[Rd] Troubles with Fortran and C (was with the function rmultinom.c of the R's Random Number Generator)

[Prof Brian Ripley]

All arguments to functions called from C by Fortran are pointers
(or should be: yours are not).  The error is within your own code.

You don't want to call rndstart and rndend around every call, only before 
the first and after the last.

This is not the list for advice om mixed Fortran/C programming, though.

On Fri, 20 Jan 2006, Sophie Ancelet wrote:

>
> Hi,
>
> I'm simulating a Markov chain in Fortran interfaced with R-2.2.1 in order
> to generate data according to a Markov Random Field called the Potts model.
>
> R Version:
> platform i686-pc-linux-gnu
> arch     i686
> os       linux-gnu
> system   i686, linux-gnu
> status
> major    2
> minor    2.1
> year     2005
> month    12
> day      20
> svn rev  36812
>
>
>
>
> Each loop of my Fortran calls the function rmultinom.c of the R's Random
> Number Generator through the wrapper:
>
> #include <R.h>
> #include <Rmath.h>
> void F77_SUB(sarmultinom)(int n,
>                          double* prob,
>                          int K,
>                          int* rN){
> rmultinom(n, prob, K, rN);}
>
>
>
> My fortran program is:
>
> subroutine testsarmultinom(n,prob,K,rN)
> implicit none
> integer n,K,rN(K)
> double precision prob(K)
>
> call rndstart()
> call sarmultinom(n,prob,K,rN)
> call rndend()
> end
>
>
> In order to understand better how the function rmultinom.c works, I have
> written an R code which calls this fortran subroutine as follows:
>
> system("R CMD SHLIB test-multinom.f wrapper.c")
> dyn.load("~/Package/test/test-multinom.so")
>
> n=1
> prob=c(0.6,0.1,0.3)
> K=3
> rN=c(1,0,0)
> res<- .Fortran("testsarmultinom",
>               as.integer(n),
>               as.double(prob),
>               as.integer(K),
>               as.integer(rN))
>
>
> Unfortunately, I have some trouble with the results. First, this command
> always returns 0 values. In other words, I always get:
>
>> res[[4]]
> [1] 0 0 0
>
>
> Moreover, if I run this R code a second time, an error message appears:
> Segmentation fault.
>
> Has somebody ever used rmultinom.c and encountered these problems? My code
> must be wrong but I don't know where. In this case, what is the correct way
> to call the C function rmultinom.c?
>
> Thanks in advance,
>
> Sophie.
>
>
>
>
> Sophie Ancelet
> ENGREF
> Laboratoire Grese
> 19 avenue du Maine
> 75 732 Paris Cedex 15
> France
> tel: 33 1 45 49 89 27
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595