C로 만드셨다면 대소문자를 가리므로 선언된 함수면과 인자개수가 정확히 맞으면 사용 가능할겁니다.
문론 외부참조용으로 하셨겠지요.
function AbCd(AAA: PChar): PChar; cdecl; external '1234.dll';
hsr///////////////////////////////////////

/*-----------------------------------------

  EDRLIB.H  헤더파일
  ----------------------------------------*/

#ifdef __cplusplus
#define EXPORT extern "C" __declspec (dllexport)
#else
#define EXPORT __declspec (dllexport)
#endif

EXPORT BOOL CALLBACK EdrCenterText (HDC, PRECT, PCSTR) ;



/*-----------------------------------------

     EDRLIB.C   ----------------------------*/


#include <windows.h>
#include "EDRLIB.H"

int APIENTRY DllMain(HINSTANCE hInstance, DWORD fdwReason, PVOID pvReserved)
{
        return TRUE;
}

EXPORT BOOL CALLBACK EdrCenterText(HDC hdc, PRECT prc, PCSTR pString)
{
  int iLength;
  SIZE size;
  
  iLength = lstrlenA(pString);
  GetTextExtentPoint32A(hdc, pString, iLength, &size);
  return TextOutA(hdc, (prc->right - prc->left - size.cx) /2,
                           (prc->bottom - prc->top - size.cy) /2,
                                           pString, iLength);
}



--------------델파이 소스코드-----------------------------
unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type
  TForm1 = class(TForm)
    procedure FormPaint(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;


function EdrCenterText(hdc:HDC;prc:TRect;pString:PChar):boolean; cdcel; external 'EDRLIB.DLL';

implementation

{$R *.dfm}

procedure TForm1.FormPaint(Sender: TObject);
var
  Rect:TRect;
begin
  Rect := Canvas.ClipRect;
  EdrCenterText(Canvas.Handle,Rect,PChar('text'));


end;

procedure TForm1.FormCreate(Sender: TObject);
begin

end;

end.