Q39526: How to Share a Structure Between C and Assembler Modules

Article: Q39526
Product(s): Microsoft Macro Assembler
Version(s): 5.0,5.1,6.0,6.0a,6.0b
Operating System(s): 
Keyword(s): 
Last Modified: 06-MAY-2001

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The information in this article applies to:

- Microsoft Macro Assembler (MASM), versions 5.0, 5.1, 6.0, 6.0a, 6.0b 
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SUMMARY
=======

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.

In C, the structure may be packed on a 1, 2, or 4-byte boundary. The 2-byte
boundary is the default. As a result, the template constructed in your MASM
module may not match the memory storage exactly, unless pad bytes are
specifically added to the template. See the example below.

In MASM 5.10 and earlier, any template you may create in with the STRUC directive
has to have unique field names throughout the 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.

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

The sample program below demonstrates this information. It consists of a C
module and an Assembler module.

Sample Code
-----------

  /* Compile options needed: none
  */ 

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

  void proc_rec( void );

  void main()
  {
     proc_rec( );
  }

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

  ; Assemble options needed: /Mx (or /Cx with 6.0 and later)

     .model small,c
     .data

  sample  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   ?
  sample  ENDS

  EXTRN      rec:word    ; can use EXTRN  rec:sample with 6.0 or later

     .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

Additional query words: kbinf 5.00 5.10 6.00 6.00a 6.00b

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM600 kbMASM500 kbMASM600a kbMASM600b
Version           : :5.0,5.1,6.0,6.0a,6.0b

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