PRODUCT  :  Borland C++                           NUMBER  :  1019
  VERSION  :  3.1
       OS  :  ALL
     DATE  :  October 19, 1993                         PAGE  :  1/2

    TITLE  :  Using COMDEFs with v3.x of BC++





      COMDEFs are an extension to C.  They enable a programmer to
  link modules containing duplicate uninitialized global variables
  that are not declared as extern and not get any duplicate symbol
  errors.

      Variables used in this manner are also known as communal
  variables.

      For example,

           FOO.C:   int i;          BAR.C:   int i;
                    main()                   test()
                    {                        {
                    }                        }

      If we have 'OPTIONS|COMPILER|ADVANCED CODE
  GENERATION|OPTIONS|GENERATE COMDEFs' enabled, then the above code
  will generate only one instance of i, and the modules will link
  together with no errors.

      With COMDEFs disabled, one of the "int i;" declarations needs
  to have the keyword extern in order to get past the linker.

  Additionally, if one of the declarations for i is initialized
  (such as "int i = 1;"), then COMDEFs will not apply for i, and
  duplicate symbols will result if declarations of i in other
  modules are not declared as extern.

      Borland has COMDEFs disabled by default since they are an
  extension to C and are not part of the ANSI C standard.  (Several
  PC Compiers have COMDEFs enabled by default).  According to ANSI,
  the above code is illegal.  However, with Borland's Generate
  COMDEFs option enabled, the above code is accepted by the linker.

      COMDEFs are not supported under C++ because uninitialized
  global variables are treated in C++ as though they were
  initialized to zero.  (All global objects have storage class
  static, and all uninitialized static variables are implicitly
  initialized to zero.)
















  PRODUCT  :  Borland C++                           NUMBER  :  1019
  VERSION  :  3.1
       OS  :  ALL
     DATE  :  October 19, 1993                         PAGE  :  2/2

    TITLE  :  Using COMDEFs with v3.x of BC++.




      For more information on C++ uninitialized global variables,
  see sections 3.5 and 8.4 in Ellis & Stroustrup's The Annotated
  C++ Reference Manual (1990, Addison-Wesley).

  The most likely problem that one would run into with communal
  variables is the error message

  ERROR: <symbol> defined in module <a> is duplicated in module <b>

      If you must use C++, you may not use COMDEFs because C++ does
      not support them.  Use the keyword 'extern' (as explained
  above) instead of using COMDEFs and multiple declarations.  If
  you don't  need to use C++ and still want to use COMDEFs, switch
  to using C.



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  subject to the terms of the No-Nonsense License Statement that
  you received with the Borland product to which this information
  pertains.