R: Foreign Function Interface

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, ENCODING)
     .Fortran(.NAME, ..., NAOK = FALSE, DUP = TRUE, PACKAGE, ENCODING)

### Arguments

`.NAME` | a character string giving the name of a C function or Fortran subroutine, or an object of class `"NativeSymbolInfo"`, `"RegisteredNativeSymbol"` or `"NativeSymbol"` referring to such a name.
`...` | arguments to be passed to the foreign function. Up to 65.
`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 if necessary before their addresses are passed to C or Fortran.
`PACKAGE` | if supplied, confine the search for a character string `.NAME` to the DLL given by this argument (plus the conventional extension, ‘.so’, ‘.dll’, ...). This is intended to add safety for packages, which can ensure by using this argument that no other package can override their external symbols, and also speeds up the search (see ‘Note’).
`ENCODING` | For back-compatibility, accepted but ignored.

### Details

These functions can be used to make calls to compiled C and Fortran 77 code. Later interfaces are `.Call` and `.External` which are more flexible and have better performance.

These functions are primitive, and `.NAME` is always matched to the first argument supplied (which should not be named). The other named arguments follow `...` and so cannot be abbreviated. For clarity, should avoid using names in the arguments passed to `...` that match or partially match `.NAME`.

### Value

A list similar to the `...` list of arguments passed in (including any names given to the arguments), but reflecting any changes made by the C or Fortran code.

### Argument types

The mapping of the types of R arguments to C or Fortran arguments 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]
raw | unsigned char * | not allowed
list | SEXP * | not allowed
other | SEXP | not allowed

Note: The C types corresponding to `integer` and `logical` are `int`, not `long` as in S. This difference matters on most 64-bit platforms, where `int` is 32-bit and `long` is 64-bit (but not on 64-bit Windows).

Note: The Fortran type corresponding to `logical` is `integer`, not `logical`: the difference matters on some Fortran compilers.

Numeric vectors in R will be passed as type `double *` to C (and as `double precision` to Fortran) unless the argument has attribute `Csingle` set to `TRUE` (use `as.single` or `single`). This mechanism is only intended to be used 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;}`. It may or may not be equivalent to the C99 `double complex` type, depending on the compiler used.

Logical values are sent as `0` (`FALSE`), `1` (`TRUE`) or `INT_MIN = -2147483648` (`NA`, but only if `NAOK = TRUE`), and the compiled code should return one of these three values: however non-zero values other than `INT_MIN` are mapped to `TRUE`.

Missing (`NA`) string values are passed to `.C` as the string "NA". As the C `char` type can represent all possible bit patterns there appears to be no way to distinguish missing strings from the string `"NA"`. If this distinction is important use `.Call`.

`.Fortran` passes the first (only) character string of a character vector as a C character array to Fortran: that may be usable as `character*255` if its true length is passed separately. Only up to 255 characters of the string are passed back. (How well this works, and even if it works at all, depends on the C and Fortran compilers and the platform.)

Lists, functions are other R objects can (for historical reasons) be passed to `.C`, but the `.Call` interface is much preferred. All inputs apart from atomic vectors should be regarded as read-only, and all apart from vectors (including lists), functions and environments are now deprecated.

### Warning

`DUP = FALSE` is dangerous. It was deprecated in R 3.1.0 and will be disabled in R 3.2.0.

People concerned about performance and especially memory usage are strongly recommended to use the `.Call` interface instead of these interfaces.

If you pass a local variable to `.C`/`.Fortran` with `DUP = FALSE`, your compiled code can alter the local variable and not just the copy in the return list. Worse, if you pass a local variable that is a formal parameter of the calling function, you may be able to change not only the local variable but the variable one level up. This will be very hard to trace.

With `DUP = FALSE`, character vectors cannot be used, and single precision values will not be returned.

It is safe to set `DUP = FALSE` provided you do not change any of the variables that might be affected, e.g.,

`.C("Cfunction", input = x, output = numeric(10))`.

In this case the `output` variable did not exist before the call so is not copied (even with `DUP = TRUE`). If the `input` variable is not changed in the C code of `Cfunction` you would be safe. Unfortunately, there is no automated check that the C function does not change its argument, and authors have frequently done so without realizing it.

In recent versions of R most unnecessary copying is avoided: on the other hand using `DUP = FALSE` can omit necessary copying.

### Fortran symbol names

All Fortran compilers known to be usable to compile R map symbol names to lower case, and so does `.Fortran`.

Symbol names containing underscores are not valid Fortran 77 (although they are valid in Fortran 9x). Many Fortran 77 compilers will allow them but may translate them in a different way to names not containing underscores. Such names will often work with `.Fortran` (since how they are translated is detected when R is built and the information used by `.Fortran`), but portable code should not use Fortran names containing underscores.

Use `.Fortran` with care for compiled Fortran 9x code: it may not work if the Fortran 9x compiler used differs from the Fortran 77 compiler used when configuring R, especially if the subroutine name is not lower-case or includes an underscore. It is possible to use `.C` and do any necessary symbol-name translation yourself.

### Copying of arguments

Character vectors are copied before calling the compiled code and to collect the results. For `DUP = TRUE` and other atomic vectors, the argument is copied before calling the compiled code if it is otherwise used in the calling code.

Non-atomic-vector objects are read-only to the C code and are never copied.

This behaviour can be changed by setting `options(CBoundsCheck = TRUE)`. In that case raw, logical, integer, double and complex vector arguments are copied both before and after calling the compiled code. The first copy made is extended at each end by guard bytes, and on return it is checked that these are unaltered. For `.C`, each element of a character vector uses guard bytes.

### Note

If one of these functions is to be used frequently, do specify `PACKAGE` (to confine the search to a single DLL) or pass `.NAME` as one of the native symbol objects. Searching for symbols can take a long time, especially when many namespaces are loaded.

You may see `PACKAGE = "base"` for symbols linked into R. Do not use this in your own code: such symbols are not part of the API and may be changed without warning.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

### See Also

`dyn.load`, `.Call`.

The ‘Writing R Extensions’ manual.

[Package base version 3.1.2 Index]