Q104645: INFO: Passing a long from C to MASM by Value & Returning a long

Article: Q104645
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 29-NOV-2001

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The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C for OS/2 
- Microsoft C/C++ for MS-DOS 
- 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 
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SUMMARY
=======

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

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

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

  long MasmSub (long, long);

  #ifdef __cplusplus
  }
  #endif

  main ()
     {
     long var1 = 98304, var2 = 147456;
     printf ("%ld + %ld = %ld", var1, var2, MasmSub (var1, var2));
     }

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, \ 
     lVar1:DWORD, \ 
     lVar2:DWORD

     mov ax, WORD PTR lVar1       ; Load the first long into DX:AX.
     mov dx, WORD PTR lVar1+2
     add ax, WORD PTR lVar2       ; Add the second long to DX:AX
     adc dx, WORD PTR lVar2+2
     ret              ; Because the function returns a long (a 4-byte
  MasmSub ENDP        ; value), C will get the return value from DX:AX.
  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, \ 
     lVar1:DWORD, \ 
     lVar2:DWORD

     mov eax, lVar1   ; Load the first long into EAX.
     add eax, lVar2   ; Add the second long to EAX.
     ret              ; Because the function returns a long (a 4-byte
  MasmSub ENDP        ; value), C will get the return value from EAX.
  END

The following is the output of the program:

  98304 + 147456 = 245760

Additional query words: mixed language

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

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