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<title>SWI-Prolog 7.3.6 Reference Manual: Section 11.2</title><link rel="home" href="index.html">
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<h2 id="sec:qsaveforeign"><a id="sec:11.2"><span class="sec-nr">11.2</span> <span class="sec-title">Runtimes
and Foreign Code</span></a></h2>
<a id="sec:qsaveforeign"></a>
<p><a id="sec:qforeign"></a> Some applications may need to use the
foreign language interface. Object code is by definition
machine-dependent and thus cannot be part of the saved program file.
<p>To complicate the matter even further there are various ways of
loading foreign code:
<p>
<ul class="latex">
<li><i>Using the library(shlib) predicates</i><br>
This is the preferred way of dealing with foreign code. It loads quickly
and ensures an acceptable level of independence between the versions of
the emulator and the foreign code loaded. It works on Unix machines
supporting shared libraries and library functions to load them. Most
modern Unixes, as well as Win32 (Windows 95/NT), satisfy this
constraint.
<li><i>Static linking</i><br>
This mechanism works on all machines, but generally requires the same C
compiler and linker to be used for the external code as is used to build
SWI-Prolog itself.
</ul>
<p>To make a runtime executable that can run on multiple platforms one
must make runtime checks to find the correct way of linking. Suppose we
have a source file <code>myextension.c</code> defining the installation
function install().
<p>If this file is compiled into a shared library, <a id="idx:loadforeignlibrary1:1926"></a><a class="pred" href="foreignlink.html#load_foreign_library/1">load_foreign_library/1</a>
will load this library and call the installation function to initialise
the foreign code. If it is loaded as a static extension, define
install() as the predicate install/0 :
<pre class="code">
static foreign_t
pl_install()
{ install();
PL_succeed;
}
PL_extension PL_extensions [] =
{
/*{ "name", arity, function, PL_FA_<flags> },*/
{ "install", 0, pl_install, 0 },
{ NULL, 0, NULL, 0 } /* terminating line */
};
</pre>
<p>Now, use the following Prolog code to load the foreign library:
<pre class="code">
load_foreign_extensions :-
current_predicate(install, install), !, % static loaded
install.
load_foreign_extensions :- % shared library
load_foreign_library(foreign(myextension)).
:- initialization load_foreign_extensions.
</pre>
<p>The path alias <code>foreign</code> is defined by <a id="idx:filesearchpath2:1927"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>.
By default it searches the directories <code><<var>home</var>>/lib/<<var>arch</var>></code>
and
<code><<var>home</var>>/lib</code>. The application can specify
additional rules for
<a id="idx:filesearchpath2:1928"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>.
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