Date: Wed, 3 Dec 1997 16:56:15 -0800 (PST)
Message-Id: <199712040056.QAA21382@adit.ap.net>
Mime-Version: 1.0
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To: eds AT gravi DOT physik DOT uni-jena DOT de (E. Schmidt), djgpp AT delorie DOT com
From: Nate Eldredge <eldredge AT ap DOT net>
Subject: Re: Simple beginner questions
Precedence: bulk

At 07:32  12/3/1997 GMT, E. Schmidt wrote:
>
>Hallo World!
>
>1. How to define user function with variable
>   number of arguments? I simply want(ed) to 
>   define my error handler printing some formatted message
>   and making some other things like closing files...
You use the `stdarg' macros, which are part of ANSI C. (va_list, va_arg,
etc.) These were somehow omitted from the libc documentation. I wrote a
replacement doc and will send you a copy under separate cover. It's a fairly
simple scheme.
>
>2. How to close the file, if I don't know if it is opened or closed?
>   Only file variable FILE * fp; is achivable.
The best way is to keep track, and only close it if it's actually opened.
One way might be to set fp=NULL when you close it, and then check whether
it's NULL before you try again. You can also use this scheme, which I think
should work:

void close_if_open(FILE *fp)
{
  fp = freopen("/dev/null", "r", fp);
  if (fp) fclose(fp);
}

`freopen()' is supposed to close the file if it's open, then reopen that
stream onto some other file. I use "/dev/null" because, on DJGPP and Unix,
it's always there.
The FILE struct probably has a flag which says whether the file is open, but
this is nonportable. Also be sure that fp is a real open or closed stream,
and not just a random pointer.
>
>3. Is it important in C to free the memory in the inverse
>   allocation order ?
>     p1=malloc(100);
>     p2=malloc(200);
>     ...
>     free(p2);
>     free(p1);
>   What happens with pieces of memory which are set free in "wrong" order?
No problems. `malloc()'s and `free()'s can be called in any order, as long
as you only `free' pointers obtained from `malloc'.
>   For above example 
>     free(p1);
>     p1=malloc(50);
>   Would these 50 bytes be allocated in the memory of previous malloc(100) ? 
All I can say is: maybe. `malloc' can do it any way it wants. If it finds
that convenient, it might, or it might not. It may just as easily allocate
it somewhere completely different. Bottom line: DON'T count on it. 
Hope this helps.

Nate Eldredge
eldredge AT ap DOT net