Foreign {base} | R Documentation

## Foreign Function Interface

### Description

Functions to make calls to compiled code that has been loaded into R.

### Usage

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

### Arguments

`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.
`PACKAGE` | if supplied, confine the search for the `name` to the DLL given by this argument (plus the conventional extension, `.so`, `.sl`, `.dll`, ...). 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 `PACKAGE="base"` for symbols linked in to R.

### Details

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.

`.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.

For details about how to write code to use with `.Call` and `.External`, see the chapter on ``System and foreign language interfaces'' in ``Writing R Extensions'' in the ``doc/manual`' subdirectory of the R source tree).

### Value

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

`.External` and `.Call` return an R object.

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

### Argument types

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

R | C | Fortran
integer | int * | integer
numeric | double * | double precision
– or – | float * | real
complex | Rcomplex * | double complex
logical | int * | integer
character | char ** | [see below]
list | SEXP * | not allowed
other | SEXP | not allowed

Numeric vectors in R will be passed as type `double *` to C (and as `double precision` to Fortran) unless (i) `.C` or `.Fortran` is used, (ii) `DUP` is false and (iii) the argument has attribute `Csingle` set to `TRUE` (use `as.single` or `single`). This mechanism is only intended to be use to facilitate the interfacing of existing C and Fortran code.

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

Note: The C types corresponding to `integer` and `logical` are `int`, not `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 `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 `SEXP`. This type is defined in ``Rinternals.h`' or the arguments can be declared as generic pointers, `void *`. Lists are passed as C arrays of `SEXP` and can be declared as `void *` or `SEXP *`.

R functions can be invoked using `call_S` or `call_R` and can be passed lists or the simple types as arguments.

### Header files for external code

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

### Note

`DUP=FALSE` is dangerous.

There are three 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.

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

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 variable is not changed in `Cfunction` you are safe.

### See Also

`dyn.load`.

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