dyn.load {base} | R Documentation

## Foreign Function Interface

### Description

Load or unload shared libraries, and test whether a C function or Fortran subroutine is available.

### Usage

    dyn.load(x, local = TRUE, now = TRUE)
    dyn.unload(x)

    is.loaded(symbol)
    symbol.C(name)
    symbol.For(name)

### Arguments

`x` | a character string giving the pathname to a DLL.
`local` | a logical value controlling whether the symbols in the DLL are stored in their own local table and not shared across DLLs, or added to the global symbol table. Whether this has any effect is system-dependent.
`now` | a logical controlling whether all symbols are resolved (and relocated) immediately the library is loaded or deferred until they are used. This control is useful for developers testing whether a library is complete and has all the necessary symbols and for users to ignore missing symbols. Whether this has any effect is system-dependent.
`symbol` | a character string giving a symbol name.
`name` | a character string giving either the name of a C function or Fortran subroutine. Fortran names probably need to be given entirely in lower case (but this may be system-dependent).

### Details

The additional arguments to `dyn.load` mirror the different aspects of the mode argument to the dlopen() routine on UNIX systems. They are available so that users can exercise greater control over the loading process for an individual library. In general, the defaults values are appropriate and one should override them only if there is good reason and you understand the implications.

The `local` argument allows one to control whether the symbols in the DLL being attached are visible to other DLLs. While maintaining the symbols in their own namespace is good practice, the ability to share symbols across related ``chapters'' is useful in many cases. Additionally, on certain platforms and versions of an operating system, certain libraries must have their symbols loaded globally to successfully resolve all symbols.

One should be careful of the potential side-effect of using lazy loading via the `now' argument as `FALSE`. If a routine is called that has a missing symbol, the process will terminate immediately and unsaved session variables will be lost. The intended use is for library developers to call specify a value `TRUE` to check that all symbols are actually resolved and for regular users to all with `FALSE` so that missing symbols can be ignored and the available ones can be called.

The initial motivation for adding these was to avoid such termination in the `_init()` routines of the Java virtual machine library. However, symbols loaded locally may not be (read probably) available to other DLLs. Those added to the global table are available to all other elements of the application and so can be shared across two different DLLs.

Some systems do not provide (explicit) support for local/global and lazy/eager symbol resolution. This can be the source of subtle bugs. One can arrange to have warning messages emitted when unsupported options are used. This is done by setting either of the options `verbose` or `warn` to be non-zero via the `options` function. Currently, we know of only 2 platforms that do not provide a value for local load (RTLD_LOCAL). These are IRIX6.4 and unpatched versions of Solaris 2.5.1.

There is a short discussion of these additional arguments with some example code available at http://cm.bell-labs.com/stat/duncan/R/dynload.

### Value

The function `dyn.load` is used for its side effect which links the specified shared library to the executing R image. Calls to `.C`, `.Fortran` and `.External` can then be used to execute compiled C functions or Fortran subroutines contained in the library.
The function `dyn.unload` unlinks the shared library.
Functions `symbol.C` and `symbol.For` map function or subroutine names to the symbol name in the compiled code: `is.loaded` checks if the symbol name is loaded and hence available for use in `.C` or `.Fortran`.

### Note

The creation of shared libraries and the runtime linking of them into executing programs is very platform dependent. In recent years there has been some simplification in the process because the C subroutine call `dlopen` has become the standard for doing this under UNIX. Under UNIX `dyn.load` uses the `dlopen` mechanism and should work on all platforms which support it. On Windows it uses the standard mechanisms for loading 32-bit DLLs.

The original code for loading DLLs in UNIX was provided by Heiner Schwarte. The compatibility code for HP-UX was provided by Luke Tierney.

### See Also

`library.dynam` to be used inside a package's `.First.lib` initialization.

`.C`, `.Fortran`, `.External`, `.Call`.

### Examples

    is.loaded(symbol.For("hcass2")) #-> probably  FALSE
    library(mva)
    is.loaded(symbol.For("hcass2")) #-> TRUE