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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
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Subject: Re: Sequencing guarantees for macro versions of standard functions
Date: Thu, 20 Jan 2022 16:35:06 -0800
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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

> 7.1.4 Use of library functions
> http://port70.net/~nsz/c/c11/n1570.html#7.1.4p1
>
> 7.1.4/1 states that any function declared in a standard header can be
> additionally implemented as a function-like macro (unless explicitly
> stated otherwise)
>
> "[...] Any function declared in a header may be additionally
> implemented as a function-like macro defined in the header [...]"
>
> It also gives some guarantees about the arguments being evaluated
> exactly once
>
> "[...] Any invocation of a library function that is implemented as a
> macro shall expand to code that evaluates each of its arguments
> exactly once, fully protected by parentheses where necessary, so it is
> generally safe to use arbitrary expressions as arguments.  [...]"
>
> However, that last statement is accompanied by a footnote
>
> "186) Such macros might not contain the sequence points that the
> corresponding function calls do"
>
> It appears that this footnote is intended to draw attention to the
> fact that normative text does not require macro implementations to
> fully match the sequencing properties of a "normal" implementation of
> the same standard function.
>
> Is that really the case?

Yes.

> Consider the following example
>
>   double x = 1.5;
>   x = modf(x, &x);
>
> This is fine in case `modf` is actually a function.  But does the
> standard intend to guarantee that this is specified and defined even
> if `modf` is implemented as a macro?

No.

> I would expect so, and it is quite possible that the following passage
>
> "[...] Likewise, those function-like macros described in the following
> subclauses may be invoked in an expression anywhere a function with a
> compatible return type could be called.  [...]"
>
> is intended to guarantee exactly that.

The quoted passage guarantees what it says:  such function-like
macros may be /invoked/ anywhere their corresponding functions
could.  It does /not/ guarantee that such invocations will work
if there are sequencing issues.

> But still, what exactly are they trying to point out in footnote 186?

If not having the same sequencing guarantees might cause a
problem (ie, if a "call" might invoke a macro rather than calling
a function), avoid using a function call expression that might
invoke a macro, as for example in this particular case

    x = (modf)( x, &x );

so that we're sure an actual function call takes place.  Hence we
know there will be no sequencing problems.  Make sense?