Q104617: HOW TO: Pass a short from C to MASM by Value & Returning short

Article: Q104617
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 kbHOWTOmaster
Last Modified: 14-DEC-2001

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

- Microsoft Visual C++, versions 1.0, 1.5, 2.0, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
-------------------------------------------------------------------------------

SUMMARY
=======

The sample code below demonstrates how to pass a short from a program written in
Microsoft C to a procedure written with the Microsoft Macro Assembler (MASM).
The MASM function also returns a short to the C program.

Registers are used to return values of simple data types. For 16-bit code, such
as an MS-DOS program, use the following conventions for returning data to a C
program:

  char                   AL
  short, int, near *     AX
  long, far *            DX:  High order portion (segment)
                            AX:  Low order portion (offset)

For 32-bit code, such as a Windows NT program, use the following conventions for
returning data to a C program:

  char                   AL
  short                  AX
  long, int, *           EAX

The samples below include one C file and two different assembly files. The two
assembly files demonstrate how to pass a variable in small model for MS-DOS and
in flat model for Windows NT. Link only the appropriate assembly module to the C
module.

Note that MASM 6.1 or later and the C/C++ 32-bit compiler that ships with Visual
C++, 32-bit Edition, are required to build the flat model Windows NT version.

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

  // Filename: CMAIN.C
  // Compile options needed: /c

  #include <stdio.h>

  #ifdef __cplusplus
  extern "C" {
  #endif

  short MasmSub(short);

  #ifdef __cplusplus
  }
  #endif

  main ()
  {
     short var = 1;
     printf ("%d\n", var);
     printf ("%d", MasmSub(var));
  }

Sample Code for MS-DOS Small Model Version
------------------------------------------

  ; Filename: MASMSUB.ASM
  ; Assemble options needed for MASM: /MX
  ; Assemble options needed for ML: /c /Cx

  .MODEL small, C
  .286
  .CODE

  MasmSub PROC, \ 
     sVar:WORD

     mov ax, sVar     ; Load the short into AX.
     add ax, 32766    ; Because the function returns a short (a 2-byte
     ret              ; value), C will get the return value from AX.
  MasmSub ENDP
  END

Sample Code for Windows NT Flat Model Version
---------------------------------------------

  ; Filename: MASMSUB.ASM
  ; Assemble options needed for ML: /c /Cx /coff

  .386
  .MODEL flat, C
  .CODE

  MasmSub PROC, \ 
     sVar:WORD

     mov ax, sVar     ; Load the short into AX.
     add ax, 32766    ; Because the function returns a short (a 2-byte
     ret              ; value), C will get the return value from AX.
  MasmSub ENDP
  END

The following is the output of the program:

 1
 32767

Additional query words: mixed language

======================================================================
Keywords          : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 kbHOWTOmaster 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc150 kbvc100 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type        : kbhowto

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