extern and linkage

[Steve Kobes]

Consider

static int x;
void f()
{
extern int x; /* internal or external linkage? */
}

C99 6.2.2.4 says that "extern" causes an identifier's declaration to
inherit a previously specified linkage when it occurs "in a scope in
which a prior declaration of that identifier is visible". One might
think this means the inner x has internal linkage. Visual C++ 6 seems
to think so too.

But the footnote immediately afterward says, "As specified in 6.2.1
[scopes], the later declaration might hide the prior declaration." If
this footnote is to make sense, the inner x would have external
linkage, because its declaration hides that of the outer x. If so, it
is a bug in VC++ (which of course would not be all that surprising).

Which interpretation is correct?

 

[Chris Torek]

Consider

static int x;
void f()
{
extern int x; /* internal or external linkage? */
}

C99 6.2.2.4 says that "extern" causes an identifier's declaration to
inherit a previously specified linkage when it occurs "in a scope in
which a prior declaration of that identifier is visible". One might
think this means the inner x has internal linkage. Visual C++ 6 seems
to think so too.

I believe this is correct.

But the footnote immediately afterward says, "As specified in 6.2.1
[scopes], the later declaration might hide the prior declaration." If
this footnote is to make sense, the inner x would have external
linkage, because its declaration hides that of the outer x. If so, it
is a bug in VC++ (which of course would not be all that surprising).

I believe this footnote refers instead to code such as this:

static int x;
void f(void) {
auto int x; /* hides "static" one; "auto" keyword for ref below */
{
extern int x;
...
}
}

Here the block-scope "auto" x has made the file-scope "x", with its
static duration, invisible. The "extern" line can no longer see it.

Note, however, that comp.lang.c is not the best group for questions
of the form: "what do these particular words in this particular
paragraph on this particular page of the C standard mean?" Along
with proposals for future standards, questions about interpretation
of current (and even past) standards is the domain of comp.std.c.