Q42110: QuickC Err Msg C2086: Redefinition Caused by Forward Reference

Article: Q42110
Product(s): See article
Version(s): 1.00 1.01 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 2-MAY-1989

The program below will produce (at warning level 0) the following
fatal error in QuickC Version 1.0x if language extensions are
disabled, and in QuickC Version 2.00 whether extensions are enabled or
not:

    C2086:
    'foo' : redefinition

The warning message will be generated in QuickC Version 1.0x if
language extensions are enabled:

    C4074:
    non standard extension used - 'redefined extern to static'

The program is as follows:

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

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

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

Microsoft has implemented ANSI compatibility as well as a number of
extensions to the C Language. The extensions allow for more flexible
compilation. The following is included in 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.

This explanation means that 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, meaning that 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 also is 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 perfectly to this guideline. Page 172 of
the "Microsoft C Language Reference Guide" states clearly that a
function definition without a storage class specifier defaults to
EXTERN. QuickC issues a diagnostic message and either terminates
translation with a fatal compiler error if language extensions are
disabled, or produces a warning message in QuickC Version 1.0x if
language extensions are enabled.

Being a one-pass compiler, QuickC has no way of determining the
storage class of a forwardly referenced function, unlike C 5.xx, and
thus deals with it in the manner described.

The language-extensions switch makes a difference because of the
extension that allows for benign type redefinitions within the same
scope. While the QuickC Compiler programmer's guide (page 171) only
mentions that benign TYPEDEF redefinitions will be allowed, this
behavior also applies to benign function type redefinitions in QuickC
Version 1.0x. QuickC Version 2.00 does not allow this type of
redefinition.

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