Loading of shared libraries and resolving of symbols (Alternative Framework)

Alternative framework for loading of shared libraries and resolving of symbols. The framework offers automatic unload management of shared libraries and provides a direct interface to the dynamic linker of the OS.

Usage

dynload(libname, auto.unload=TRUE)
dynsym(libhandle, symname, protect.lib=TRUE)
dynunload(libhandle)
dynpath(libhandle)
dyncount(libhandle)
dynlist(libhandle)

Arguments

libname: character string giving the pathname to a shared library in OS-specific notation.
libhandle: external pointer representing a handle to an opened library.
symname: character string specifying a symbolic name to be resolved.
auto.unload: logical, if TRUE a finalizer will be registered that will automatically unload the library.
protect.lib: logical, if TRUE resolved external pointers protect library handles from finalization.

Value

dynload returns an external pointer libhandle on success. Otherwise NULL is returned, if the library is not found or the linkage failed.

dynsym returns an external pointer address on success. Otherwise NULL is returned, if the address was invalid or the symbol has not been found.

dynunload always returns NULL.

dynpath returns a single string.

dyncount returns a single integer.

dynlist returns a character vector.

Details

dynload loads a shared library into the current R process using the OS-specific dynamic linker interface. The libname is passed as-is directly to the dynamic linker and thus is given in OS-specific notation - see below for details. On success, a handle to the library represented as an external pointer R objects is returned, otherwise NULL. If auto.unload is TRUE, a finalizer function is registered that will unload the library on garbage collection via dynunload.

dynsym looks up symbol names in loaded libraries and resolves them to memory addresses returned as external pointer R objects. Otherwise NULL is returned. If protect.lib is TRUE, the library handle is protected by resolved address external pointers from unloading.

dynpath returns the full path of the loaded library specified by libhandle.

dyncount returns the number of symbols in the loaded library specified by libhandle.

dynlist returns all symbol names in the loaded library specified by libhandle.

dynunload explicitly unreferences the loaded library specified by libhandle.

Setting both auto.unload and protect.lib to TRUE, libraries remain loaded as long as resolved symbols are in use, and they get automatic unloaded when no resolved symbols remain.

Dynamic linkers usually hold an internal link count, such that a library can be opened multiple times via dynload - with a balanced number of calls to dynunload that decreases the link count to unload the library again.

Similar functionality is available via dyn.load and getNativeSymbolInfo, except that path names are filtered and no automatic unloading of libraries is supported.

Shared library

Shared libraries are single files that contain compiled code, data and meta-information. The code and data can be loaded and mapped to a process at run-time once. Operating system platforms have slightly different schemes for naming, searching and linking options.

Platform	Binary format	File Extension
Linux, BSD derivates and Sun Solaris	ELF format	`so`
Darwin / Apple Mac OS X	Mach-O format	`dylib`
Microsoft Windows	PE format	`dll`

Library search on Posix platforms (Linux,BSD,Sun Solaris)

The following text is taken from the Linux dlopen manual page:

These search rules will only be applied to path names that do not contain an embedded '/'.

If the LD_LIBRARY_PATH environment variable is defined to contain a colon-separated list of directories, then these are searched.
The cache file /etc/ld.so.cache is checked to see whether it contains an entry for filename.
The directories /lib and /usr/lib are searched (in that order).

If the library has dependencies on other shared libraries, then these are also automatically loaded by the dynamic linker using the same rules.

Library search on Darwin (Mac OS X) platforms

The following text is taken from the Mac OS X dlopen manual page:

dlopen() searches for a compatible Mach-O file in the directories specified by a set of environment variables and the process's current working directory. When set, the environment variables must contain a colon-separated list of directory paths, which can be absolute or relative to the current working directory. The environment variables are $LD_LIBRARY_PATH, $DYLD_LIBRARY_PATH, and $DYLD_FALLBACK_LIBRARY_PATH. The first two variables have no default value. The default value of $DYLD_FALLBACK_LIBRARY_PATH is $HOME/lib;/usr/local/lib;/usr/lib. dlopen() searches the directories specified in the environment variables in the order they are listed.

When path doesn't contain a slash character (i.e. it is just a leaf name), dlopen() searches the following until it finds a compatible Mach-O file: $LD_LIBRARY_PATH, $DYLD_LIBRARY_PATH, current working directory, $DYLD_FALLBACK_LIBRARY_PATH.

When path contains a slash (i.e. a full path or a partial path) dlopen() searches the following the following until it finds a compatible Mach-O file: $DYLD_LIBRARY_PATH (with leaf name from path ), current working directory (for partial paths), $DYLD_FALLBACK_LIBRARY_PATH (with leaf name from path ).

Library search on Microsoft Windows platforms

The following text is taken from the Window SDK Documentation:

If no file name extension is specified [...], the default library extension .dll is appended. However, the file name string can include a trailing point character (.) to indicate that the [shared library] module name has no extension. When no path is specified, the function searches for loaded modules whose base name matches the base name of the module to be loaded. If the name matches, the load succeeds. Otherwise, the function searches for the file in the following sequence:

The directory from which the application loaded.
The current directory.
The system directory. Use the GetSystemDirectory Win32 API function to get the path of this directory.
The 16-bit system directory. There is no function that obtains the path of this directory, but it is searched. Windows Me/98/95: This directory does not exist.
The Windows directory. Use the GetWindowsDirectory Win32 API function to get the path of this directory.
The directories that are listed in the PATH environment variable.

Windows Server 2003, Windows XP SP1: The default value of

HKLM\System\CurrentControlSet\Control\Session Manager\SafeDllSearchMode

is 1 (current directory is searched after the system and Windows directories).

Windows XP: If

HKLM\System\CurrentControlSet\Control\Session Manager\SafeDllSearchMode

is 1, the current directory is searched after the system and Windows directories, but before the directories in the PATH environment variable. The default value is 0 (current directory is searched before the system and Windows directories).

The first directory searched is the one directory containing the image file used to create the calling process. Doing this allows private dynamic-link library (DLL) files associated with a process to be found without adding the process's installed directory to the PATH environment variable.

The search path can be altered using the SetDllDirectory function. This solution is recommended instead of using SetCurrentDirectory or hard-coding the full path to the DLL.

If a path is specified and there is a redirection file for the application, the function searches for the module in the application's directory. If the module exists in the application's directory, the LoadLibrary function ignores the specified path and loads the module from the application's directory. If the module does not exist in the application's directory, LoadLibrary loads the module from the specified directory. For more information, see Dynamic Link Library Redirection from the Windows SDK Documentation.

Portability

The implementation is based on the dynload library (part of the DynCall project) which has been ported to all major R platforms (ELF (Linux,BSD,Solaris), Mach-O (Mac OS X) and Portable Executable (Win32/64)).