Q39852: Error C2086: Redefinition Caused by Forward Reference

Article: Q39852
Product(s): See article
Version(s): 1.00 1.01 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 24-JAN-1989

When compiled with any version of the QuickC compiler, the program
below will produce (at warning level 0) the fatal error.

     C2086:
     'foo': redefinition.

This error will occur if foo() is typed as being of any class other
than int, the default return type.

The following program demonstrates this situation:

    main()
        {
        int i;
        i = foo();
        }

    static int foo()
        {
        return(1);
        }

This is expected behavior for the following reason. The following
information is from the ANSI draft standard (13 May 1988):

    I. If the declaration of an identifier for an object or a function
       contains the storage class specifier "extern", the identifier
       has the same linkage as any _visible_ declaration of the
       identifier with file scope.

   II. If the declaration of an identifier for a function has no
       storage class specifier (as is the case with implicit [first
       use] function uses), its linkage is determined exactly as if it
       were declared with the storage class specifier "extern".

  III. If the declaration of an identifier for an object or a function
       has file scope and contains the storage class specifier
       "static", the identifier has internal linkage.

   IV. If, within a translation unit, the same identifier appears with
       both internal and external linkage, the behavior is undefined.

A forward reference to a function not yet visible to the compiler will
be treated as of type EXTERN because the compiler is unaware of its
storage class. Because it is of type EXTERN, it has external linkage,
i.e., it is an identifier associated with a function defined
outside the current module. When the compiler then encounters the
definition of this function with the STATIC storage class, it
interprets it as a redefinition of that function. Code has already
been generated treating it as EXTERN; now it has been declared as
STATIC. Being STATIC, it then has internal linkage. At this point, the
identifier has both internal and external linkage and behavior is,
according to the draft standard, undefined.

The following information is also from the draft standard:

   Undefined behavior - behavior, upon use of a nonportable or
   erroneous construct of erroneous data, or of indeterminately-valued
   objects for which the standard imposes no requirements.
   Permissible undefined behavior ranges from ignoring the situation
   completely with unpredictable results, to behaving during
   translation or program execution in a documented manner
   characteristic of the environment (with or without the issuance of
   a diagnostic message), to terminating a translation or execution
   (with the issuance of a diagnostic message).

QuickC's behavior conforms precisely to this guideline. Page 172 of
the "Microsoft C Language Reference Guide" states that a function
definition without a storage class specifier defaults to EXTERN.
QuickC issues a diagnostic message and terminates translation (fatal
compiler error). Being a one-pass compiler, QuickC has no way of
determining the storage class of a forward-referenced function,
unlike C Versions 5.x, and thus deals with it in the manner described.

This behavior also applies to QuickC Version 2.00.

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