[Rd] cerr and cout not working calling c++ from R

[Dirk Eddelbuettel]

On 6 May 2011 at 22:09, Simon Urbanek wrote:
| Sean,
| 
| the path form the console is not under your control and it depends heavily on the internal settings in R (is console enabled, is R interactive, is the output a tty or a file ...) - that's why you have to use Rprint/REprintf - that's the only path that is guaranteed to work. Anything else is settings-specific and not guaranteed to work (it may appear to work in limited settings, but will not work in others). Note that, for example, WriteConsole is only used if R is not fed via I/O pipes - that may be what baffled you in that case.
| 
| So AFAICS the only C++ solution (other than using Rprintf/REprintf directly) is to create streams to pipes whose ends will call Rprintf/REprintf with the content. Anything else is not general. You may want to ask Rcpp people (Dirk is CC'd anyway ;)) - I'd imagine they must have thought of it as it seems very natural to request in a C++ interface ... (but I don't use Rcpp so I don't know).

Rcpp makes no claims about reimplementing IO, or providing new IO facilities.
Rcpp is clearly bound by the R API upon which it builds; the R documentation
(which I quoted in my earlier email in this thread) suggests to expect nasty
things when one mixes IO systems between R and C++.

And that seems to be what Sean is experiencing; I for one am not surprised.
It was never clear from Seans's mails exactly how much time he has had for
reading of the available documentation, his approach seems to me mostly
driven by a vast and vague set of assumption on his part (eg the rout/rerr
business he comes back to), as well as a lot energy and a willingness to just
try this.  Which is perfectly fine, but it can also fail.

I'd suggest to start small with as little IO as possible. Maybe even comment
all IO out as much as possible. Or redo it as strstream and feed that to
Rprintf.

Dirk
 

| Cheers,
| Simon
| 
| 
| 
| On May 6, 2011, at 3:19 PM, Sean Robert McGuffee wrote:
| 
| > Hi,
| > 
| > Thanks for the comments so far.
| > 
| > I've been going through the code extensively and seem to be having trouble
| > reproducing what R is doing, so this confuses me. For example, when I write
| > out the pointer values for ptr_R_WriteConsole type functions, I can't find a
| > function that matches what is actually being called.
| > 
| > REprintf("Rstd_WriteConsole=%ld, ptr_R_WriteConsole=%ld,
| > R_WriteConsole=%ld\n", Rstd_WriteConsole, ptr_R_WriteConsole,
| > R_WriteConsole);
| > 
| > It might be something like RTcl_WriteConsole too. However, I can't seem to
| > find the headers I would need to try to find it's pointer value. Anyway,
| > these are often hidden/protected/static which is an aspect of code that I
| > only understand in concept. The concept I understand about this type of
| > stuff is that if you provide a better way of doing things, then maybe you
| > hide some stuff that doesn't involve the better way. However, in this case,
| > the hidden stuff is just making it difficult for me to debug and get to
| > bottom of what is going on. Anyway, I get different results if I call on the
| > functions that simply use stdout when trying to reproduce Rprintf
| > functionality. I'm only doing that to try to get to the bottom of what R is
| > doing with stdout and stderr. I also am having a similar issue with trying
| > to understand what GNU GCC is doing with how it sync's cout and cerr with
| > stdout and stderr. Maybe R is intercepting some of these components. Anyway,
| > 
| > I was hoping to get around the
| > "5.6 Interfacing C++ code
| >   ========================   [...]
| >      Using C++ iostreams, as in this example, is best avoided.  There is
| >   no guarantee that the output will appear in the R console, and indeed it
| >   will not on the R for Windows console.  Use R code or the C entry points
| >   (*note Printing::) for all I/O if at all possible."
| > if at all possible. My reason is that I have 76386 lines of c++ code that
| > I'm trying to interface with R, and it literally might be faster for me to
| > update R rather than modify all of that code to suit R which seems to be
| > lacking a c++ ostream interface.
| > 
| > Ideally, I'd write rout and rerr versions of cout and cerr for R to become
| > more compatible with c++. I have had some trouble figuring a lot of this
| > hidden stuff out. Strategies I have considered are the following:
| > 1) Write macros that conditionally replace cerr and cout with Rprintf
| > somehow throughout all of my c++ code that I'm trying to interface with R.
| > 2) Recreate the std::cerr and std::cout type of classes but maybe add some
| > extra functionality for callbacks to when i/o events take place to somehow
| > try to fix things.
| > 3) Make a scratch version of R source code that has no hidden aspects to try
| > to investigate what is going on a little better.
| > 
| > Also, I should mention that I found an unanswered post from someone who had
| > a similar problem when trying to use cerr and cout when accessing functions
| > from dynamically linked libraries. I wonder, is it possible that R hasn't
| > completely connected to these libraries? I mean, it's doing some sort of
| > magic where it finds a pointer to a function, but maybe c++ has a lot going
| > on in the background related to these ostream types that could be getting
| > messed up. Maybe some aspects of the global environment might be different
| > somehow? I directly link to my libraries via a c++ compiler when I get the
| > identical functions to work from main in c++.
| > 
| > One final thing I'm considering is that the R manual also mentions something
| > about garbage collection and some circumstances under which it's important
| > to explicitly request pointers to be maintained. Is it possible that R is
| > eating global pointers used by std::ostream ?
| > 
| > All suggestions are valued and if anyone has recommendations on strategy,
| > please let me know.
| > 
| > Thanks,
| > Sean
| > 
| > 
| > On 5/6/11 2:50 PM, "Davor Cubranic" <cubranic at stat.ubc.ca> wrote:
| > 
| >> On 2011-05-06, at 11:41 AM, Dirk Eddelbuettel wrote:
| >> 
| >>> | I’m trying to call some of my c++ code from R and seem to be having an
| >>> issue
| >>> | with streams, although that’s just one obvious sign of something different
| >>> | in performance between calling the same function from main in c++ vs.
| >>> | calling the same function from R. [...]
| >>> In a nutshell, R 'owns' your console. That is part of the Faustian bargain.
| >> 
| >> But isn't he redirecting cout to a file, so the C++ output is not to the
| >> console?
| >> 
| >> Davor
| > 
| > 
| > On 5/6/11 2:41 PM, "Dirk Eddelbuettel" <edd at debian.org> wrote:
| > 
| >> 
| >> On 6 May 2011 at 14:21, Sean Robert McGuffee wrote:
| >> | Hi,
| >> | 
| >> | Sorry, I just tried posting this but I had it in the wrong format of text,
| >> | so this is a cleared format repost:
| >> | 
| >> | I’m trying to call some of my c++ code from R and seem to be having an issue
| >> | with streams, although that’s just one obvious sign of something different
| >> | in performance between calling the same function from main in c++ vs.
| >> | calling the same function from R. I’m not getting any signs of errors from
| >> | R, but the behavior is different somehow and in a way that breaks my
| >> | algorithms. In both cases of launching a function from c++ or R, I redirect
| >> | cout to a file using a streambuf. In particular, I can see progress stop
| >> | when launching from R at a point when cout gets truncated when starting to
| >> | write out iterator values in a std::vector<string>. Then, I find no way to
| >> | write anything else to cout. When I write those values to Rprintf, they are
| >> | there and they are as expected. I’ve tried setting
| >> | ios_base::sync_with_stdio(false) in case R is messing with stdout and stderr
| >> | somehow, but that made no difference. I’m not quite sure how R uses those C
| >> | FILE* streams. Anyway, I am wondering if anyone has a hunch as to what might
| >> | be going on. At this point I don’t know if it might be an issue with R not
| >> | completely handling c++ from a library or if it is an issue with cout in
| >> | particular or if there is something else I’m missing. Please let me know if
| >> | you have any suggestions for me.
| >> 
| >> Doug Bates sometimes remarks that, once all other alternatives are exhausted,
| >> one could consider reading the manual. And indeed, 'Writing R Extensions' has
| >> this to say:
| >> 
| >>   5.6 Interfacing C++ code
| >>   ========================
| >> 
| >>   [...]
| >> 
| >>      Using C++ iostreams, as in this example, is best avoided.  There is
| >>   no guarantee that the output will appear in the R console, and indeed it
| >>   will not on the R for Windows console.  Use R code or the C entry points
| >>   (*note Printing::) for all I/O if at all possible.
| >> 
| >> In a nutshell, R 'owns' your console. That is part of the Faustian bargain.
| >> 
| >> Dirk
| >> 
| >> -- 
| >> Gauss once played himself in a zero-sum game and won $50.
| >>                      -- #11 at http://www.gaussfacts.com
| > 
| > ______________________________________________
| > R-devel at r-project.org mailing list
| > https://stat.ethz.ch/mailman/listinfo/r-devel
| > 
| > 
| 

-- 
Gauss once played himself in a zero-sum game and won $50.
                      -- #11 at http://www.gaussfacts.com