Passing a Pointer to a Structure from C to Assembly (62626)

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