Re: confused about extern use

On 21 Feb, 08:36, "Lalatendu Das" <lalat...@xxxxxxxxx> wrote:
On Feb 21, 12:22 pm, Ian Collins <ian-n...@xxxxxxxxxxx> wrote:>
Lalatendu Das wrote:

Here in the above example I am confused about, what extra the coder
going to achieve by declaring it as extern in the header file a.h.

it would have helped if you'd left the example in...

So it can be used in another compilation unit.

Actually what u mean by another compilation unit.

abbreviations like "u" don't add to the clarity.

"another compilation" unit is basically another C file (actually it's
the C file and its associated includes). A complete program is
composed on one or more compilation units.

If my assumption is
not wrong do u mean If I will try to compile another C-program no need
to include this header file or what ?

you'll need the header file to included in each C file that
uses the shared variable.


I don't think so it will work in that case. And in any case if I have
to include this header file to define one variable of structure type
"abc" then why to declare it extern there.
Actually I might have some wrong notion, please explain through
example if u want to?
thanks for ur replay.

each external object may have many declaration but only one
definition. Essentially you *declare* the type wherever it is
needed (often in an H file) but *define* the storage used in
only one place (usually a C file).


a.h
-------

/* declare struct abc type */
struct abc {
unsigned long a;
unsigned long b;
};

/* declare abc_var to be type struct abc */
extern struct abc abc_var;


file.c
-------

/* pull in declarations */
#include <a.h>

/* define abc_var in one place */
struct abc abc_var;

void f()
{
/* use abc_var */
abc_var.a = 1;
}


/* another compilation unit */
file2.c
-------

/* pull in declarations */
#include <a.h>

/* DON'T re-define abc_var */


void g()
{
/* use abc_var again */
abc_var.a = 0;
}


the program consists of two compilation units that both
access abc_var. How they are joined together ("linked")
is implementaion dependent (see your compiler
documentation).



--
Nick Keighley






.

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