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<h5 class="subsubsection">3.2.2.8 Malloc Tunable Parameters</h5>

<p>You can adjust some parameters for dynamic memory allocation with the
<code>mallopt</code> function.  This function is the general SVID/XPG
interface, defined in <samp><span class="file">malloc.h</span></samp>. 
<a name="index-malloc_002eh-277"></a>

<div class="defun">
&mdash; Function: int <b>mallopt</b> (<var>int param, int value</var>)<var><a name="index-mallopt-278"></a></var><br>
<blockquote><p>When calling <code>mallopt</code>, the <var>param</var> argument specifies the
parameter to be set, and <var>value</var> the new value to be set.  Possible
choices for <var>param</var>, as defined in <samp><span class="file">malloc.h</span></samp>, are:

          <dl>
<dt><code>M_TRIM_THRESHOLD</code><dd>This is the minimum size (in bytes) of the top-most, releasable chunk
that will cause <code>sbrk</code> to be called with a negative argument in
order to return memory to the system. 
<br><dt><code>M_TOP_PAD</code><dd>This parameter determines the amount of extra memory to obtain from the
system when a call to <code>sbrk</code> is required.  It also specifies the
number of bytes to retain when shrinking the heap by calling <code>sbrk</code>
with a negative argument.  This provides the necessary hysteresis in
heap size such that excessive amounts of system calls can be avoided. 
<br><dt><code>M_MMAP_THRESHOLD</code><dd>All chunks larger than this value are allocated outside the normal
heap, using the <code>mmap</code> system call.  This way it is guaranteed
that the memory for these chunks can be returned to the system on
<code>free</code>.  Note that requests smaller than this threshold might still
be allocated via <code>mmap</code>. 
<br><dt><code>M_MMAP_MAX</code><dd>The maximum number of chunks to allocate with <code>mmap</code>.  Setting this
to zero disables all use of <code>mmap</code>. 
<br><dt><code>M_PERTURB</code><dd>If non-zero, memory blocks are filled with values depending on some
low order bits of this parameter when they are allocated (except when
allocated by <code>calloc</code>) and freed.  This can be used to debug the
use of uninitialized or freed heap memory. 
</dl>

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