From owner-freebsd-chat  Fri Feb 25 21:16:39 2000
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Date: Fri, 25 Feb 2000 21:46:17 -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?
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* 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.

-Alfred


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