DOCUMENT:Q39520  06-MAY-2001  [masm]
TITLE   :Passing an Integer from C to MASM, Returning a Double
PRODUCT :Microsoft Macro Assembler
PROD/VER::5.1,6.0,6.0a,6.0b
OPER/SYS:
KEYWORDS:

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft Macro Assembler (MASM), versions 5.1, 6.0, 6.0a, 6.0b 
-------------------------------------------------------------------------------

SUMMARY
=======

The following MASM program shows how to receive an integer from a C program,
then pass the value back to the C program as a double. Note: This routine was
composed for the large-memory model and assumes that a coprocessor exists on the
system.

MORE INFORMATION
================

Below is the C program that calls the MASM routine. It should be compiled for
large-memory model and either 8087 or emulator-math library.

Also shown is the MASM routine. The main piece to this program is the FILD
instruction that transforms the integer to a floating-point real, then pushes
the value on the co-processor stack. The other point is to obtain the segment
and offset of the __fac variable. A value requiring more than 4 bytes cannot be
stored in the DX and AX registers for return, so a double is returned by storing
its address in DX:AX.

Sample Code
-----------

   /* Compile options needed: /AL /FPi87 (or /FPi)
   */ 

   #include <stdio.h>

   extern double abc( int x );

   void main()
   {
      int x;
      double y;

      x = 7;
      y = abc( x );
      printf( "%The value of y is %lf.\n", y );
   }

   ----------------------------------------------------------------------

   ; Assemble options needed: /Mx

           .MODEL LARGE,C    ; so Masm will use C naming conventions
           .DATA
           .CODE
   EXTRN _fac:QWORD          ; __fac is the global variable used by C
                             ; for storing floating point accumulations
   PUBLIC  abc
   abc     PROC    FAR
           push    bp
           mov     bp,sp
           fild    WORD PTR [bp+6]
           mov     dx,SEG _fac
           mov     es,dx
           fstp    QWORD PTR es:_fac  ; this pops the value off of
           mov     ax,OFFSET _fac     ; the co-processor and puts it
           fwait                      ; in __fac
           mov     sp,bp
           pop     bp
           ret

   abc     ENDP
           END

Program Output:

The value of y is 7.000000.

Additional query words: kbinf 5.10 6.00 6.00a 6.00b

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM600a kbMASM600b
Version           : :5.1,6.0,6.0a,6.0b

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 2001.