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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"> |
| — 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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