[Rd] .Call ref card [was Re: R-devel Digest, Vol 109, Issue 22]

Ramon Diaz-Uriarte rdiaz02 at gmail.com
Thu Mar 22 16:15:32 CET 2012

On Thu, 22 Mar 2012 10:38:55 -0400,Simon Urbanek <simon.urbanek at r-project.org> wrote:

> On Mar 22, 2012, at 9:45 AM, Terry Therneau <therneau at mayo.edu> wrote:

> > 
> >> 
> >>> 
> >>>>  strongly disagree. I'm appalled to see that sentence here.
> >>> > 
> >>> > Come on!
> >>> > 
> >>>> >> The overhead is significant for any large vector and it is in particular unnecessary since in .C you have to allocate *and copy* space even for results (twice!). Also it is very error-prone, because you have no information about the length of vectors so it's easy to run out of bounds and there is no way to check. IMHO .C should not be used for any code written in this century (the only exception may be if you are passing no data, e.g. if all you do is to pass a flag and expect no result, you can get away with it even if it is more dangerous). It is a legacy interface that dates way back and is essentially just re-named .Fortran interface. Again, I would strongly recommend the use of .Call in any recent code because it is safer and more efficient (if you don't care about either attribute, well, feel free ;)).
> >>> > 
> >>> > So aleph will not support the .C interface? ;-)
> >>> > 
> >> It will look at the timestamp of the source file and delete the package if it is not before 1980 ;). Otherwise it will send a request for punch cards with ".C is deprecated, please upgrade to .Call" stamped out :P At that point I'll be flaming about using the native Aleph interface and not the R compatibility layer ;)
> >> 
> >> Cheers,
> >> S
> > I'll dissent -- I don't think .C is inherently any more dangerous than .Call and prefer it's simplicity in many cases.  Calling C at all is what is inherently dangerous -- I can reference beyond the end of a vector, write over objects that should be read only, and branch to random places using either interface. 

> You can always do so deliberately, but with .C you have no way of preventing it since you don't even know what is the length! That is certainly far more dangerous than .Call where you can simply loop over the length, check that the lengths are compatible etc. Also for types like strings .C is a minefield that is hard to not blow up whereas .Call it is even more safe than scalar arrays. You can do none of that with .C which relies entirely on conventions with no recorded semantics.

> > If you are dealing with large objects and worry about memory efficiency then .Call puts more tools at your disposal and is worth the effort.  However, I did not find the .Call interface at all easy to use at first

> I guess this depends on the developer and is certainly a
> factor. Personally, I find the subset of the R API needed for .Call
> fairly small and intuitive (in particular when you are just writing a
> safer replacement for .C), but I'm obviously biased. Maybe in a separate
> thread we could discuss this - I'd be happy to write a ref card or cheat
> sheet if I find out what people find challenging on .Call. Nonetheless,
> my point is that it is more than worth investing the effort both in
> safety and performance.

After your previous email I made a mental note "try to finally learn to
use .Call since I often deal with large objects". So, yes, I'd love to see
a ref card and cheat sheet: I have tried learning to use .Call a few
times, but have always gone back to .C since (it seems that) all I needed
to know are just a couple of conventions, and the rest is "C as usual".

You say "if I find out what people find challenging on
.Call". Hummm... can I answer "basically everything"?  I think Terry
Thereneau says, "the things I needed to know are scattered about in
multiple places". When I see the convolve example (5.2 in Writing R
extensions) I understand the C code; when I see the convolve2 example in
5.10.1 I think I can guess what lines "PROTECT(a ..." to "xab =
NUMERIC_POINTER ..."  might be doing, but I would not know how to do that
on my own. Yes, I can go to 5.9.1 to read about PROTECT, then search for
... But, at that point, I've gone back to .C. Of course, this might just
be my laziness/incompetence/whatever.




> > and we should keep that in mind before getting too pompous in our lectures to the "sinners of .C".  (Mostly because the things I needed to know are scattered about in multiple places.)
> > 
> > I might have to ask for an exemption on that timestamp -- the first bits of the survival package only reach back to 1986.  And I've had to change source code systems multiple times which plays hob with the file times, though I did try to preserve the changelog history to forstall some future litigious soul who claims they wrote it first  (sccs -> rcs -> cvs -> svn -> mercurial).   :-)
> > 

> ;) Maybe the rule should be based on the date of the first appearance of the package, fair enough :)

> Cheers,
> Simon
> 	[[alternative HTML version deleted]]

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Ramon Diaz-Uriarte
Department of Biochemistry, Lab B-25
Facultad de Medicina 
Universidad Autónoma de Madrid 
Arzobispo Morcillo, 4
28029 Madrid

Phone: +34-91-497-2412

Email: rdiaz02 at gmail.com
       ramon.diaz at iib.uam.es


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