Q62626: Passing a Pointer to a Structure from C to Assembly

Article: Q62626
Product(s): Microsoft Macro Assembler
Version(s): 5.1,5.1a,6.0,6.0a,6.0b
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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

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