Q104618: HOW TO: Pass a char from C to MASM by Value & Returning a char

Article: Q104618
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 kbHOWTOmaster
Last Modified: 14-NOV-2001

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

- Microsoft C for OS/2 
- Microsoft C for MS-DOS 
- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++, versions 1.0, 1.5, 2.0, 2.1, 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 char from a program written in
Microsoft C to a procedure written with the Microsoft Macro Assembler (MASM).
The MASM function also returns a char 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

  char MasmSub (char);

  #ifdef __cplusplus
  }
  #endif

  main ()
  {
     char var = 'a';
     printf ("%c\n", var);
     printf ("%c", 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, \ 
     cVar:BYTE

     mov al, cVar     ; Load the char into AL.
     add al, 25       ; Because the function returns a char (a 1-byte
     ret              ; value), C will get the return value from AL.
  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, \ 
     cVar:BYTE

     mov al, cVar     ; Load the char into AL.
     add al, 25       ; Because the function returns a char (a 1-byte
     ret              ; value), C will get the return value from AL.
  MasmSub ENDP
  END

The following is the output of the program:

 a
 z

Additional query words:

======================================================================
Keywords          : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 kbHOWTOmaster 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC200 kbVC210 kbVC32bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,2.1,4.0,5.0,6.0
Issue type        : kbhowto

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

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 1986-2002.