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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Set the result of void function
Date: Fri, 29 Sep 2017 09:15:04 -0700
Organization: None to speak of
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David Kleinecke <dkleinecke@gmail.com> writes:
> On Thursday, September 28, 2017 at 6:45:18 PM UTC-7, Keith Thompson wrote:
>> David Kleinecke <dkleinecke@gmail.com> writes:
[...]
>> > As nearly as I can tell it would be possible to use
>> > non-prototyped functions where we now use variadic
>> > functions. I think that is a bad idea. But ...
>> 
>> The behavior would be undefined.
>> 
>> Given:
>> 
>>     printf("%d\n", 42);
>>     printf("%s\n", "hello");
>> 
>> there is no way to declare and define printf such that both calls will
>> have defined behavior other than by using the ", ..." syntax.
>> 
>> Any call to a variadic function has undefined behavior unless a correct
>> prototype for that function is visible.
>  
> I don't want to defend the undefensible. But the context
> would be 
>    int printf () { ...}
> which is not the library printf.

So it has nothing to do with the actual printf, which has to accept a
variable number of arguments.

Using a different name to avoid confusion:

    int not_printf() { /* ... */ }

empty parentheses in a function *definition* mean that the function has
no parameters.  The difference between that and

    int not_printf(void) { /* ... */ }

is that the latter imposes a constraint on callers, so that calling it
with one or more arguments must be diagnosed.

>                                  With no prototype this
> should work - provided the implementation gets the right
> arguments in the right place. 
>
> As I said before I don't like this idea. But it seems 
> legal.

No, calling not_printf() with one or more arguments has undefined
behavior but need not be diagnosed.

This is why we use prototypes.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"