Re: keyword extern
From: John Ilves (johnilves_at_adelphia.net)
Date: 06/27/04
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Date: Sun, 27 Jun 2004 10:07:41 -0400
tweak wrote:
> Jack Klein wrote:
>
>> On Sat, 26 Jun 2004 10:19:23 -0700, tweak <xbwaichunasx@cox.net> wrote
>> in comp.lang.c:
>>
>>
>>> Emmanuel Delahaye wrote:
>>>
>>>> In 'comp.lang.c', tweak <xbwaichunasx@cox.net> wrote:
>>>>
>>>>
>>>>
>>>>> What's the best way to use extern when using multiplefiles that is
>>>>> easiest to maintain?
>>>>
>>>>
>>>>
>>>> First of all, try to avoid the use of global scope variables.
>>>> Read-only are acceptable, but read/write are source of trouble.
>>>> However, if you insist ...
>>>>
>>>>
>>>>
>>>>> Is it best to declare:
>>>>>
>>>>> extern int a;
>>>>>
>>>>> in a header file and include the header file in all files except
>>>>> where it's defined.
>>>>
>>>>
>>>>
>>>> Almost. The header should also be included in the definition source
>>>> file. This is the only way to check the match between the
>>>> declaration and the definition.
>>>>
>>>> /* data.h */
>>>> /* usual guards ommited */
>>>> extern int x;
>>>> /* data.c */
>>>> #include "data.h"
>>>> long x; /* ERR! */
>>>>
>>>> It also allows nice things like :
>>>>
>>>> /* data.h */
>>>> /* usual guards ommited */
>>>> extern int a[128];
>>>> /* data.c */
>>>> #include "data.h"
>>>> int a[];
>>>>
>>>> The size definition is now unique, public and centralized. It helps
>>>> reading and maintenance.
>>>
>>>
>>> Thanks. I'm just trying to understand best practice use of the
>>> keyword extern since I do not program for a living, but I would like
>>> to start a project at source forge this year. And I am just double
>>> checking the basics.
>>>
>>> So the best practice would be to use global static variables in practice
>>
>>
>>
>> Note that the word "global" does not exist in the C standard. The
>> term you are looking for here is "file scope" which is well defined by
>> the C standard.
>>
>> All declarations and definitions written outside of any function in a
>> file have "file scope". Their scope lasts from the point of the
>> definition or declaration until the end of the translation unit being
>> compiled.
>>
>> File scope definitions and declarations have external linkage by
>> default, unless the keyword static is added.
>>
>> One never needs to use the extern keyword with a function definition,
>> prototype or declaration. All references to functions have external
>> linkage by default.
>>
>> One needs to us the extern keyword for declarations (not definitions)
>> of data objects that are defined elsewhere. That is because of the
>> tentative definition feature in C.
>>
>> If, outside of any block in a file, you have the three file scope
>> declarations:
>>
>> extern int x;
>> int y = 0;
>> int z;
>>
>> ...then here is how a C compiler understands them.
>>
>> -- extern int x; If there is no other file scope declaration of this
>> object in the source file, the compiler will treat it as a simple
>> external declaration. If any code in the source file access 'x', the
>> compiler will generate an external reference for it. In the final
>> stage of creating the executable program, usually called linking, the
>> int 'x' must be provided by another source file or library.
>>
>> -- int y = 0; This is a definition of an int named 'y' that has
>> static storage duration, external linkage, and file scope. It will be
>> initialized with the value 0 before main() is called, and its lifetime
>> will be the entire execution time of the program. Even if none of the
>> code in the source file access 'y', the compiler will generate an
>> external definition for it. If there is more than one external
>> definition of this object in the final program, the result is
>> undefined.
>>
>> --- int z; This is a 'tentative definition'. If there is no other
>> declaration for this object, then at the end of the file the compiler
>> will automatically create a definition "int z = 0'" and create the
>> object.
>>
>>
>>> rather than modifiable extern variables? How should extern be used
>>> then?
>>
>>
>>
>> As to the use of variables with external linkage, modifiable or not, I
>> will say this: All absolute rules are rubbish, including this one.
>>
>> Modifiable objects with wide scope, whether they have external linkage
>> or not, can be and often are abused and become the cause of hard to
>> find program defects. Nevertheless, like almost every other feature
>> of the language, they can be used properly and sometimes they are the
>> best solution to a problem. On the other hand, many times there are
>> better solutions possible.
>
>
> Thanks for being very clear.
>
> I am trying to avoid problems of maintenance before I finish writing my
> project. I see too many open source projects, where I cannot easily
> navigate through the code, and I'm trying to avoid that from the onset.
>
> I saw extern as being a probable maintenance problem since tracking down
> definitions has never been any fun.
Thought I'd mention that cscope can be very useful for that:
http://cscope.sourceforge.net/
>
> Thanks Again,
>
> Brian
>
> P.S. I'm still learning.
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