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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Unexpected and difficult to understand SIGSEGV
Date: Sun, 27 Sep 2015 15:54:44 -0700
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James Kuyper <jameskuyper@verizon.net> writes:
[...]
> Because you're making the mistake of insisting on using C90, you've
> enabled the "implicit int" rule that caused so much trouble that it was
> removed in C99. As a result, if you use a function in test.c without
> first declaring it, the compiler acts as though a implicit declaration
> was provided that takes the specified number of arguments, and returns
> an int value. If the parameters of your functions have types compatible
> with the promoted types of the corresponding arguments, and the return
> value is in fact 'int' - or if the return value isn't used, then the
> behavior of your code is well defined. Even if the return value isn't
> 'int', if it's the same size as 'int', on many platforms your code will
> still appear to work - which is bad for you, because you've missed an
> opportunity to detect a code defect.
[...]

If the return type isn't int, you'll have undefined behavior even if the
result isn't used.

For example:

    double func(void) { return 0.0) }

    /*
    * In another source file where no declaration of func() is visible:
    */
    func();

In C90, the call is not a constraint violation, but the behavior is
undefined, because the type of the function is not compatible with the
type of the prefix expression.

-- 
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"