

   FFoorreeiiggnn FFuunnccttiioonn IInntteerrffaaccee

              .C(name, ..., NAOK=FALSE, DUP=TRUE)
        .Fortran(name, ..., NAOK=FALSE, DUP=TRUE)
        .External(name, ...)

   AArrgguummeennttss::

       name: a character string giving the name of a C function
             or Fortran subroutine.

        ...: arguments to be passed to the foreign function.

       NAOK: if `TRUE' then any `NA' or `NaN' or `Inf' values
             in the arguments are passed on to the foreign
             function.  If `FALSE', the presence of `NA'  or
             `NaN' or `Inf' values is regarded as an error.

        DUP: if `TRUE' then arguments are ``duplicated'' before
             their address is passed to C or Fortran.

   DDeessccrriippttiioonn::

        The functions `.C' and `.Fortran' can be used to make
        calls to C and Fortran code.

        `.External' can be used to call compiled code that uses
        R objects in the same way as internal R functions.
        There is no documentation to help you write this sort
        of code.

   VVaalluuee::

        The functions `.C' and `.Fortran' return a list similar
        to the `...'  list of arguments passed in, but reflect-
        ing any changes made by the C or Fortran code.

        `.External' returns an R object.

        These calls are typically made in conjunction with
        `dyn.load' which links DLLs to R.

   AArrgguummeenntt ttyyppeess::

        The mapping of the types of R arguments to C or Fortran
        arguments in `.C' or `.Fortran' is

            R              C                  Fortran
         integer       int *              integer
         real          double *           double precision
         complex       complex *          double precision
         logical       int *              integer
         character     char **            [compiler dependent]
         list          void * (SEXP *)
         other         void * (SEXP)

        The C type `complex' is defined in `Complex.h' as a
        `typedef struct {double r; double i;}'.

        Note: The C types corresponding to `integer' and `logi-
        cal' are `int', not `long' as in S.

        Character strings are passed as C arrays of character
        strings to Fortran: the first string may be usable if
        its length is passed separately.

        Lists, functions, expressions, environments and other
        language elements are passed as generic pointers (`void
        *'). R functions can be invoked using `call_S' or
        `call_R' and can be passed lists or the simple types as
        arguments.

   HHeeaaddeerr ffiilleess ffoorr eexxtteerrnnaall ccooddee::

        Writing code for use with `.External' will typically
        use internal R structures. If possible use just those
        in `Rinternals.h', as other header files are not
        installed and are even more likely to be changed.

   NNoottee::

        `DUP=FALSE' is dangerous.

        There are two important dangers with `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 `.C'/`.Fortran' with
        `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.

        1.  If your C/Fortran routine calls back any R function
        including `S_alloc'/`R_alloc' then do not use
        `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 `DUP=FALSE' if you don't change any
        of the variables that might be affected, e.g.,

        `.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 vari-
        able is not changed in `Cfunction' you are safe.

   SSeeee AAllssoo::

        `dyn.load'.

