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<title>Foreign Pointers (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Foreign-Pointers">
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<p>
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Next: <a href="Foreign-Types.html" accesskey="n" rel="next">Foreign Types</a>, Previous: <a href="Foreign-Extensions.html" accesskey="p" rel="prev">Foreign Extensions</a>, Up: <a href="Foreign-Function-Interface.html" accesskey="u" rel="up">Foreign Function Interface</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
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<h4 class="subsection" id="Foreign-Pointers-1"><span>6.19.3 Foreign Pointers<a class="copiable-link" href="#Foreign-Pointers-1"> ¶</a></span></h4>
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<p>Foreign libraries are essentially key-value mappings, where the keys are
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names of definitions and the values are the addresses of those
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definitions. To look up the address of a definition, use
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<code class="code">foreign-library-pointer</code> from the <code class="code">(system foreign-library)</code>
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module.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-foreign_002dlibrary_002dpointer"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">foreign-library-pointer</strong> <var class="def-var-arguments">lib name</var><a class="copiable-link" href="#index-foreign_002dlibrary_002dpointer"> ¶</a></span></dt>
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<dd><p>Return a “wrapped pointer” for the symbol <var class="var">name</var> in the shared
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object referred to by <var class="var">lib</var>. The returned pointer points to a C
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object.
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</p>
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<p>As a convenience, if <var class="var">lib</var> is not a foreign library, it will be
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passed to <code class="code">load-foreign-library</code>.
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</p></dd></dl>
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<p>If we continue with the <code class="code">bessel.so</code> example from before, we can get
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the address of the <code class="code">init_math_bessel</code> function via:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (system foreign-library))
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(define init (foreign-library-pointer "bessel" "init_math_bessel"))
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init
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⇒ #<pointer 0x7fb35b1b4688>
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</pre></div>
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<p>A value returned by <code class="code">foreign-library-pointer</code> is a Scheme wrapper
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for a C pointer. Pointers are a data type in Guile that is disjoint
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from all other types. The next section discusses ways to dereference
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pointers, but before then we describe the usual type predicates and so
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on.
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</p>
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<p>Note that the rest of the interfaces in this section are part of the
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<code class="code">(system foreign)</code> library:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (system foreign))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-pointer_002daddress"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">pointer-address</strong> <var class="def-var-arguments">pointer</var><a class="copiable-link" href="#index-pointer_002daddress"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fpointer_005faddress"><span class="category-def">C Function: </span><span><strong class="def-name">scm_pointer_address</strong> <var class="def-var-arguments">(pointer)</var><a class="copiable-link" href="#index-scm_005fpointer_005faddress"> ¶</a></span></dt>
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<dd><p>Return the numerical value of <var class="var">pointer</var>.
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</p>
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<div class="example">
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<pre class="example-preformatted">(pointer-address init)
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⇒ 139984413364296 ; YMMV
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</pre></div>
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</dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dpointer"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-pointer</strong> <var class="def-var-arguments">address [finalizer]</var><a class="copiable-link" href="#index-make_002dpointer"> ¶</a></span></dt>
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<dd><p>Return a foreign pointer object pointing to <var class="var">address</var>. If
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<var class="var">finalizer</var> is passed, it should be a pointer to a one-argument C
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function that will be called when the pointer object becomes
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unreachable.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-pointer_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">pointer?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-pointer_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">obj</var> is a pointer object, or <code class="code">#f</code>
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otherwise.
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</p></dd></dl>
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<dl class="first-defvr">
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<dt class="defvr" id="index-_0025null_002dpointer"><span class="category-def">Scheme Variable: </span><span><strong class="def-name">%null-pointer</strong><a class="copiable-link" href="#index-_0025null_002dpointer"> ¶</a></span></dt>
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<dd><p>A foreign pointer whose value is 0.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-null_002dpointer_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">null-pointer?</strong> <var class="def-var-arguments">pointer</var><a class="copiable-link" href="#index-null_002dpointer_003f"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">pointer</var> is the null pointer, <code class="code">#f</code> otherwise.
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</p></dd></dl>
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<p>For the purpose of passing SCM values directly to foreign functions, and
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allowing them to return SCM values, Guile also supports some unsafe
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casting operators.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-scm_002d_003epointer"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">scm->pointer</strong> <var class="def-var-arguments">scm</var><a class="copiable-link" href="#index-scm_002d_003epointer"> ¶</a></span></dt>
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<dd><p>Return a foreign pointer object with the <code class="code">object-address</code>
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of <var class="var">scm</var>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-pointer_002d_003escm"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">pointer->scm</strong> <var class="def-var-arguments">pointer</var><a class="copiable-link" href="#index-pointer_002d_003escm"> ¶</a></span></dt>
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<dd><p>Unsafely cast <var class="var">pointer</var> to a Scheme object.
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Cross your fingers!
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</p></dd></dl>
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<p>Sometimes you want to give C extensions access to the dynamic FFI. At
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that point, the names get confusing, because “pointer” can refer to a
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<code class="code">SCM</code> object that wraps a pointer, or to a <code class="code">void*</code> value. We
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will try to use “pointer object” to refer to Scheme objects, and
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“pointer value” to refer to <code class="code">void *</code> values.
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</p>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005ffrom_005fpointer"><span class="category-def">C Function: </span><span><code class="def-type">SCM</code> <strong class="def-name">scm_from_pointer</strong> <code class="def-code-arguments">(void *ptr, void (*finalizer) (void*))</code><a class="copiable-link" href="#index-scm_005ffrom_005fpointer"> ¶</a></span></dt>
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<dd><p>Create a pointer object from a pointer value.
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</p>
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<p>If <var class="var">finalizer</var> is non-null, Guile arranges to call it on the pointer
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value at some point after the pointer object becomes collectible.
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</p></dd></dl>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fto_005fpointer"><span class="category-def">C Function: </span><span><code class="def-type">void*</code> <strong class="def-name">scm_to_pointer</strong> <code class="def-code-arguments">(SCM obj)</code><a class="copiable-link" href="#index-scm_005fto_005fpointer"> ¶</a></span></dt>
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<dd><p>Unpack the pointer value from a pointer object.
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</p></dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Foreign-Types.html">Foreign Types</a>, Previous: <a href="Foreign-Extensions.html">Foreign Extensions</a>, Up: <a href="Foreign-Function-Interface.html">Foreign Function Interface</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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