Date: Sat, 26 Feb 2000 00:37:41 -0500 From: "Crist J. Clark" <cjc@cc942873-a.ewndsr1.nj.home.com> To: Alfred Perlstein <bright@wintelcom.net> Cc: cjclark@home.com, Marco Molteni <molter@csl.sri.com>, freebsd-chat@FreeBSD.ORG Subject: Re: how to do this C preprocessor trick? Message-ID: <20000226003741.C20702@cc942873-a.ewndsr1.nj.home.com> In-Reply-To: <20000225214616.U21720@fw.wintelcom.net>; from bright@wintelcom.net on Fri, Feb 25, 2000 at 09:46:17PM -0800 References: <20000225182432.A5017@sofia.csl.sri.com> <20000226001121.A20702@cc942873-a.ewndsr1.nj.home.com> <20000225214616.U21720@fw.wintelcom.net>
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On Fri, Feb 25, 2000 at 09:46:17PM -0800, Alfred Perlstein wrote:
> * Crist J. Clark <cjc@cc942873-a.ewndsr1.nj.home.com> [000225 21:36] wrote:
> > On Fri, Feb 25, 2000 at 06:24:32PM -0800, Marco Molteni wrote:
> > > Hi all,
> > >
> > > I have a function that takes a variable number of arguments:
> > >
> > > void d_printf(const char *format, ...)
> > >
> > > I would like to make it print automatically the function name
> > > from which it is called, eg instead of doing
> > >
> > > f() { d_printf("f: blabla", x, y, z); }
> > >
> > > doing simply
> > >
> > > f() { d_printf("blabla", x, y, z); }
> > >
> > > To do that, I though of wrapping d_printf() around a macro like
> > >
> > > #define dprintf(x) d_printf(__FUNCTION__, x)
> > >
> > > but whatever combination I use (also with #), the thing is not going to work:
> > >
> > > main.c:231: macro `d_printf' used with too many (4) args
> > >
> > > Is it possible to trick the C preprocessor to do what I want?
> >
> > Yeah, I use the same type of thing to produce error messages. I'm
> > having a little bit of trouble understanding exactly what you are
> > trying to do above, so I'll just show my solution to my problem.
> >
> > I wanted to just be able to do,
> >
> > errmsg(char fmt, ...)
> >
> > But have it print,
> >
> > cmd(file:line)- Error message
> >
> > Where 'cmd' is the name of the program (the tail of argv[0]), 'file'
> > is the C source file name, and 'num' is the line number.
> >
> > char *cmd
> >
> > void _errmsg(char *fmt, ... )
> > {
> > va_list ap;
> >
> > va_start(ap,fmt);
> > vfprintf(stderr,fmt,ap);
> > va_end(ap);
> > }
> >
> > #define errmsg fprintf(stderr,"%s(%s:%d)- ",cmd,__FILE__,__LINE__); _errmsg
> >
> >
> > Gets me around the varargs in the precompiler by not using _any_
> > args in the macro. So,
> >
> > errmsg("cannot fine file: %s\n",str);
> >
> > Expands to,
> >
> > fprintf(stderr,"%s(%s:%d)- ",cmd,__FILE__,__LINE__); _errmsg("cannot fine file: %s\n",str);
> >
> > And you know, it works. Big help in debugging big apps. When it's sent
> > bound for users, I make the messages a bit less verbose, but only
> > takes the one change.
>
> One of the nasty side effects is that this makes the macro expand to
> multiple statements.
>
> what's so bad about that?
>
> if (foo < 0)
> errmsg("foo < 0");
>
> Macros that expand to multiple statements ought to be enclosed in a
> do { } while(0) loop.
>
> Although the extra parens are ugly, it things a bit safer/cleaner.
Why a,
do { <stuff> } while(0)
Rather than just,
{ <stuff> }
That's how I group multi-statement macros, but that does not work for
this one.
I just saw your answer and I guess it boils down to which is more ugly
and which is easier to forget to do properly,
Yours,
d_printf((fmt,arg1,arg2));
Or mine,
{ d_printf(fmt,arg1,arg2); }
Extra pair of parenthesis or extra pair of curly brackets? ;)
Or am I overlooking another vulnerability?
--
Crist J. Clark cjclark@home.com
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