The EXTERNDEF directive combines the functionality of the EXTERN/EXTRN and PUBLIC directives. It provides a uniform way to declare global symbols that are to be shared across multiple modules.
Syntax
EXTERNDEF [Language-Name] Name:Type [, ...]
Where Type is one of:
A symbol declared with EXTERNDEF is treated as PUBLIC if a definition for the symbol is encountered during the assembly, otherwise the symbol is assumed to be defined in another module and is treated as if it were declared with the EXTERN/EXTRN directive.
The following example shows how a declaration for the ReturnCode symbol can be shared between two modules (Main.asm and FileErr.asm) by way of a common header file (ErrNum.inc):
; ----------------------------------------------------------------------
; ErrNum.inc
RETCODE_T typedef DWORD
RC_NoError equ 0
RC_FileNotFound equ 1
RC_SystemError equ 3
EXTERNDEF ReturnCode:RETCODE_T ; declaration
; ----------------------------------------------------------------------
; FileErr.asm
.386
.MODEL FLAT
INCLUDE ErrNum.inc ; bring in error number definitions
; and declaration for ReturnCode
.CODE
; Tell the user about the file error,
; then make sure the program has a non-zero exit status
FileError proc
...
mov ReturnCode, RC_FileNotFound
ret
FileError endp
end
; ----------------------------------------------------------------------
; Main.asm
.386
.MODEL FLAT
INCLUDE ErrNum.inc ; bring in error number definitions
; and declaration for ReturnCode
EXTERNDEF FileError:PROC ; This could be in a common header too
.DATA
ReturnCode RETCODE_T RC_NoError ; actual definition of ReturnCode
.CODE
Main proc
...
...
call FileError ; hypothetical error condition
...
...
mov eax, ReturnCode ; load the exit status
call Exit ; and shutdown the program
Main endp
end Main