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shlib.pl -- Utility library for loading foreign objects (DLLs, shared objects)
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This section discusses the functionality of the (autoload) library(shlib), providing an interface to manage shared libraries. We describe the procedure for using a foreign resource (DLL in Windows and shared object in Unix) called mylib.

First, one must assemble the resource and make it compatible to SWI-Prolog. The details for this vary between platforms. The swipl-ld(1) utility can be used to deal with this in a portable manner. The typical commandline is:

swipl-ld -o mylib file.{c,o,cc,C} ...

Make sure that one of the files provides a global function install_mylib() that initialises the module using calls to PL_register_foreign(). Here is a simple example file mylib.c, which creates a Windows MessageBox:

#include <windows.h>
#include <SWI-Prolog.h>

static foreign_t
pl_say_hello(term_t to)
{ char *a;

  if ( PL_get_atom_chars(to, &a) )
  { MessageBox(NULL, a, "DLL test", MB_OK|MB_TASKMODAL);



{ PL_register_foreign("say_hello", 1, pl_say_hello, 0);

Now write a file mylib.pl:

:- module(mylib, [ say_hello/1 ]).
:- use_foreign_library(foreign(mylib)).

The file mylib.pl can be loaded as a normal Prolog file and provides the predicate defined in C.

Source use_foreign_library(+FileSpec) is det
Source use_foreign_library(+FileSpec, +Entry:atom) is det
Load and install a foreign library as load_foreign_library/1,2 and register the installation using initialization/2 with the option now. This is similar to using:
:- initialization(load_foreign_library(foreign(mylib))).

but using the initialization/1 wrapper causes the library to be loaded after loading of the file in which it appears is completed, while use_foreign_library/1 loads the library immediately. I.e. the difference is only relevant if the remainder of the file uses functionality of the C-library.

As of SWI-Prolog 8.1.22, use_foreign_library/1,2 is in provided as a built-in predicate that, if necessary, loads library(shlib). This implies that these directives can be used without explicitly loading library(shlib) or relying on demand loading.

Source find_library(+LibSpec, -Lib, -Delete) is det[private]
Find a foreign library from LibSpec. If LibSpec is available as a resource, the content of the resource is copied to a temporary file and Delete is unified with true.
Source lib_to_file(+Lib0, -Lib, -Copy) is det[private]
If Lib0 is not a regular file we need to copy it to a temporary regular file because dlopen() and Windows LoadLibrary() expect a file name. On some systems this can be avoided. Roughly using two approaches (after discussion with Peter Ludemann):
  • On FreeBSD there is shm_open() to create an anonymous file in memory and than fdlopen() to link this.
  • In general, we could redefine the system calls open(), etc. to make dlopen() work on non-files. This is highly non-portably though.
  • We can mount the resource zip using e.g., fuse-zip on Linux. This however fails if we include the resources as a string in the executable.
See also
- https://github.com/fancycode/MemoryModule for Windows
Source zipper_members_(+Zipper, -Members) is det[private]
Simplified version of zipper_members/2 from library(zip). We already have a lock on the zipper and by moving this here we avoid dependency on another library.
To be done
- : should we cache this?
Source compatible_architecture_lib(+Entries, +Name, -CompatibleLib) is det[private]
Entries is a list of entries in the zip file, which are already filtered to match the shared library identified by Name. The filtering is done by entries_for_name/3.

CompatibleLib is the name of the entry in the zip file which is compatible with the current architecture. The compatibility is determined according to the description in qsave_program/2 using the qsave:compat_arch/2 hook.

The entries are of the form 'shlib(Arch, Name)'

Source qsave:compat_arch(Arch1, Arch2) is semidet[multifile]
User definable hook to establish if Arch1 is compatible with Arch2 when running a shared object. It is used in saved states produced by qsave_program/2 to determine which shared object to load at runtime.
See also
- foreign option in qsave_program/2 for more information.
Source load_foreign_library(:FileSpec) is det
Source load_foreign_library(:FileSpec, +Entry:atom) is det
Load a shared object or DLL. After loading the Entry function is called without arguments. The default entry function is composed from =install_=, followed by the file base-name. E.g., the load-call below calls the function install_mylib(). If the platform prefixes extern functions with =_=, this prefix is added before calling.
FileSpec- is a specification for absolute_file_name/3. If searching the file fails, the plain name is passed to the OS to try the default method of the OS for locating foreign objects. The default definition of file_search_path/2 searches <prolog home>/lib/<arch> on Unix and <prolog home>/bin on Windows.
See also
- use_foreign_library/1,2 are intended for use in directives.
Source unload_foreign_library(+FileSpec) is det
Source unload_foreign_library(+FileSpec, +Exit:atom) is det
Unload a shared object or DLL. After calling the Exit function, the shared object is removed from the process. The default exit function is composed from =uninstall_=, followed by the file base-name.
Source current_foreign_library(?File, ?Public)
Query currently loaded shared libraries.
Source reload_foreign_libraries
Reload all foreign libraries loaded (after restore of a state created using qsave_program/2.
Source unload_foreign(+File)[private]
Unload the given foreign file and all `spontaneous' foreign predicates created afterwards. Handling these spontaneous predicates is a bit hard, as we do not know who created them and on which library they depend.
Source win_add_dll_directory(+AbsDir) is det[private]
Add AbsDir to the directories where dependent DLLs are searched on Windows systems. This call uses the AddDllDirectory() API when provided. On older Windows systems it extends %PATH%.
- existence_error(directory, AbsDir) if the target directory does not exist.
- domain_error(absolute_file_name, AbsDir) if AbsDir is not an absolute file name.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

Source use_foreign_library(Arg1, Arg2)
Source load_foreign_library(Arg1, Arg2)
Source unload_foreign_library(Arg1, Arg2)