Re: keyword extern

From: Emmanuel Delahaye (emdelYOURBRA_at_noos.fr)
Date: 06/26/04 

Date: 26 Jun 2004 09:29:44 GMT

In 'comp.lang.c', umbs.sairam@gmail.com wrote:

> One of the purposes of using extern in multiple files is restricting
> the visibility of the variable only to the file that needs to "see"
> it.

It certainely is a bad practice. What if the type changes (int to long, float
to double, etc.) ? Because the compiler has no way to check the consistency
of the code, if you miss a change the code becomes inconsitent. This practice
is obviously wrong.

If you are concerned with the visibility of the global variables, which
is a good thing, there are other options:

- don't use global at all. (Use ADT's)
- give the user no right to change the data (a const pointer to the data)

/* data.h */
extern int const *px;

/* data.c */
#include "data.h"
static int x;
int const *px = &x;

Of course, in real life, data are gathered in some structure and a single
'context' pointer is enough.

Note that this 'protection' is just an intention. There is no way to prevent
an sloopy programmer to write and compile a code that uses a typecast to
force the pointer to a read/write access to the data. But it will invoke an
undefined behaviour. The ADT solution is safer.

> In your question, in both the cases the variable "a" is visible in
> all the files. In such a situation, there is no benefit in using the
> keyword "extern. You might as well declare the variable as global.

Sorry, I failed to understand your point. 'extern' doesn't make global a
variable. It's the lack of 'static' that makes it. 

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