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From: gwyn@smoke.BRL.MIL (Doug Gwyn)
Newsgroups: comp.lang.c
Subject: Re: Function-types: compatability, and typedefs
Message-ID: <10955@smoke.BRL.MIL>
Date: 7 Sep 89 03:59:25 GMT
References: <KERS.89Aug31163340@cdollin.hpl.hp.com> <660059@hpclwjm.HP.COM>
Reply-To: gwyn@brl.arpa (Doug Gwyn)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 44

In article <660059@hpclwjm.HP.COM> walter@hpclwjm.HP.COM (Walter Murray) writes:
>>>    extern int example( MonadicStringFunction arg );
>>>Would I need to add the * to ensure the argument is treated as a function 
>>>pointer...
>>I think so.  You want a pointer to a function, you should declare it as such.
>>The automatic conversion of identifier designating a function to a pointer
>>occurs only in expressions, not in declarations.  
>As I read the dpANS, the * is not necessary.  There is an adjustment that
>occurs in function declarations and definitions, analogous to the
>automatic conversion that occurs in expressions.  See 3.5.4.3 and 3.7.1. 
>(In the dpANS of 12/7/88, see page 69, lines 22-24, and page 83,
>lines 23-26.)

It's true that in 3.7.1 it's made clear that when a function is called
with an argument of function type, the argument is converted to pointer
to function.  That makes Walter's interpretation a consistent one.  All
that 3.5.4.3 covers is when two function types are compatible, which
doesn't so far as I can see cover the question.  However, I'm willing
to take 3.7.1 as providing a sufficient "as if" constraint to make what
I would consider the "wrong" declaration completely interchangeable
with the "right" one.

Even if the * is not necessary, it is allowed and I think more clearly
documents the interface.

>>>Can I also use the typedef in a *definition*:
>>>    MonadicStringFunction boring( char *arg ) { return arg; }
>>With the first (non-pointer) typedef, you're supposed to be able to.
>I don't think so, if I am understanding the question right.  It is
>forbidden by a constraint in 3.7.1.  See footnote 76 on page 82.

I agree with this; I missed the constraint Walter mentions.
The key is that in the syntax
	function_definition ::= declaration_specifiers{opt} declarator
				declaration_list{opt} compound_statement
the declaration_specifiers are entirely associated with the function
return type, so the above example would be an attempt to declare a
function that returns a MonadicStringFunction, which is of course illegal.
The "declarator" is basically constrained to include one identifier (the
name of the function), a bunch of parentheses, possibly parameter list,
*s, etc. but no place to introduce the desired typedef.  The optional
declaration_list is for old-style parameter declarations.  Thus there's
no place that you could use a typedef for the actual function type.  You
could use one for the function return type (only).