From oliver at first.in-berlin.de  Thu Jun  9 17:15:04 2011
From: oliver at first.in-berlin.de (oliver)
Date: Thu, 9 Jun 2011 17:15:04 +0200
Subject: [Rd] "warning: assignment discards qualifiers from pointer
 target type"
In-Reply-To: <4DF0DEA4.9000602@gmail.com>
References: <alpine.LFD.2.02.1106071606010.26985@gannet.stats.ox.ac.uk>
	<20110608160839.GA11746@siouxsie>
	<B68723F7-F66B-40FD-AE38-0A88C0C51B82@r-project.org>
	<20110609001731.GA3565@siouxsie> <20110609003209.GA4839@siouxsie>
	<86C9E656-BCF4-48C8-8016-877DFAA0D158@r-project.org>
	<20110609112715.GA3855@siouxsie> <4DF0B1D8.4080605@gmail.com>
	<20110609132817.GA5438@siouxsie> <4DF0DEA4.9000602@gmail.com>
Message-ID: <20110609151504.GB7827@siouxsie>

On Thu, Jun 09, 2011 at 10:54:28AM -0400, Duncan Murdoch wrote:
> On 09/06/2011 9:28 AM, oliver wrote:
> >On Thu, Jun 09, 2011 at 07:43:20AM -0400, Duncan Murdoch wrote:
> >>  On 11-06-09 7:27 AM, oliver wrote:
> >>  >On Wed, Jun 08, 2011 at 08:35:34PM -0400, Simon Urbanek wrote:
> >>  >>
> >>  >>On Jun 8, 2011, at 8:32 PM, oliver wrote:
> >>  >>
> >>  >>>On Thu, Jun 09, 2011 at 02:17:31AM +0200, oliver wrote:
> >>  >>>[...]
> >>  >>>>OK, I looked at this now.
> >>  >>>>
> >>  >>>>LENGTH() checks the length of the vector.
> >>  >>>>
> >>  >>>>Good to know this.
> >>  >>>>
> >>  >>>>So the problem of a vector of length 0 can be with any arguments of type SEXP,
> >>  >>>>hence I will need to check ANY arg on it's length.
> >>  >>>>
> >>  >>>>This is vital to stability under any situation.
> >>  >>>>
> >>  >>>>Thanks for this valuable hint!
> >>  >>>>
> >>  >>>>I will add checks for all my SEXP-args.
> >>  >>>[...]
> >>  >>>
> >>  >>>Hey, LENGTH() does not work with String-vectors! :(
> >>  >>>
> >>  >>
> >>  >>Of course it does ...
> >>  >>
> >>  >>
> >>  >
> >>  >It does not so on my R 2.10.1 installation.
> >>  >
> >>  >
> >>  >In the R-Shell I get:
> >>  >
> >>  >    ==============================
> >>  >    >   length(c())
> >>  >    [1] 0
> >>  >    >
> >>  >    ==============================
> >>  >
> >>  >So c() is vec of length 0.
> >>  >
> >>  >When I feed my readjpeg() with c() as filename arg,
> >>  >
> >>  >testing with:
> >>  >====================================================
> >>  >    if( LENGTH( filename_sexp )<   1 )
> >>  >    {
> >>  >      error("LENGTH( filename_sexp )<   1");
> >>  >      //error("filename can't be vector of length 0");
> >>  >    }
> >>  >    else
> >>  >    {
> >>  >      error("LENGTH( filename_sexp ) is not<   1");
> >>  >    }
> >>  >====================================================
> >>  >
> >>  >
> >>  >I got:
> >>  >    Error in readjpeg(filename = c()) : LENGTH( filename_sexp ) is not<   1
> >>  >
> >>  >
> >>  >You can explain why?
> >>
> >>  c() doesn't create a STRSXP, it is NULL, which is a NILSXP.
> >>  LENGTH() doesn't work on that object.  (I'd recommend using length()
> >>  rather than LENGTH(); it's a function, not a macro, and it does give
> >>  the expected answer.)
> >[...]
> >
> >Interestingly, c() as value for an integer value
> >can be testes correctly with LENGTH().
> 
> Presumably you converted it to an INTSXP.  c() is NULL.
[...]

Ooops, yes. In the R-part. :-)
I did it, because when I don't get an int from the user's call,
I have set it to NA (default in the R-function definition).

I made
  width <- as.integer(width)
for easy check inside the C-part, on integer.

(I really only need int, no floating point values.)

I thought this makes the C-code easier at that part.

Or I may check on NA inside the C-part.

How would I do that correctly?

After so much different opinions I read here, I'm rather puzzled on that now.

If there is already a R-extensions-FAQ I could collect the informations
from the mail sof the last days, so that the different opinions and the discussion
can result in a some answers...


> 
> >So the question arises: is c() always creating NILSXP,
> >or is it NILSXP only for STRSXP, and 0 otherwise?
> 
> That question makes no sense at all.

It make sense for the picky.
As NULL is rather (void*)0  and a pointer is not an int (just can be coerced normally) ;)


Ciao,
  Oliver



[Rd] "warning: assignment discards qualifiers from pointer target type"
oliver