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Date: Sat, 26 Feb 2000 00:11:22 -0500
From: "Crist J. Clark" <cjc@cc942873-a.ewndsr1.nj.home.com>
To: Marco Molteni <molter@csl.sri.com>
Cc: freebsd-chat@FreeBSD.ORG
Subject: Re: how to do this C preprocessor trick?
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In-Reply-To: <20000225182432.A5017@sofia.csl.sri.com>; from molter@csl.sri.com on Fri, Feb 25, 2000 at 06:24:32PM -0800
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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.
-- 
Crist J. Clark                           cjclark@home.com


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