Q39526

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Q39526: How to Declare External struct in MASM

Article: Q39526
Version(s): 5.00 5.10 | 5.10
Operating System: MS-DOS    | OS/2
Flags: ENDUSER | struc external
Last Modified: 12-JAN-1989

Question:

In mixed-language programming with MASM and C, how can I declare an
external variable that is of a "struct" type declared in the C module?

Response:

You may declare a type with STRUC directive in your MASM module with
the same memory-storage template as the "struct" type declared in your
C module. You also can declare the variable as external with WORD type
if it is a near data, or DWORD type if it is a far data.

Note: In C, the packed struct type and the unpacked struct type have
different storage in memory. As a result, the template constructed in
your MASM module has to match the memory storage exactly. (See the
sample program below.) Please refer to the "Microsoft Mixed-Language
Programming Guide" (Section 9.2, "Structure, Records, and User-Defined
Types") for more specific information regarding the C storage method.

You can access the fields of the structure in the assembly module by
using the "." operator.

The sample program below demonstrates this information:

/* sample program, c module */

struct record {
    char byte_1 ;
    unsigned int word_1 ;
    char byte_2 ;
    unsigned int word_2 ;
    } rec = {0x41, 0xffff, 0x42, 0xeeee} ;

void proc_rec(void) ;

main()
{
proc_rec() ;
}

/* end of the C module */

Any template you may create in MASM with STRUC directive has to
have unique field names through the MASM module. The field names
represent the offset relative to the beginning of the structure. They
do not have to be literally the same field names of the structure
defined in the C module.

The following sample program demonstrates this information:

; sample program, MASM module

        .model small
        DOSSEG
        .data
foo     struc
byte_1  db   ?      ; if dw is used, next padding byte is not needed.
junk_1  db   ?      ; necessary padding, unused byte
word_1  dw   ?
byte_2  db   ?
junk_2  db   ?      ; necessary padding, unused byte
word_2  dw   ?
foo     ends

extrn   _rec:word       ; extrn  _rec:foo   is illeage

        .code
        public _proc_rec
_proc_rec  proc  near
        mov ax, _rec.word_1
        mov bx, _rec.word_2
        mov cl, _rec.byte_1
        mov ch, _rec.byte_2
        ret
_proc_rec  endp
        end

; end of Masm module