Doubts about Linkage Rules

fctk

source: http://rm-f.net/~orange/devel/specif...t.html#3.1.2.2

there are two passages in this paragraph i can't fully understand:

1) "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. If there is no visible declaration with file scope, the
identifier has external linkage."

in particular: "the identifier has the same linkage as /any/ /visible/
declaration of the identifier with file scope"

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

i can't imagine an example of an identifier having both internal and
external linkage...

Apr 17 '06 #1

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2096

Diomidis Spinellis

fctk wrote:

source: http://rm-f.net/~orange/devel/specif...t.html#3.1.2.2

there are two passages in this paragraph i can't fully understand:

1) "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. If there is no visible declaration with file scope, the
identifier has external linkage."

in particular: "the identifier has the same linkage as /any/ /visible/
declaration of the identifier with file scope"
It means that the following is a legal declaration of foo with file
linkage scope:

static int foo;
extern int foo;

and this a legal declaration of bar with external linkage scope:

int bar;
extern int bar;

The rationale document for C89 explains how this requirement allows the
implementation of a one-pass compiler that generates intermediate
assembly code.

Note that C99 clarifies the *visible* part of the specification as
follows (6.2.2):

"For an identifier declared with the storage-class specifier extern in a
scope in which a *prior* declaration of that identifier is visible, if
the prior declaration specifies internal or external linkage, the
linkage of the identifier at the later declaration is the same as the
linkage specified at the prior declaration. If no prior declaration is
visible, or if the prior declaration specifies no linkage, then the
identifier has external linkage."
2) "If, within a translation unit, the same identifier appears with both
internal and external linkage, the behavior is undefined."

i can't imagine an example of an identifier having both internal and
external linkage...

This is not allowed:

static int bar;
int bar;

--
Diomidis Spinellis
Code Quality: The Open Source Perspective (Addison-Wesley 2006)
http://www.spinellis.gr/codequality

Apr 17 '06 #2

lawrence.jones

fctk <-> wrote:

there are two passages in this paragraph i can't fully understand: [...] in particular: "the identifier has the same linkage as /any/ /visible/
declaration of the identifier with file scope"
Visibility of identifiers is discussed in 6.2.1p2. Consider:

extern int i;
static int i;

The second declaration is not yet visible at the first declaration, so
the first declaration declares i to have external linkage.
i can't imagine an example of an identifier having both internal and
external linkage...

See above. :-)

If the declarations were reversed, all would be well since the "static"
declaration would be visible at the "extern" declaration and thus the
"extern" declaration would declare "i" with internal linkage, too.

-Larry Jones

How am I supposed to learn surgery if I can't dissect anything? -- Calvin

Apr 17 '06 #3

fctk

Diomidis Spinellis ha scritto:

fctk wrote:
2) "If, within a translation unit, the same identifier appears with
both internal and external linkage, the behavior is undefined."

i can't imagine an example of an identifier having both internal and
external linkage...

This is not allowed:

static int bar;
int bar;

i can't understand why there is a problem with:

static int bar;
int bar;

let me expand the previous example to something as:

static int bar; /* first declaration of `bar' */

void f1(void) {
bar;
}

int bar; /* second declaration of `bar' */

void f2(void) {
bar;
}

first of all let's highlight the scopes of the two declarations of
`bar'; i will put an /* 1 */ in front of all lines that belongs to the
scope of the first declaration, and an /* 2 */ in front of all lines
that belong to the scope of the second declaration:

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
int bar; /* second declaration of `bar' */
/* 2 */
/* 2 */ void f2(void) {
/* 2 */ bar;
/* 2 */ }

both declarations of `bar' have file-scope, becouse they are put outside
of any block and outside of any list of parameter declarations in a
function definition.

please note that, as far as i know, given a certain identifier `foo',
the scope of a declaration of `foo' and the scope of another declaration
of `foo' *can't* overlap. overlapping is possible only for the scopes of
*different* identifiers declarations.

the relevant rules here for the linkage are:

1) 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.

2) If the declaration of an identifier for an object has file scope and
no storage-class specifier, its linkage is external.

so for rule 1) all instances of identifier `bar' within the scope of the
first declaration of `bar' have internal-linkage, while for rule 2) all
instances of identifier `bar' within the scope of the second declaration
of `bar' have external-linkage:

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar; /* this instance has internal-linkage */
/* 1 */ }
/* 1 */
int bar; /* second declaration of `bar' */
/* 2 */
/* 2 */ void f2(void) {
/* 2 */ bar; /* this instance has external-linkage */
/* 2 */ }

so as far as i know there is no instance of some identifier with *both*
internal-linkage and external-linkage.

of course my reasoning is wrong in some points, but i can't understand
where.

sorry for the long message.

Apr 18 '06 #4

Vladimir S. Oka

fctk wrote:

Diomidis Spinellis ha scritto:
fctk wrote:
2) "If, within a translation unit, the same identifier appears with
both internal and external linkage, the behavior is undefined."

i can't imagine an example of an identifier having both internal and
external linkage...

This is not allowed:

static int bar;
int bar;

i can't understand why there is a problem with:

<snip>
first of all let's highlight the scopes of the two declarations of
`bar'; i will put an /* 1 */ in front of all lines that belongs to the
scope of the first declaration, and an /* 2 */ in front of all lines
that belong to the scope of the second declaration:

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
int bar; /* second declaration of `bar' */
/* 2 */
/* 2 */ void f2(void) {
/* 2 */ bar;
/* 2 */ }

<snip>

I think your problem is in not understanding scopes properly. The above
example should read (using your notation):

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
/* 1 */ int bar; /* second declaration of `bar' */ /* BANG! */
/* 1 */
/* 1 */ void f2(void) {
/* 1 */ bar;
/* 1 */ }

As file scope spans the whole of the file (hence the name).

Functions `f1()` and `f2()` have inner scopes of their own, where you
can declare `bar` again, but that `bar` then is local to the function,
and masks the file scope one from the line where it's declared till the
closing brace of the function.

Apr 18 '06 #5

fctk

Vladimir S. Oka ha scritto:

I think your problem is in not understanding scopes properly. The above
example should read (using your notation):

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
/* 1 */ int bar; /* second declaration of `bar' */ /* BANG! */
/* 1 */
/* 1 */ void f2(void) {
/* 1 */ bar;
/* 1 */ }

As file scope spans the whole of the file (hence the name).

did you mean perhaps something as:

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
/* 1 */ int bar; /* second declaration of `bar' */
/* 1 */ /* 2 */
/* 1 */ /* 2 */ void f2(void) {
/* 1 */ /* 2 */ bar;
/* 1 */ /* 2 */ }

?

with the previous notation i mean: the scope of the first declaration
overlaps with the scope of the second declaration from line 8 to line
11, and the second declaration "masks" the first declaration from line 8
to line 11.

Apr 18 '06 #6

Vladimir S. Oka

fctk wrote:

Vladimir S. Oka ha scritto:
I think your problem is in not understanding scopes properly. The above
example should read (using your notation):

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
/* 1 */ int bar; /* second declaration of `bar' */ /* BANG! */
/* 1 */
/* 1 */ void f2(void) {
/* 1 */ bar;
/* 1 */ }

As file scope spans the whole of the file (hence the name).

did you mean perhaps something as:

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
/* 1 */ int bar; /* second declaration of `bar' */
/* 1 */ /* 2 */
/* 1 */ /* 2 */ void f2(void) {
/* 1 */ /* 2 */ bar;
/* 1 */ /* 2 */ }

?

with the previous notation i mean: the scope of the first declaration
overlaps with the scope of the second declaration from line 8 to line
11, and the second declaration "masks" the first declaration from line 8
to line 11.

Yes, but you can't have the above. The second declaration is illegal.
That's why I did not include /* 2 */ in my example (I've added a /*
BANG! */, though).

Apr 18 '06 #7

fctk

Vladimir S. Oka ha scritto:

fctk wrote:

did you mean perhaps something as:

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
/* 1 */ int bar; /* second declaration of `bar' */
/* 1 */ /* 2 */
/* 1 */ /* 2 */ void f2(void) {
/* 1 */ /* 2 */ bar;
/* 1 */ /* 2 */ }

?

with the previous notation i mean: the scope of the first declaration
overlaps with the scope of the second declaration from line 8 to line
11, and the second declaration "masks" the first declaration from line 8
to line 11.

Yes, but you can't have the above. The second declaration is illegal.
That's why I did not include /* 2 */ in my example (I've added a /*
BANG! */, though).

ok, now i think i understand why the second instance of `bar' in my
example has both internal and external linkage.

anyway i don't understand why the following piece of code (without f1
and f2 functions) is wrong:

static int bar;
int bar;

the previous gets re-written as:

static int bar; /* first declaration of identifier `bar' */
/* 1 */
/* 1 */ int bar; /* second declaration of identifier `bar' */
/* 1 */ /* 2 */

there is *no* instance of identifier `bar' in the scope of some
declaration. so it is false that there is an instance of `bar' with both
external and internal linkage.

when trying to compile, i get:

error: non-static declaration of 'bar' follows static declaration
error: previous declaration of 'bar' was here

i'm sure there is something else i haven't fully understood yet.

Apr 18 '06 #8

Vladimir S. Oka

fctk wrote:

Vladimir S. Oka ha scritto:
fctk wrote:

did you mean perhaps something as:

static int bar; /* first declaration of `bar' */
/* 1 */
/* 1 */ void f1(void) {
/* 1 */ bar;
/* 1 */ }
/* 1 */
/* 1 */ int bar; /* second declaration of `bar' */
/* 1 */ /* 2 */
/* 1 */ /* 2 */ void f2(void) {
/* 1 */ /* 2 */ bar;
/* 1 */ /* 2 */ }

?

with the previous notation i mean: the scope of the first declaration
overlaps with the scope of the second declaration from line 8 to line
11, and the second declaration "masks" the first declaration from line 8
to line 11.

Yes, but you can't have the above. The second declaration is illegal.
That's why I did not include /* 2 */ in my example (I've added a /*
BANG! */, though).

ok, now i think i understand why the second instance of `bar' in my
example has both internal and external linkage.

anyway i don't understand why the following piece of code (without f1
and f2 functions) is wrong:

static int bar;
int bar;

the previous gets re-written as:

static int bar; /* first declaration of identifier `bar' */
/* 1 */
/* 1 */ int bar; /* second declaration of identifier `bar' */
/* 1 */ /* 2 */

there is *no* instance of identifier `bar' in the scope of some
declaration. so it is false that there is an instance of `bar' with both
external and internal linkage.

when trying to compile, i get:

error: non-static declaration of 'bar' follows static declaration
error: previous declaration of 'bar' was here

i'm sure there is something else i haven't fully understood yet.

Maybe you should try thinking about this particular example in this
way:

static int bar;

Tells the compiler that `bar` is "visible" only internally to the file
in question.

int bar;

Then tells the compiler that it is "visible" externally as well.

Logically, both can't be true at the same time, hence the error.

Apr 18 '06 #9

fctk

Vladimir S. Oka ha scritto:

Maybe you should try thinking about this particular example in this
way:

static int bar;

Tells the compiler that `bar` is "visible" only internally to the file
in question.

int bar;

Then tells the compiler that it is "visible" externally as well.

Logically, both can't be true at the same time, hence the error.

mmh... but i think there is another error. `static int bar' and `int
bar' are two *definitions* other than two declarations. they are two
definitions of the *same* object.

it is as if i wrote:

static void f1(void) {}
extern void f1(void) {}

isn't it?

Apr 18 '06 #10

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