Date: Fri, 25 Feb 2000 23:49:09 -0800 From: Alfred Perlstein <bright@wintelcom.net> To: cjclark@home.com Cc: Marco Molteni <molter@csl.sri.com>, freebsd-chat@FreeBSD.ORG Subject: Re: how to do this C preprocessor trick? Message-ID: <20000225234909.W21720@fw.wintelcom.net> In-Reply-To: <20000226003741.C20702@cc942873-a.ewndsr1.nj.home.com>; from cjc@cc942873-a.ewndsr1.nj.home.com on Sat, Feb 26, 2000 at 12:37:41AM -0500 References: <20000225182432.A5017@sofia.csl.sri.com> <20000226001121.A20702@cc942873-a.ewndsr1.nj.home.com> <20000225214616.U21720@fw.wintelcom.net> <20000226003741.C20702@cc942873-a.ewndsr1.nj.home.com>
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* Crist J. Clark <cjc@cc942873-a.ewndsr1.nj.home.com> [000225 22:02] wrote:
> 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> }
I really don't remeber offhand, I know a lot of macros in FreeBSD
are moved from
#define foo { bar; baz; }
to
#define foo do{ bar; baz; }while(0)
I think it has to do with the fact that '{ .. }' is not a statement,
but a block, it will break in certain constructs.
Check the commitlogs and cross reference the code that was fixed by
macros being changed to do/while in the tree.
> That's how I group multi-statement macros, but that does not work for
> this one.
You should use do/while
>
> 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?
It's easy to forget the braces and it will slip by most likely without
a compile time warning, it's safer to do it our way.
--
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
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