PublicShow -- Utility library for loading foreign objects (DLLs, shared objects)

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

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

The file 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, +Options:list) 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 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, +Options:list) 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. Options provided are below. Other options are passed to open_shared_object/3.
Installation function to use. Default is default(install), which derives the function from FileSpec.
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.

Undocumented predicates

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

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