt, ...)

In Delphi, this would become :

procedure printf(const fmt: PChar); varargs;

My question is : How can I get to the contents of the stack when implementing a method which is defined with the 'varargs' directive?

I would expect that some tooling for this exists, like Dephi translations of the va_start(), va_arg() and va_end() functions, but I can't find this anywhere.

Please help!

PS: Please don't drift off in discussions about the 'why' or the 'array of const' alternative - I need this to write C-like patches for functions inside Xbox games (see the Delphi Xbox emulator project 'Dxbx' on sourceforge for details).

Answer 1

OK, I see the clarification in your question to mean that you need to implement a C import in Delphi. In that case, you need to implement varargs yourself.

The basic knowledge needed is the C calling convention on the x86: the stack grows downwards, and C pushes arguments from right to left. Thus, a pointer to the last declared argument, after it is incremented by the size of the last declared argument, will point to the tail argument list. From then, it's simply a matter of reading the argument out and incrementing the pointer by an appropriate size to move deeper into the stack. The x86 stack in 32-bit mode is 4-byte aligned generally, and this also means that bytes and words are passed as 32-bit integers.

Anyhow, here's a helper record in a demo program that shows how to read out data. Note that Delphi seems to be passing Extended types in a very odd way; however, you likely won't have to worry about that, as 10-byte floats aren't generally widely used in C, and aren't even implemented in the latest MS C, IIRC.

{$apptype console}

type  
  TArgPtr = record
  private
    FArgPtr: PByte;
    class function Align(Ptr: Pointer; Align: Integer): Pointer; static;
  public
    constructor Create(LastArg: Pointer; Size: Integer);
    // Read bytes, signed words etc. using Int32
    // Make an unsigned version if necessary.
    function ReadInt32: Integer;
    // Exact floating-point semantics depend on C compiler.
    // Delphi compiler passes Extended as 10-byte float; most C
    // compilers pass all floating-point values as 8-byte floats.
    function ReadDouble: Double;
    function ReadExtended: Extended;
    function ReadPChar: PChar;
    procedure ReadArg(var Arg; Size: Integer);
  end;

constructor TArgPtr.Create(LastArg: Pointer; Size: Integer);
begin
  FArgPtr := LastArg;
  // 32-bit x86 stack is generally 4-byte aligned
  FArgPtr := Align(FArgPtr + Size, 4);
end;

class function TArgPtr.Align(Ptr: Pointer; Align: Integer): Pointer;
begin
  Integer(Result) := (Integer(Ptr) + Align - 1) and not (Align - 1);
end;

function TArgPtr.ReadInt32: Integer;
begin
  ReadArg(Result, SizeOf(Integer));
end;

function TArgPtr.ReadDouble: Double;
begin
  ReadArg(Result, SizeOf(Double));
end;

function TArgPtr.ReadExtended: Extended;
begin
  ReadArg(Result, SizeOf(Extended));
end;

function TArgPtr.ReadPChar: PChar;
begin
  ReadArg(Result, SizeOf(PChar));
end;

procedure TArgPtr.ReadArg(var Arg; Size: Integer);
begin
  Move(FArgPtr^, Arg, Size);
  FArgPtr := Align(FArgPtr + Size, 4);
end;

procedure Dump(const types: string); cdecl;
var
  ap: TArgPtr;
  cp: PChar;
begin
  cp := PChar(types);
  ap := TArgPtr.Create(@types, SizeOf(string));
  while True do
  begin
    case cp^ of
      #0: 
      begin
        Writeln;
        Exit;
      end;

      'i': Write(ap.ReadInt32, ' ');
      'd': Write(ap.ReadDouble, ' ');
      'e': Write(ap.ReadExtended, ' ');
      's': Write(ap.ReadPChar, ' ');
    else
      Writeln('Unknown format');
      Exit;
    end;
    Inc(cp);
  end;
end;

type
  PDump = procedure(const types: string) cdecl varargs;
var
  MyDump: PDump;

function AsDouble(e: Extended): Double;
begin
  Result := e;
end;

function AsSingle(e: Extended): Single;
begin
  Result := e;
end;

procedure Go;
begin
  MyDump := @Dump;

  MyDump('iii', 10, 20, 30);
  MyDump('sss', 'foo', 'bar', 'baz');

  // Looks like Delphi passes Extended in byte-aligned
  // stack offset, very strange; thus this doesn't work.
  MyDump('e', 2.0);
  // These two are more reliable.
  MyDump('d', AsDouble(2));
  // Singles passed as 8-byte floats.
  MyDump('d', AsSingle(2));
end;

begin
  Go;
end.