KB104616 - INFO: Sharing Common C Types with MASM

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The samples below include one C file and two different assembly files. The two assembly files demonstrate how to share variables 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

void MasmFunc(void);
char charvar = 'a';       // Declaring variables outside
short shortvar  = 1;      // of a function definition
long longvar = 32768L;    // makes them public.

#ifdef __cplusplus
}
#endif

main ()
{
   while (shortvar < 11)  // Display and increment variables 11 times
   {
      printf ("%c %d %ld\n", charvar, shortvar, longvar) ;
       MasmFunc () ;
   }
}

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

.DATA                            ; NOTE: You can put these as EXTERNDEF
                                 ; in .INC file and include it here.
  EXTRN charvar:BYTE        ; The EXTRN directive enables a MASM
  EXTRN shortvar:WORD        ; procedure to access public variables.
  EXTRN longvar:DWORD

.CODE
MasmFunc PROC
   inc charvar
   inc shortvar
   add WORD PTR longvar, 1
   adc WORD PTR longvar+2, 0
   ret
MasmFunc 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

.DATA                             ; NOTE: You can put these as EXTERNDEF
                                  ; in .INC file and include it here.
   EXTERNDEF charvar:BYTE      ; The EXTRN directive enables a MASM
   EXTERNDEF shortvar:WORD     ; procedure to access public variables.
   EXTERNDEF longvar:DWORD

.CODE
MasmFunc PROC
   inc charvar
   inc shortvar
   inc longvar
   ret
MasmFunc ENDP
END

The following is the output of the program:

INFO: Sharing Common C Types with MASM

SUMMARY

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Sample Code

Sample Code for MS-DOS Small Model Version

Sample Code for Windows NT Flat Model Version