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 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’, ‘.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` | optional name for an encoding to be assumed for character vectors. Allowed but ignored for `.Fortran`, deprecated for `.C`. See ‘Details’.

### Details

These functions can be used to make calls to compiled C and Fortran 77 code. Later interfaces are `.Call` and `.External`.

Character strings will be translated from the value of `ENCODING` or any declared encoding (see `Encoding`) to the current locale before being passed to the compiled C code. They will be returned encoded in the current locale unless `ENCODING` was specified, when the output strings are translated to the specified encoding. This is deprecated: convert code to use `iconv`.

These functions are all primitive, and `.NAME` is always matched to the first argument supplied (which if named must partially match `.NAME`). 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

Numeric vectors in R will be passed as type `double *` to C (and as `double precision` to Fortran) unless (i) `DUP` is true and (ii) 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`.

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.

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 is passed 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.

People concerned about 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.

Character vectors cannot be used with `DUP = FALSE`.

It is safe and useful to set `DUP = FALSE` if 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 it cannot cause trouble. If the `input` variable is not changed in the C code of `Cfunction` you are safe.

Note that if `DUP = TRUE` there are up to two copies involved. Prior to R 2.15.1 this was always the case for vectors (one before calling the compiled code and one to collect the results), and this is still the case for character vectors. For other atomic vectors, the argument is copied before calling the compiled code if it is not otherwise used in the calling code (such as `output` in the example above). Non-atomic-vector objects are read-only to the C code and are never copied.

### 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 also possible to use `.C` and do any necessary symbol-name translation yourself.

### 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.

The way pairlists were passed by `.C` prior to R 2.15.0 was not as documented. This has been corrected, but the `.Call` and `.External` interfaces are much preferred for passing anything other than atomic vectors.

### 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 2.15.1 Index]