Passing Double Precision Data Between QuickBASIC and MPW C (60419)

MORE INFORMATION

When doing interlanguage calling with Microsoft QuickBASIC, the other language can make calls to the BASIC runtime to get the address of passed arguments. This is accomplished with the GetNextLibArg statement.

GetNextLibArg returns a pointer to a variant record containing another pointer to each possible type of QuickBASIC variable (single precision, double precision, integer, etc.). Usually a double indirection of pointers is used to get the actual passed data.

Using double indirection of pointers with LSC or Apple MPW C, however, does not return the correct value. This is a QuickBASIC header file problem. The routine below demonstrates how an inline assembly-language routine can be used in place of the double indirection to work around the problem.

More information on interlanguage calling with Microsoft QuickBASIC can be found in the "Microsoft QuickBASIC for Apple Macintosh: Language Reference" starting on page 444.

Compile and link PrintDbl.c from Apple MPW C version 3.00 as follows: Note: The file BasicLib.a.o comes with Microsoft QuickBASIC for the Apple Macintosh on the Examples disk in the User Libraries:MBPC Rsrcs:MPWP PCR Folder.

Once compiled and linked, PrintDbl is a pure code resource that can be used with the RetDbl QuickBASIC program.

Note: PrintDbl uses the record structure BigRec to pass the double precision number to the assembly language subroutine. This is because MPW automatically converts single and double precision numbers to an extended 10-byte format when passing them. Using the record structure stops MPW from performing this conversion.

Code Example

The following QuickBASIC program is RetDbl, which invokes an Apple MPW C routine to return series of double precision numbers:

   LIBRARY "PrintDbl"

   A# = 0 : B# = 0 : C# = 0
   CALL PrintDbl(A#, B#, C#)
   PRINT A#, B#, C#

   WHILE INKEY$ = "" : WEND
				

The following Apple MPW C routine is PrintDbl.c, which accepts a series of passed double precision numbers from a Microsoft QuickBASIC program and returns a constant value in them:

/**************************************************************
 * PrintDbl.c (c) 1989 Microsoft Corporation                  *
 **************************************************************
 * Description: Example to show how to pass double precision  *
 *              numbers from QuickBASIC to MPW C and back.    *
 **************************************************************/ 

#include "BasicMPWC.h"

struct BigRec {
   DOUBLE dblnum;
};

pascal void AssignDbl(LIBARGPTR ptr, struct BigRec val) = {
   0x225F,  /* movea.l (a7)+,a1                             */ 
   0x2E19,  /* move.l  (a1)+,d7   ;get Long value           */ 
   0x2C19,  /* move.l  (a1)+,d6   ; get second part         */ 
   0x205f,  /* movea.l (a7)+,a0   ;get ptr to Int variable  */ 
   0x20C7,  /* move.l   d7, (a0)+ ;assign the Int value     */ 
   0x20C6   /* move.l   d6, (a0)+ ;assign the Int value     */ 
};

/*------------------------------------------------------------*
 * A routine to pass back to BASIC double precision constants *
 *    Called from BASIC as:                                   *
 *    CALL PrintDbl (< double precision argument list >)      *
 *------------------------------------------------------------*/ 

PUBLIC VOID MAIN() {
   INT16         tempflag, argtype;
   LIBARGPTR     valptr;
   struct BigRec tempdbl;

   argtype = GetNextLibArg(&valptr, &tempflag);

   while (argtype != _ARGSEND) {
      if (argtype != _NULLARG)
         if (argtype == _DBLARG) {
            tempdbl.dblnum = 123.456;
            AssignDbl(valptr, tempdbl);
         }
      argtype = GetNextLibArg(&valptr, &tempflag);
   }

} /* MAIN */ 
				

Running RetDbl produces the following output: