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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Scopes of identifiers
Date: Thu, 11 Apr 2019 14:00:00 -0700
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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:
> On 4/10/2019 8:58 PM, Keith Thompson wrote:
>> For example, an old "hello, world" program might look something like
>> this:
>> 
>>      main() {
>>          int printf();
>>          printf("hello, world\n");
>>      }
>> 
>> but in modern C you'd just #include <stdio.h>.  (Actually in ancient C
>> you probably wouldn't have bothered to declare printf, since it returns
>> an int and the call would be resolved correctly.)
>
> This is not entirely accurate. A variadic function (like `printf`) has 
> to be declared *with prototype*. Calling a variadic function without a 
> declaration or with a non-prototype declaration results in undefined 
> behavior. This requirement is present in the language since C89.
>
> The correct version would be
>
>    int main()
>    {
>      int printf(const char *, ...);
>      printf("hello, world\n");
>    }

Of course I'm aware of that (and I should have mentioned it).  It was
meant to be an example of a pre-ANSI C program, written when the
language didn't have prototypes or explicitly variadic functions.

In modern C, you could write:

    int main(void) {
        int printf(const char *restrict format, ...);
        printf("hello, world\n");
    }

but it's a lot easier just to write:

    #include <stdio.h>
    int main(void) {
        printf("hello, world\n");
    }

(and not have to worry about whether "restrict" is supported and/or
needed).  But in pre-ANSI C, this:
    printf();
or this:
    int printf();
would have been the only available way to declare printf.

As someone pointed out, a floating-point function might have made for a
better example:

    /* This is pre-ANSI */
    main() {
        double sqrt();
        printf("%f\n", sqrt(2.0));
    }

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */