A Little Quibble

[Doug Gwyn]

In article <25200002 at uicsrd.csrd.uiuc.edu> mcdaniel at uicsrd.csrd.uiuc.edu writes:
>#include <stdio.h>
>int a = 1;
	EXTERNAL LINKAGE
	FILE SCOPE
	STATIC STORAGE DURATION
	TYPE int
	NAME "a"
	INITIALIZED TO THE VALUE 1 BEFORE PROGRAM EXECUTION STARTS
>main()
>    {
>    int a = 2; /* Y */
	NO LINKAGE
	BLOCK Y SCOPE (hides file-scope declaration of "a")
	AUTOMATIC STORAGE DURATION
	TYPE int
	NAME "a"
	RUN-TIME INITIALIZED TO THE VALUE 2 WHENEVER THIS POINT IS EXECUTED
>        {
>        extern int a; /* Z */
	EXTERNAL LINKAGE (since no visible file-scope declaration of "a")
	BLOCK Z SCOPE (hides both other declarations of "a")
	STATIC STORAGE DURATION
	TYPE int
	NAME "a"
	NO INITIALIZATION PERFORMED
>        printf("%d\n", a);
	This identifier "a" must be the block-Z declared one.
	Further, all external-linkage "a"s in all combined translation
	units and libraries must denote the same object.
	Therefore, "1" followed by a new-line must be printed.
>        }
	Block-Y-scope declared "a" is back in scope now.
>    }
	File-scope declared "a" is back in scope now.

>On the VAX BSD 4.3 compiler, it complaints about line Z (redeclaration of a).

That should be a lint-like usage warning, not a true diagnostic message.
The usage is legal C, but possibly a slip-up by the programmer.

>What if I change line Z to "extern const int a;"?

Qualified and unqualified versions of a type are distinct types belonging
to the same type category.  They do not have compatible types.  Therefore
the behavior is undefined.

>    #include <stdio.h>
>    main() {
>	int a; a = 1;
>	{ int a = a; printf("a=%d\n", a); }
>	exit(0);
>	}
>The BSD 4.3 compiler apparently evaluates the right-hand side of
>	int a = a;
>in the context of the current block (as if it were
>	int a; a = a;
>); this program does not output "1".

Yes, that's correct.  The identifier's scope starts right after its
declarator (i.e. before its initializer).  The initializer must use
the newly-introduced "a", not one that is not currently in scope.
Since the new object named "a" has not been given a value, random
junk may be printed (or other undefined behavior may occur).

All this stuff is covered in section 3.1.2 of the draft proposed
ANSI C standard.