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<title>Memory Blocks (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Memory-Management.html" rel="up" title="Memory Management">
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<div class="subsection-level-extent" id="Memory-Blocks">
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<p>
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Next: <a href="Weak-References.html" accesskey="n" rel="next">Weak References</a>, Previous: <a href="Garbage-Collection-Functions.html" accesskey="p" rel="prev">Function related to Garbage Collection</a>, Up: <a href="Memory-Management.html" accesskey="u" rel="up">Memory Management and Garbage Collection</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="Memory-Blocks-1"><span>6.17.2 Memory Blocks<a class="copiable-link" href="#Memory-Blocks-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-automatically_002dmanaged-memory"></a>
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<a class="index-entry-id" id="index-GC_002dmanaged-memory"></a>
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<a class="index-entry-id" id="index-conservative-garbage-collection"></a>
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<p>In C programs, dynamic management of memory blocks is normally done
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with the functions malloc, realloc, and free. Guile has additional
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functions for dynamic memory allocation that are integrated into the
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garbage collector and the error reporting system.
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</p>
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<p>Memory blocks that are associated with Scheme objects (for example a
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foreign object) should be allocated with <code class="code">scm_gc_malloc</code> or
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<code class="code">scm_gc_malloc_pointerless</code>. These two functions will either
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return a valid pointer or signal an error. Memory blocks allocated this
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way may be released explicitly; however, this is not strictly needed,
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and we recommend <em class="emph">not</em> calling <code class="code">scm_gc_free</code>. All memory
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allocated with <code class="code">scm_gc_malloc</code> or <code class="code">scm_gc_malloc_pointerless</code>
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is automatically reclaimed when the garbage collector no longer sees any
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live reference to it<a class="footnote" id="DOCF18" href="#FOOT18"><sup>18</sup></a>.
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</p>
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<p>When garbage collection occurs, Guile will visit the words in memory
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allocated with <code class="code">scm_gc_malloc</code>, looking for live pointers. This
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means that if <code class="code">scm_gc_malloc</code>-allocated memory contains a pointer
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to some other part of the memory, the garbage collector notices it and
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prevents it from being reclaimed<a class="footnote" id="DOCF19" href="#FOOT19"><sup>19</sup></a>. Conversely, memory allocated with
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<code class="code">scm_gc_malloc_pointerless</code> is assumed to be “pointer-less” and
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is not scanned for pointers.
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</p>
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<p>For memory that is not associated with a Scheme object, you can use
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<code class="code">scm_malloc</code> instead of <code class="code">malloc</code>. Like
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<code class="code">scm_gc_malloc</code>, it will either return a valid pointer or signal
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an error. However, it will not assume that the new memory block can
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be freed by a garbage collection. The memory must be explicitly freed
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with <code class="code">free</code>.
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</p>
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<p>There is also <code class="code">scm_gc_realloc</code> and <code class="code">scm_realloc</code>, to be used
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in place of <code class="code">realloc</code> when appropriate, and <code class="code">scm_gc_calloc</code>
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and <code class="code">scm_calloc</code>, to be used in place of <code class="code">calloc</code> when
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appropriate.
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</p>
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<p>The function <code class="code">scm_dynwind_free</code> can be useful when memory should be
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freed with libc’s <code class="code">free</code> when leaving a dynwind context,
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See <a class="xref" href="Dynamic-Wind.html">Dynamic Wind</a>.
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</p>
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<dl class="first-deftypefn">
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<dt class="deftypefn" id="index-scm_005fmalloc"><span class="category-def">C Function: </span><span><code class="def-type">void *</code> <strong class="def-name">scm_malloc</strong> <code class="def-code-arguments">(size_t <var class="var">size</var>)</code><a class="copiable-link" href="#index-scm_005fmalloc"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-scm_005fcalloc"><span class="category-def">C Function: </span><span><code class="def-type">void *</code> <strong class="def-name">scm_calloc</strong> <code class="def-code-arguments">(size_t <var class="var">size</var>)</code><a class="copiable-link" href="#index-scm_005fcalloc"> ¶</a></span></dt>
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<dd><p>Allocate <var class="var">size</var> bytes of memory and return a pointer to it. When
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<var class="var">size</var> is 0, return <code class="code">NULL</code>. When not enough memory is
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available, signal an error. This function runs the GC to free up some
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memory when it deems it appropriate.
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</p>
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<p>The memory is allocated by the libc <code class="code">malloc</code> function and can be
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freed with <code class="code">free</code>. There is no <code class="code">scm_free</code> function to go
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with <code class="code">scm_malloc</code> to make it easier to pass memory back and forth
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between different modules.
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</p>
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<p>The function <code class="code">scm_calloc</code> is similar to <code class="code">scm_malloc</code>, but
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initializes the block of memory to zero as well.
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</p>
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<p>These functions will (indirectly) call
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<code class="code">scm_gc_register_allocation</code>.
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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_005frealloc"><span class="category-def">C Function: </span><span><code class="def-type">void *</code> <strong class="def-name">scm_realloc</strong> <code class="def-code-arguments">(void *<var class="var">mem</var>, size_t <var class="var">new_size</var>)</code><a class="copiable-link" href="#index-scm_005frealloc"> ¶</a></span></dt>
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<dd><p>Change the size of the memory block at <var class="var">mem</var> to <var class="var">new_size</var> and
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return its new location. When <var class="var">new_size</var> is 0, this is the same
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as calling <code class="code">free</code> on <var class="var">mem</var> and <code class="code">NULL</code> is returned. When
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<var class="var">mem</var> is <code class="code">NULL</code>, this function behaves like <code class="code">scm_malloc</code>
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and allocates a new block of size <var class="var">new_size</var>.
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</p>
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<p>When not enough memory is available, signal an error. This function
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runs the GC to free up some memory when it deems it appropriate.
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</p>
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<p>This function will call <code class="code">scm_gc_register_allocation</code>.
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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_005fgc_005fmalloc"><span class="category-def">C Function: </span><span><code class="def-type">void *</code> <strong class="def-name">scm_gc_malloc</strong> <code class="def-code-arguments">(size_t <var class="var">size</var>, const char *<var class="var">what</var>)</code><a class="copiable-link" href="#index-scm_005fgc_005fmalloc"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-scm_005fgc_005fmalloc_005fpointerless"><span class="category-def">C Function: </span><span><code class="def-type">void *</code> <strong class="def-name">scm_gc_malloc_pointerless</strong> <code class="def-code-arguments">(size_t <var class="var">size</var>, const char *<var class="var">what</var>)</code><a class="copiable-link" href="#index-scm_005fgc_005fmalloc_005fpointerless"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-scm_005fgc_005frealloc"><span class="category-def">C Function: </span><span><code class="def-type">void *</code> <strong class="def-name">scm_gc_realloc</strong> <code class="def-code-arguments">(void *<var class="var">mem</var>, size_t <var class="var">old_size</var>, size_t <var class="var">new_size</var>, const char *<var class="var">what</var>);</code><a class="copiable-link" href="#index-scm_005fgc_005frealloc"> ¶</a></span></dt>
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<dt class="deftypefnx def-cmd-deftypefn" id="index-scm_005fgc_005fcalloc"><span class="category-def">C Function: </span><span><code class="def-type">void *</code> <strong class="def-name">scm_gc_calloc</strong> <code class="def-code-arguments">(size_t <var class="var">size</var>, const char *<var class="var">what</var>)</code><a class="copiable-link" href="#index-scm_005fgc_005fcalloc"> ¶</a></span></dt>
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<dd><p>Allocate <var class="var">size</var> bytes of automatically-managed memory. The memory
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is automatically freed when no longer referenced from any live memory
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block.
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</p>
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<p>When garbage collection occurs, Guile will visit the words in memory
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allocated with <code class="code">scm_gc_malloc</code> or <code class="code">scm_gc_calloc</code>, looking for
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pointers to other memory allocations that are managed by the GC. In
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contrast, memory allocated by <code class="code">scm_gc_malloc_pointerless</code> is not
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scanned for pointers.
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</p>
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<p>The <code class="code">scm_gc_realloc</code> call preserves the “pointerlessness” of the
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memory area pointed to by <var class="var">mem</var>. Note that you need to pass the old
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size of a reallocated memory block as well. See below for a motivation.
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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_005fgc_005ffree"><span class="category-def">C Function: </span><span><code class="def-type">void</code> <strong class="def-name">scm_gc_free</strong> <code class="def-code-arguments">(void *<var class="var">mem</var>, size_t <var class="var">size</var>, const char *<var class="var">what</var>)</code><a class="copiable-link" href="#index-scm_005fgc_005ffree"> ¶</a></span></dt>
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<dd><p>Explicitly free the memory block pointed to by <var class="var">mem</var>, which was
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previously allocated by one of the above <code class="code">scm_gc</code> functions. This
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function is almost always unnecessary, except for codebases that still
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need to compile on Guile 1.8.
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</p>
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<p>Note that you need to explicitly pass the <var class="var">size</var> parameter. This
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is done since it should normally be easy to provide this parameter
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(for memory that is associated with GC controlled objects) and help keep
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the memory management overhead very low. However, in Guile 2.x,
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<var class="var">size</var> is always ignored.
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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_005fgc_005fregister_005fallocation"><span class="category-def">C Function: </span><span><code class="def-type">void</code> <strong class="def-name">scm_gc_register_allocation</strong> <code class="def-code-arguments">(size_t <var class="var">size</var>)</code><a class="copiable-link" href="#index-scm_005fgc_005fregister_005fallocation"> ¶</a></span></dt>
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<dd><p>Informs the garbage collector that <var class="var">size</var> bytes have been allocated,
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which the collector would otherwise not have known about.
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</p>
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<p>In general, Scheme will decide to collect garbage only after some amount
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of memory has been allocated. Calling this function will make the
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Scheme garbage collector know about more allocation, and thus run more
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often (as appropriate).
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</p>
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<p>It is especially important to call this function when large unmanaged
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allocations, like images, may be freed by small Scheme allocations, like
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foreign objects.
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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_005fdynwind_005ffree-1"><span class="category-def">C Function: </span><span><code class="def-type">void</code> <strong class="def-name">scm_dynwind_free</strong> <code class="def-code-arguments">(void *mem)</code><a class="copiable-link" href="#index-scm_005fdynwind_005ffree-1"> ¶</a></span></dt>
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<dd><p>Equivalent to <code class="code">scm_dynwind_unwind_handler (free, <var class="var">mem</var>,
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SCM_F_WIND_EXPLICITLY)</code>. That is, the memory block at <var class="var">mem</var> will be
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freed (using <code class="code">free</code> from the C library) when the current dynwind is
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left.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-malloc_002dstats"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">malloc-stats</strong><a class="copiable-link" href="#index-malloc_002dstats"> ¶</a></span></dt>
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<dd><p>Return an alist ((<var class="var">what</var> . <var class="var">n</var>) ...) describing number
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of malloced objects.
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<var class="var">what</var> is the second argument to <code class="code">scm_gc_malloc</code>,
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<var class="var">n</var> is the number of objects of that type currently
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allocated.
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</p>
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<p>This function is only available if the <code class="code">GUILE_DEBUG_MALLOC</code>
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preprocessor macro was defined when Guile was compiled.
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</p></dd></dl>
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</div>
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<div class="footnotes-segment">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h5 class="footnote-body-heading"><a id="FOOT18" href="#DOCF18">(18)</a></h5>
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<p>In Guile up to version 1.8, memory
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allocated with <code class="code">scm_gc_malloc</code> <em class="emph">had</em> to be freed with
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<code class="code">scm_gc_free</code>.</p>
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<h5 class="footnote-body-heading"><a id="FOOT19" href="#DOCF19">(19)</a></h5>
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<p>In Guile up to 1.8, memory
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allocated with <code class="code">scm_gc_malloc</code> was <em class="emph">not</em> visited by the
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collector in the mark phase. Consequently, the GC had to be told
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explicitly about pointers to live objects contained in the memory block,
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e.g., <i class="i">via</i> SMOB mark functions (see <a class="pxref" href="Smobs.html"><code class="code">scm_set_smob_mark</code></a>)</p>
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</div>
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<hr>
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<p>
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Next: <a href="Weak-References.html">Weak References</a>, Previous: <a href="Garbage-Collection-Functions.html">Function related to Garbage Collection</a>, Up: <a href="Memory-Management.html">Memory Management and Garbage Collection</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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