Load or unload DLLs (also known as shared objects), and test whether a C function or Fortran subroutine is available.
dyn.load(x, local = TRUE, now = TRUE, ...)
dyn.unload(x)is.loaded(symbol, PACKAGE = "", type = "")
a character string giving the pathname to a DLL, also known as a dynamic shared object. (See ‘Details’ for what these terms mean.)
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. It is ignored on Windows.
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. It is ignored on Windows.
other arguments for future expansion. See section ‘Windows’ below.
a character string giving a symbol name.
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 by using this argument
that no other package can override their external symbols. This is
used in the same way as in .C, .Call,
.Fortran and .External functions.
The type of symbol to look for: can be any ("", the
default), "Fortran", "Call" or "External".
The function dyn.load is used for its side effect which links
the specified DLL to the executing R image. Calls to .C,
.Call, .Fortran and .External can then be used to
execute compiled C functions or Fortran subroutines contained in the
library. The return value of dyn.load is an object of class
DLLInfo. See getLoadedDLLs for information about
this class.
The function dyn.unload unlinks the DLL. Note that unloading a
DLL and then re-loading a DLL of the same name may or may not work: on
Solaris it uses the first version loaded. Note also that some DLLs cannot
be safely unloaded at all: unloading a DLL which implements C finalizers
but does not unregister them on unload causes R to crash.
is.loaded checks if the symbol name is loaded and
searchable and hence available for use as a character string value
for argument .NAME in .C or .Fortran or
.Call or .External. It will succeed if any one of the
four calling functions would succeed in using the entry point unless
type is specified. (See .Fortran for how Fortran
symbols are mapped.) Note that symbols in base packages are not
searchable, and other packages can be so marked.
The ‘standard mechanisms for loading DLLs’ include a search order for where a DLL is found (if not given as an absolute path, which is preferred), and of where its dependent DLLs will be found. This search path depends on the version of Windows and its security settings, but for versions since Windows XP SP1 it is
The directory from which the application was launched.
The various system directories,
e.g.c:/Windows/system32, c:/Windows/system and
c:/Windows.
The current directory.
Along the search path for executables given by the environment
variable PATH.
Packages often want to supply dependent DLLs in their libs
directory, and do this by setting the PATH variable
(library.dynam does that automatically in recent
versions of R), but the DLL search order means that DLLs in the
launch directory and in system directories will be preferred. On
Windows XP SP1 and later there is a way to modify the search order.
If argument DLLpath is supplied to dyn.load, the latter
makes use of the Windows system call SetDllDirectory to insert
the value of DLLpath in second place, and removes the current
directory, for the duration of that dyn.load call. (Note that
only one directory can be inserted in this way.)
Users have been confused by messages like
error: unable to load shared object '.../library/rJava/libs/x64/rJava.dll': LoadLibrary failure: The specified module could not be found.
The final line is a Windows (not R) diagnostic: the ‘module’
that could not be found is not rJava.dll but something else
Windows is looking for (here most likely Java DLLs): if you are lucky
there will be a dialog box with more details.
Do not use dyn.unload on a DLL loaded by
library.dynam: use library.dynam.unload.
This is needed for system housekeeping.
The objects dyn.load loads are called ‘dynamically
loadable libraries’ (abbreviated to ‘DLL’) on all platforms
except macOS, which uses the term for a different sort
of object. On Unix-alikes they are also called ‘dynamic
shared objects’ (‘DSO’), or ‘shared objects’ for
short. (The POSIX standards use ‘executable object file’,
but no one else does.)
See ‘See Also’ and the ‘Writing R Extensions’ and ‘R Installation and Administration’ manuals for how to create and install a suitable DLL.
Unfortunately a very few platforms (e.g., Compaq Tru64) do not handle
the PACKAGE argument correctly, and may incorrectly find
symbols linked into R.
The additional arguments to dyn.load mirror the different
aspects of the mode argument to the dlopen() routine on POSIX
systems. They are available so that users can exercise greater control
over the loading process for an individual library. In general, the
default values are appropriate and you 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. The intended use is for library developers to call with
value TRUE to check that all symbols are actually resolved and
for regular users to call 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 (very old) 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.
There is a short discussion of these additional arguments with some example code available at http://www.stat.ucdavis.edu/~duncan/R/dynload/.
External code must not change the floating point control word, but
many DLLs do so. Common changes are to set it to use 53 bit
precision instead of R's default 64 bit precision, or to unmask
some exceptions. dyn.load detects such changes,
and restores R's control word to its default value of hex 8001F.
This may cause the DLL to malfunction; if so, it should be rewritten
to save and restore the control word itself. If warn.FPU
is set to TRUE using the options function,
a warning will be printed. (If the warning says
that the control word was changed from some other value than 8001F,
please report the circumstances to the Windows maintainers:
that probably indicates an internal bug.)
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
library.dynam to be used inside a package's
.onLoad initialization.
SHLIB for how to create suitable DLLs.
# NOT RUN {
## expect all of these to be false in R >= 3.0.0.
is.loaded("supsmu") # Fortran entry point in stats
is.loaded("supsmu", "stats", "Fortran")
is.loaded("PDF", type = "External") # pdf() device in grDevices
# }
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