Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!decwrl!labrea!rutgers!cmcl2!lanl!unm-la!unmvax!nmtsun!warner%hydrovax@nmt.edu
From: warner%hydrovax@nmt.edu (M. Warner Losh)
Newsgroups: comp.lang.c
Subject: Descriptors in VMS C (was Re: VMS C  &  records in files)
Message-ID: <1003@nmtsun.nmt.edu>
Date: 24 Aug 88 21:03:37 GMT
References: <612@philmds.UUCP>
Sender: news@nmtsun.nmt.edu
Organization: New Mexico Tech Hydrology Program
Lines: 44

In article <612@philmds.UUCP>, leo@philmds.UUCP (Leo de Wit) writes...
> 
>Don't know which version of VMS C you're using, but this is from
>descrip.h on our system, the last 5 lines (maybe you overlooked them):
>
[$DESCRITOR macro deleted]

This macro can only be used when you know what the string is that you are 
dealing with in advance.  While this is true for a large number of cases, 
it isn't true in ALL cases.

For example, if you want to translate the logical name TT, you can do 
something like:
	$DESCRIPTOR (tt_dsc, "TT);

	/* system call to translate logical name */

but if you want to see if some user is on the system (not know at compile
time) for a program like TALK or PHONE then you have to generate the
descriptor somehow.  You can use the $DESCRIPTOR macro to set up the 
storage, but you'd still have to fill in an address and length.  You can
use the function I posted, or you could write a macro that looks something 
like:

#define fill_in_desc(d,s)	{d->dsc$w_length = strlen(s); \
				 d->dsc$a_address = s;}
and use it like this:

func(str)
char *str;
{
	$DESCRIPTOR(user_string_dsc, "");

	fill_in_desc(user_string_dsc, str);
}

and that would also take care of the problem of freeing stuff up, since it 
is done by the 'C' compiler via automatic variables.  So this would be an
even better solution than the function that I posted.  Leo, thank you for 
pointing out this macro.  It made me rethink some of the methods that I was 
using.

Warner		warner%hydrovax%nmt@relay.cs.net
My spelling and views are my own.