DOCUMENT:Q62626  06-MAY-2001  [masm]
TITLE   :Passing a Pointer to a Structure from C to Assembly
PRODUCT :Microsoft Macro Assembler
PROD/VER::5.1,5.1a,6.0,6.0a,6.0b
OPER/SYS:
KEYWORDS:

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

 - Microsoft Macro Assembler (MASM), versions 5.1, 5.1a, 6.0, 6.0a, 6.0b 
-------------------------------------------------------------------------------

SUMMARY
=======

The code sample below illustrates how to pass a structure from a C module to an
assembly module. This code makes it possible to access the structure fields
within the assembly module using the "." field operator.

Within the assembly module a structure is declared. This structure is used as a
template by the assembler to resolve accessing the fields of the structure. The
structure may be given any label name.

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

The following is an example. Note that the syntax for accessing structure
members will be different with MASM 6.0 and later.

Sample Code:
------------

   /* Compile options needed: none
   */ 

   #include <stdio.h>

   struct node
   {
      int IntVar;
      char Array[5];
   };

   struct node var;
   extern void IncStruc( struct node * );

   void main()
   {
      var.Array[0] = 'H';
      var.Array[2] = 'I';
      var.IntVar = 0x0b12;
      IncStruc( &var );

   /* Incremented values on returning to C */ 

      printf( "var.IntVar is now %x\n",  var.IntVar );
      printf( "var.Array[0] is now %c\n", var.Array[0] );
      printf( "var.Array[2] is now %c\n", var.Array[2] );
   }

   ----------------------------------------------------------------------

   ; Assemble options needed: none

   ;         OPTION OLDSTRUCTS      ; for MASM 5.1 behavior from MASM 6.0

            .model small, c

      delta STRUC
            IntVar DW ?
            Array  DB 5 dup (?)
      delta ENDS

            .code

   IncStruc PROC
            PUSH   BP
            MOV    BP, SP

            MOV    bx, [bp+4]
            INC    [bx].IntVar
            INC    [bx].Array
            INC    [bx+2].Array

            POP    bp
            RET
   IncStruc ENDP

            END

The output is as follows:

   var.IntVar is now b13
   var.Array[0] is now I
   var.Array[2] is now J

Additional query words: kbinf 5.10 5.10a 6.00 6.00a 6.00b

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM600a kbMASM510a kbMASM600b
Version           : :5.1,5.1a,6.0,6.0a,6.0b

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

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