Re: What's it with this kind of definition?

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Date: Sat, 19 Jun 2004 23:40:53 +0100

In article <c520b56a.0406191251.5d250ab4@posting.google.com>,
 estudiantin@rediffmail.com (Curious Student) wrote:

> Some places till now, I've seen function prototypes within functions
> instead of in the global declaration space, which I thought was the
> way.
>
> I thought it was <I>only</I>:
>
> int myfunction(int, int);
>
> int main(void)
> {
> int a,b;
> return myfunction(a,b);
> }
>
> But I've also seen:
>
> int main(void)
> {
> int a, b, myfunction(int, int);
> return myfunction(a,b);
> }
> I've seen this in K&R as well, but haven't come accross a piece that
> talks about this kinda declaration.
> What's the deal here? I understand it as follows:
>
> It defines the scope of usage. If defined in the global space, much
> like extern variables, the function can be called from anywhere within
> the file. If declared within another function, it may be called only
> from within that function in which it is defined.
>
> How true?

That is probably correct. However, it is an extremely bad programming
style. Here is why:

You write functions that are called only from within the file containing
them (to be more precise: From the same compilation unit), and functions
that are called from other places as well. For the first type, you
should use static functions. For the second type, you use extern
functions.

Every caller needs to know how to call a function. Since for extern
functions the caller usually cannot see the definition of the function,
you write a function declaration and put it into a header file, and
every caller should include that header file. You might change the
function parameters at some point in the future; in that case you also
change the declaration in the header file. If the new declaration is not
compatible with the old one, then the compiler will refuse to compile
callers of the function that have not been adapted to the new
parameters.

In the example you wrote, the declaration of myfunction is within main.
If someone changes the definition of myfunction, and also changes the
declaration in the header file, there is a good chance that the
declaration in main is missed. I could have changed it to "myfunction
(double, double, double)", but the compiler would still assume that
myfunction takes two int arguments, so it will not tell you that the
call to myfunction is incorrect, and things will go badly wrong.

You should never have the declaration of a function anywhere but in
exactly _one_ header file.

• Previous message: Christian Bau: "Re: restrict keyword from ISO C99?"
• In reply to: Curious Student: "What's it with this kind of definition?"
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