R: Obtain a Description of one or more Native (C/Fortran) Symbols

getNativeSymbolInfo {base} | R Documentation

## Obtain a Description of one or more Native (C/Fortran) Symbols

### Description

This finds and returns as comprehensive a description of one or more dynamically loaded or ‘exported’ built-in native symbols. For each name, it returns information about the name of the symbol, the library in which it is located and, if available, the number of arguments it expects and by which interface it should be called (i.e `.Call`, `.C`, `.Fortran`, or `.External`). Additionally, it returns the address of the symbol and this can be passed to other C routines which can invoke. Specifically, this provides a way to explicitly share symbols between different dynamically loaded package libraries. Also, it provides a way to query where symbols were resolved, and aids diagnosing strange behavior associated with dynamic resolution.

This is now vectorized in the `name` argument so can process multiple symbols in a single call. The result is a list that can be indexed by the given symbol names.

### Usage

    getNativeSymbolInfo(name, PACKAGE, unlist = TRUE,
                        withRegistrationInfo = FALSE)

### Arguments

`name` | the name(s) of the native symbol(s) as used in a call to `is.loaded`, etc. Note that Fortran symbols should be supplied as-is, not wrapped in `symbol.For`.
`PACKAGE` | an optional argument that specifies to which dynamically loaded library we restrict the search for this symbol. If this is `"base"`, we search in the R executable itself.
`unlist` | a logical value which controls how the result is returned if the function is called with the name of a single symbol. If `unlist` is `TRUE` and the number of symbol names in `name` is one, then the `NativeSymbolInfo` object is returned. If it is `FALSE`, then a list of `NativeSymbolInfo` objects is returned. This is ignored if the number of symbols passed in `name` is more than one. To be compatible with earlier versions of this function, this defaults to `TRUE`.
`withRegistrationInfo` | a logical value indicating whether, if `TRUE`, to return information that was registered with R about the symbol and its parameter types if such information is available, or if `FALSE` to return the address of the symbol.

### Details

This uses the same mechanism for resolving symbols as is used in all the native interfaces (`.Call`, etc.). If the symbol has been explicitly registered by the shared library in which it is contained, information about the number of arguments and the interface by which it should be called will be returned. Otherwise, a generic native symbol object is returned.

### Value

Generally, a list of `NativeSymbolInfo` elements whose elements can be indexed by the elements of `name` in the call. Each `NativeSymbolInfo` object is a list containing the following elements:

`name` | the name of the symbol, as given by the `name` argument.
`address` | if `withRegistrationInfo` is `FALSE`, this is the native memory address of the symbol which can be used to invoke the routine, and also to compare with other symbol addresses. This is an external pointer object and of class `NativeSymbol`. If `withRegistrationInfo` is `TRUE` and registration information is available for the symbol, then this is an object of class `RegisteredNativeSymbol` and is a reference to an internal data type that has access to the routine pointer and registration information. This too can be used in calls to `.Call`, `.C`, `.Fortran` and `.External`.
`package` | a list containing 3 elements: name the short form of the library name which can be used as the value of the `PACKAGE` argument in the different native interface functions. path the fully qualified name of the shared library file. dynamicLookup a logical value indicating whether dynamic resolution is used when looking for symbols in this library, or only registered routines can be located.
`numParameters` | the number of arguments that should be passed in a call to this routine.

Additionally, the list will have an additional class, being `CRoutine`, `CallRoutine`, `FortranRoutine` or `ExternalRoutine` corresponding to the R interface by which it should be invoked.
If any of the symbols is not found, an error is immediately raised.
If `name` contains only one symbol name and `unlist` is `TRUE`, then the single `NativeSymbolInfo` is returned rather than the list containing that one element.

### Note

One motivation for accessing this reflectance information is to be able to pass native routines to C routines as function pointers in C. This allows us to treat native routines and R functions in a similar manner, such as when passing an R function to C code that makes callbacks to that function at different points in its computation (e.g., `nls`). Additionally, we can resolve the symbol just once and avoid resolving it repeatedly or using the internal cache. In the future, one may be able to treat `NativeSymbol` objects directly as callback objects.

### Author(s)

Duncan Temple Lang

### References

For information about registering native routines, see “In Search of {C/C++} & {FORTRAN} Routines”, R News, volume 1, number 3, 2001, p20–23 (http://CRAN.R-project.org/doc/Rnews/).

### See Also

`getDLLRegisteredRoutines`, `is.loaded`, `.C`, `.Fortran`, `.External`, `.Call`, `dyn.load`.

### Examples

    library(stats) # normally loaded
    getNativeSymbolInfo("dansari")

    getNativeSymbolInfo("hcass2")  # a Fortran symbol

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