"extern" inside a block

[G Patel]

I've seen some code with extern modifiers in front of variables
declared inside blocks. Are these purely definitions (no definition)
or are they definitions with static duration but external linkage?

Not much on this in the FAQ or tutorials.

 

[G Patel]

I've seen some code with extern modifiers in front of variables
declared inside blocks. Are these purely definitions (no definition)

correction: purely declarations

 

[Eric Sosman]

correction: purely declarations

They are declarations of objects or functions
defined elsewhere with external linkage. (IMHO they
are also stylistically repugnant, but that's another
discussion.)

 

[E. Robert Tisdale]

I've seen some code with extern modifiers
in front of variables declared inside blocks.
Are these purely [declarations] (no definition)
Yes.

or are they definitions with static duration but external linkage?
No.

Not much on this in the FAQ or tutorials.

Not surpizing.
There is seldom call for this.
I use it when I want to keep declarations private:

> cat main.c

#include <stdio.h>

int main(int argc, char* argv[]) {
extern void* malloc(size_t);
extern void free(void*);
const size_t n = 13;
int* p = (int*)malloc(n*sizeof(int));
free((void*)p);
return 0;
}

When, for example, I want to use a private helper function.

 

[G Patel]

They are declarations of objects or functions
defined elsewhere with external linkage. (IMHO they
are also stylistically repugnant, but that's another
discussion.)

I've seen things like this too:

{
extern int foo = 10;

/* rest of block */
}

So if extern means it's a declaration, would that line really mean:

{
extern in foo; /* declare an already defined object */
foo = 10; /* assign to that object */

/* rest of block */
}

 

[Eric Sosman]

I've seen things like this too:

{
extern int foo = 10;

/* rest of block */
}

If you've seen them and the compiler didn't complain,
the compiler was being operated in a non-conforming mode:

6.7.8 Initialization
/5/ If the declaration of an identifier has block
scope, and the identifier has external or internal
linkage, the declaration shall have no initializer
for the identifier.

If you've seen `extern int foo = 10;' at file scope,
outside a block, that's fine: it's a definition of `foo'
with an initializer, specifying (unnecessarily) external
linkage. But it's not permitted to do this inside a block.

 

[G Patel]

If you've seen them and the compiler didn't complain,
the compiler was being operated in a non-conforming mode:

6.7.8 Initialization
/5/ If the declaration of an identifier has block
scope, and the identifier has external or internal
linkage, the declaration shall have no initializer
for the identifier.

If you've seen `extern int foo = 10;' at file scope,
outside a block, that's fine: it's a definition of `foo'
with an initializer, specifying (unnecessarily) external
linkage. But it's not permitted to do this inside a block.

You're right, I've only seen extern with initializer at the top of a
file. This is weird, so with an intializer it's a definition, without
an initializer it's a declaraction.

I have one more question. In multi source file programs, I put extern
declaractions for variables I want to share amongst many source files
in a headder, and include that in all those source files. I never used
to include that in the file I defined the file-scope variable. I
tried it after a suggestion and it works. But is this 100% proper (not
undefined)?

Technically, after preprocessing that source file has this at the top:


extern T var; /* came in from #include */

T var; /* was in the .c file */



I know the behaviour when extern declaration follows a non-extern
declaraction is okay, because the extern takes the
linkage/characteristics of the non-extern declaration (either static or
implicity external). But how does this work when extern declaration
PRECEDES a non-extern declaration?

Am I safe to include the header like this?

 

[Beta What]

--snipped--


I know the behaviour when extern declaration follows a non-extern
declaraction is okay, because the extern takes the
linkage/characteristics of the non-extern declaration (either static or
implicity external). But how does this work when extern declaration
PRECEDES a non-extern declaration?

Am I safe to include the header like this?

You need to get yourself a copy of K&R2.

 

[Luke Wu]

I have one more question. In multi source file programs, I put extern
declaractions for variables I want to share amongst many source files
in a headder, and include that in all those source files. I never used
to include that in the file I defined the file-scope variable. I
tried it after a suggestion and it works. But is this 100% proper (not
undefined)?

No. This is perfectly fine to do. This is actually more convenient
(add the header in every source file that requires the use of the
identifier) and allows the compiler to better catch descrepancies
between the single definition and the form of the declarations.

Technically, after preprocessing that source file has this at the top:


extern T var; /* came in from #include */

T var; /* was in the .c file */



I know the behaviour when extern declaration follows a non-extern
declaraction is okay, because the extern takes the
linkage/characteristics of the non-extern declaration (either static or
implicity external).

Yes. The standard says...

6.2.2 Linkage of Identifiers
+++++++++++++++++++++++++++++
(4) For an identifier with the storage-class specifier extern in a
scope in which a prior declaration of that identifier is visible [and
has internal or external linkage] ... the linkage of that later
declaration is the same ... (continued below)


So this explains a situation such as:

T var; /* has external linkage */
extern T var; /* has same linkage - external */

OR

static T var; /* has internal linkage */
extern T var; /* has same linkage - internal */

Note: "NO LINKAGE" doesn't have the same affect on later extern
declarations.

You said you already knew this, which is good because it will help
understand what happens when the order is switched.

But how does this work when extern declaration
PRECEDES a non-extern declaration?

The standard says...

6.2.2 Linkage of Identifiers
+++++++++++++++++++++++++++++
(4 continued from above)... If no prior declaration is visible [for
declaration with extern storage-class specifier], or if the prior
declaration specifies no linkage, then the identifier has external
linkage.

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


This gives you the answer to your question. The first sentence tells
us that extern declaraed identifiers have external linkage if there is
no previous identical identifier with external/internal linkage. The
second sentence tells us that a declaration at file scope without a
storage class specifier has external linkage.

So in your question:

/* external linkage - no previous internal/external linkage */
extern T var;

T var; /* external linkage - no storage class specifier & file scope */


So both become declaration with external linkage, and specify the same
objects (linkage works when identifiers are in the same scope , or
different scopes.)

 

[Luke Wu]

static

You need to get yourself a copy of K&R2.

K&R2 wouldn't help the OP much in this case (appendix *might* provide
*some* help).

 

[Richard Bos]

#include <stdio.h>

int main(int argc, char* argv[]) {
extern void* malloc(size_t);

This loses (use the headers, damn it!)...

extern void free(void*);

....this loses...

const size_t n = 13;
int* p = (int*)malloc(n*sizeof(int));

....this loses majorly (and you know it)...

free((void*)p);

....and now you're just being silly.

return 0;
}

When, for example, I want to use a private helper function.

There is no such thing in C.

Richard

 

[Giorgos Keramidas]

I've seen some code with extern modifiers
in front of variables declared inside blocks.
Are these purely [declarations] (no definition)
Yes.

or are they definitions with static duration but external linkage?
No.

Not much on this in the FAQ or tutorials.

Not surpizing.
There is seldom call for this.

You mean 'extremely rarely' or 'never', of course.

I use it when I want to keep declarations private:

cat main.c

#include <stdio.h>

int main(int argc, char* argv[]) {
extern void* malloc(size_t);
extern void free(void*);
const size_t n = 13;
int* p = (int*)malloc(n*sizeof(int));
free((void*)p);
return 0;
}

When, for example, I want to use a private helper function.

All that because it is easier to type two extra lines with 'prototypes'
for malloc() and free(), just for the fun of making your life difficult?

You could have just included <stdlib.h> you know

 

[CBFalconer]

On 2005-02-23 09:56, E. Robert Tisdale wrote:
.... snip ...

I use it when I want to keep declarations private:

cat main.c

#include <stdio.h>

int main(int argc, char* argv[]) {
extern void* malloc(size_t);
extern void free(void*);
const size_t n = 13;
int* p = (int*)malloc(n*sizeof(int));
free((void*)p);
return 0;
}

When, for example, I want to use a private helper function.

All that because it is easier to type two extra lines with 'prototypes'
for malloc() and free(), just for the fun of making your life difficult?

You could have just included <stdlib.h> you know

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