INFO: Sharing Common C Types with MASM

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 6.0, 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

SUMMARY

The sample code below demonstrates how to share variables of type char, short, and long between a Microsoft C program and a Microsoft Macro Assembler (MASM) program. This is accomplished by declaring the C variables outside the a function definition, which makes them public. The MASM subprogram can gain access to these public variables by declaring them with the EXTRN directive. The EXTRN directive has the following format

   EXTRN <name>:<type>

MORE INFORMATION

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

   a 1 32768
   b 2 32769
   c 3 32770
   d 4 32771
   e 5 32772
   f 6 32773
   g 7 32774
   h 8 32775
   i 9 32776
   j 10 32777