The R Project SVN R

\name{Foreign}
\title{Foreign Function Interface}
\usage{
       .C(name, \dots, NAOK=FALSE, DUP=TRUE, PACKAGE)
 .Fortran(name, \dots, NAOK=FALSE, DUP=TRUE, PACKAGE)
.External(name, \dots)
    .Call(name, \dots)
}
\alias{.C}
\alias{.Fortran}
\alias{.External}
\alias{.Call}
\arguments{
  \item{name}{a character string giving the name of a C function or
    Fortran subroutine.}
  \item{\dots}{arguments to be passed to the foreign function.}
  \item{NAOK}{if \code{TRUE} then any \code{NA} or \code{NaN} or
    \code{Inf} values in the arguments are passed on to the foreign
    function.  If \code{FALSE}, the presence of \code{NA}  or \code{NaN}
    or \code{Inf} values is regarded as an error.}
  \item{DUP}{if \code{TRUE} then arguments are ``duplicated'' before
    their address is passed to C or Fortran.}
  \item{PACKAGE}{if supplied, confine the search for the \code{name} to
    the DLL given by this argument (plus the conventional extension,
    \code{.so}, \code{.sl}, \code{.dll}, \dots).  This is intended to
    add safety for packages, which can ensure that no other package can
    override their external symbols by using this argument.  Use
    \code{PACKAGE="base"} for symbols linked in to \R.} 
}
\description{
  The functions \code{.C} and \code{.Fortran} can be used to make calls
  to C and Fortran code.

  \code{.External} can be used to call compiled code that uses \R
  objects in the same way as internal \R functions. 

  \code{.Call} can be used call compiled code which makes use of
  internal \R objects.  The arguments are passed to the C code as a
  sequence of \R objects.  It is included to provide compatibility with
  S version 4.
}
\details{
  For details about how to write code to use with \code{.Call} and
  \code{.External} see the \file{R-external} manual which can be made
  from files in the \R source tree.
}
\value{
  The functions \code{.C} and \code{.Fortran} return a list similar to
  the \code{\dots} list of arguments passed in, but reflecting any
  changes made by the C or Fortran code.

  \code{.External} and \code{.Call} return an \R object.

  These calls are typically made in conjunction with
  \code{\link{dyn.load}} which links DLLs to \R.
}
%%-- This note based on BDR's understanding, edited TSL
\section{Argument types}{
    The mapping of the types of \R arguments to C or Fortran arguments
    in \code{.C} or \code{.Fortran} is
    \tabular{lll}{
\    R \tab     C \tab     Fortran\cr
integer \tab int * \tab integer\cr
numeric \tab double * \tab double precision\cr
-- or -- \tab float * \tab real\cr
complex \tab complex * \tab double complex\cr
logical \tab int * \tab integer \cr
character \tab char ** \tab [see below]\cr
list \tab SEXP *\tab not allowed\cr
other \tab SEXP\tab not allowed\cr
}
Numeric vectors in \R will be passed as type \code{double *} to C
(and as \code{double precision} to Fortran) unless (i) \code{.C} or
\code{.Fortran} is used, (ii) \code{DUP} is false and (iii) the argument
has attribute \code{Csingle} set to \code{TRUE} (use \code{\link{as.single}}
or \code{\link{single}}).  This mechanism is only intended to be use to
facilitate the interfacing of existing C and Fortran code.

The C type \code{complex} is defined in \file{Complex.h} as a
\code{typedef struct {double r; double i;}}. Fortran type
\code{double complex} is an extension to the Fortran standard, and
the availibility of a mapping of \code{complex} to Fortran may be
compiler dependent.

\emph{Note:} The C types corresponding to \code{integer} and
\code{logical} are \code{int}, not \code{long} as in S.

The first character string of a character vector is passed as a C
character array to Fortran: that string may be usable
as \code{character*255} if its true length is passed separately. Only up
to 255 characters of the string are passed back.

Functions, expressions, environments and other language
elements are passed as the internal \R pointer type \code{SEXP}. This type is
defined in \code{Rinternals.h} or the arguments can be declared
as generic pointers, \code{void *}. Lists are passed as C arrays of
\code{SEXP} and can be declared as \code{void *} or \code{SEXP *}.

R functions can be invoked using \code{call_S} or \code{call_R} and
can be passed lists or the simple types as arguments.
}
%%-- This note by Thomas Lumley, (minimally edited by MM):
\note{\emph{\code{DUP=FALSE} is dangerous.}
    
There are three important dangers with \code{DUP=FALSE}. The first is that
garbage collection may move the object, resulting in the pointers
pointing nowhere useful and causing hard-to-reproduce bugs.

The second is that if you pass a formal parameter of the calling
function to \code{.C}/\code{.Fortran} with \code{DUP=FALSE}, it may not
necessarily be copied.  You may be able to change not only the local
variable but the variable one level up.  This will also be very hard to
trace.

The third is that lists are passed as a single \R \code{SEXP} with
\code{DUP=FALSE}, not as an array of \code{SEXP}. This means the
accessor macros in \code{Rinternals.h} are needed to get at the list
elements and the lists cannot be passed to \code{call_S}/\code{call_R}.

1.  If your C/Fortran routine calls back any \R function including
\code{S_alloc}/\code{R_alloc} then do not use \code{DUP=FALSE}.  Do not
even think about it.  Calling almost any \R function could trigger
garbage collection.

2.  If you don't trigger garbage collection it is safe and useful to set
\code{DUP=FALSE} if you don't change any of the variables that might be
affected, e.g.,

\code{.C("Cfunction", input=x, output=numeric(10))}.

In this case the output variable didn't exist before the call so it can't
cause trouble. If the input variable is not changed in \code{Cfunction} you are
safe.
 }

\section{Header files for external code}{
    Writing code for use with \code{.External} and \code{.Call} will
    use internal \R structures. If possible use just those defined in
    \file{Rinternals.h} and/or the macros in \file{Rdefines.h},
    as other header files are not installed and are even more
    likely to be changed.
}
    
\seealso{\code{\link{dyn.load}}.}
\keyword{programming}