Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!wuarchive!udel!princeton!notecnirp!nfs
From: nfs@notecnirp.Princeton.EDU (Norbert Schlenker)
Newsgroups: comp.lang.c
Subject: Re: ANSI/Non-ANSI Function Declaration Macros
Message-ID: <22402@princeton.Princeton.EDU>
Date: 14 Dec 89 15:45:15 GMT
References: <4603@itivax.iti.org> <244@isgtec.UUCP>
Sender: news@princeton.Princeton.EDU
Reply-To: nfs@notecnirp.UUCP (Norbert Schlenker)
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Organization: Dept. of Computer Science, Princeton University
Lines: 58

In article <244@isgtec.UUCP| peter@isgtec.UUCP (Peter Curran) writes:
|...
|I have solved this problem in two parts.
|1.  Define a symbol PROTOTYPES as 1 if your current compiler supports
|	prototypes, 0 otherwise...
|2.  Define a macro P(x) as follows
|#if PROTOTYPES
|#	define P(x)		x
|#else
|#	define P(x)		()
|#	endif
|3.  Now you can declare functions as, for example
|		int foo		P((FILE *file, int abc));
|	Note the double parentheses - they are important.  If PROTOTYPES
|	is 1, this preprocesses to
|		int foo (FILE *file, int abc):
|	Otherwise it preprocesses to
|		int foo ();
|	This is obviously uglier than using a real compiler, but only
|	three characters extra - a lot better than millions of #if's.
|3.  Write function headers in the old style:
|
|int foo (file, abc)
|FILE *file;
|int abc;....
|
|	This is obsolescent, but all standard-conforming compilers are
|	required to support it, and presumably will for the foreseeable
|	future.  This means I get the benefits of prototypes whenever
|	possible, but can port to obsolete compilers without too much
|	pain.  Of course, once UNIX catches up with the rest of the
|	world, I will abandoned the obsolescent form entirely.

But there is a problem with this too!  With a K&R compiler, things work
just fine using this scheme.  But with an ANSI compiler, any function
taking a small argument (like char or short) will end up with a conflict
between the prototype and the actual file header.  For example:

extern void func(char c, short n);

void func(c, n)
char c;
short n;
{}

won't be accepted.  The K&R definition is going to be treated as if
it says:

void func(c, n)
int c;
int n;
{}

since argument widening is automatic in K&R.  Then there is a terrible
mismatch between the prototype and the definition, which every ANSI
compiler that I have seen complains bitterly about.  So now you need to
conditionally compile function headers, and that will always be less
pleasant since it can't be done automagically by the preprocessor.