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Q85227: FIX: Using EXTERNDEF w/ a label May Work Incorrectly

Article: Q85227
Product(s): Microsoft Macro Assembler
Version(s): MS-DOS: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 6.0, 6.0a, 6.0b 
-------------------------------------------------------------------------------

SYMPTOMS
========

In the Microsoft Macro Assembler (MASM) initializing a data structure with the
offset of a label declared with EXTERNDEF or PROTO can cause incorrect results.

RESOLUTION
==========

Use EXTERN to perform the same task as the EXTERNDEF; this method will work
properly. Also, instead of using OFFSET to initialize a WORD field in the
structure, use the label to initialize a NEAR PTR field.

STATUS
======

Microsoft has confirmed this to be a problem in MASM versions 6.0, 6.0a, and
6.0b. This problem was corrected in MASM for MS-DOS version 6.1.

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

EXTERNDEF was introduced as a convenient way to declare shared data structures,
variables, symbols, and labels inside a header file. The same functionality can
be obtained by using EXTERN and PUBLIC. To replace "EXTERNDEF <name>":

- If <name> is used in the current module but is not defined, use "EXTERN
  <name>:<qualified type>".

- If <name> is defined in the current module, use "PUBLIC <name>".

Using this method, each EXTERN and PUBLIC will be specific to certain source
files, which prevents the use of one global include file.

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

  ; Assemble options needed:  /c /Zi
  ; Link options needed: /CO
  ; If SOURCE1.ASM and SOURCE2.ASM are assembled and linked together,
  ; The structure fields an_st.func2 and an_st.func3 will be improperly
  ; initialized with the offset 0x0000.

  ;===========================<source1.asm>=============================
  .MODEL small

  st_st STRUCT 1b
     func1 WORD ?
     func2 WORD ?
     func3 WORD ?
     func4 WORD ?
  st_st ENDS

  ; Uncomment the line below to have fn_2a properly marked for external
  ; linkage. Note that if the EXTERNDEF statement is not commented out,
  ; the EXTERN statement must come first for this to work properly.
  ;EXTERN fn_2a:NEAR

  EXTERN fn_1a:NEAR
  EXTERNDEF fn_2a:NEAR
  fn_3a PROTO NEAR

  .DATA
  an_st st_st { \ 
     offset fn_1a,
     offset fn_2a, ; This doesn't work.
     offset fn_3a, ; This doesn't work either.
     offset fn_4a}

  .CODE
     .startup
     .exit 0
  fn_4a:
     ret
  END

  ;===========================<source2.asm>=============================
  .MODEL small
  EXTERNDEF fn_1a:NEAR
  EXTERNDEF fn_2a:NEAR
  EXTERNDEF fn_3a:NEAR

  .CODE
  fn_1a:
     ret
  fn_2a:
     ret
  fn_3a:
     ret
  END

Additional query words: 6.00 6.00a 6.00b buglist6.00 buglist6.00a buglist6.00b fixlist6.10

======================================================================
Keywords          :  
Technology        : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM600a kbMASM600b
Version           : MS-DOS:6.0,6.0a,6.0b
Solution Type     : kbfix

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

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