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<h1>Module <a href="type_Gc.html">Gc</a></h1>
<pre><span id="MODULEGc"><span class="keyword">module</span> Gc</span>: <code class="code"><span class="keyword">sig</span></code> <a href="Gc.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info module top">
<div class="info-desc">
<p>Memory management control and statistics; finalised values.</p>
</div>
</div>
<hr width="100%">
<pre><code><span id="TYPEstat"><span class="keyword">type</span> <code class="type"></code>stat</span> = {</code></pre><table class="typetable">
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.minor_words">minor_words</span> : <code class="type">float</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of words allocated in the minor heap since
the program was started. This number is accurate in
byte-code programs, but only an approximation in programs
compiled to native code.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.promoted_words">promoted_words</span> : <code class="type">float</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of words allocated in the minor heap that
survived a minor collection and were moved to the major heap
since the program was started.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.major_words">major_words</span> : <code class="type">float</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of words allocated in the major heap, including
the promoted words, since the program was started.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.minor_collections">minor_collections</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of minor collections since the program was started.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.major_collections">major_collections</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of major collection cycles completed since the program
was started.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.heap_words">heap_words</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Total size of the major heap, in words.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.heap_chunks">heap_chunks</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of contiguous pieces of memory that make up the major heap.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.live_words">live_words</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of words of live data in the major heap, including the header
words.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.live_blocks">live_blocks</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of live blocks in the major heap.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.free_words">free_words</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of words in the free list.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.free_blocks">free_blocks</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of blocks in the free list.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.largest_free">largest_free</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Size (in words) of the largest block in the free list.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.fragments">fragments</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of wasted words due to fragmentation. These are
1-words free blocks placed between two live blocks. They
are not available for allocation.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.compactions">compactions</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Number of heap compactions since the program was started.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.top_heap_words">top_heap_words</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Maximum size reached by the major heap, in words.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTstat.stack_size">stack_size</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Current size of the stack, in words.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 3.12.0</li>
</ul>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
}
<div class="info ">
<div class="info-desc">
<p>The memory management counters are returned in a <code class="code">stat</code> record.</p>
<p>The total amount of memory allocated by the program since it was started
is (in words) <code class="code">minor_words + major_words - promoted_words</code>. Multiply by
the word size (4 on a 32-bit machine, 8 on a 64-bit machine) to get
the number of bytes.</p>
</div>
</div>
<pre><code><span id="TYPEcontrol"><span class="keyword">type</span> <code class="type"></code>control</span> = {</code></pre><table class="typetable">
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span><span id="TYPEELTcontrol.minor_heap_size">minor_heap_size</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The size (in words) of the minor heap. Changing
this parameter will trigger a minor collection. Default: 256k.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span><span id="TYPEELTcontrol.major_heap_increment">major_heap_increment</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>How much to add to the major heap when increasing it. If this
number is less than or equal to 1000, it is a percentage of
the current heap size (i.e. setting it to 100 will double the heap
size at each increase). If it is more than 1000, it is a fixed
number of words that will be added to the heap. Default: 15.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span><span id="TYPEELTcontrol.space_overhead">space_overhead</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The major GC speed is computed from this parameter.
This is the memory that will be "wasted" because the GC does not
immediately collect unreachable blocks. It is expressed as a
percentage of the memory used for live data.
The GC will work more (use more CPU time and collect
blocks more eagerly) if <code class="code">space_overhead</code> is smaller.
Default: 80.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span><span id="TYPEELTcontrol.verbose">verbose</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>This value controls the GC messages on standard error output.
It is a sum of some of the following flags, to print messages
on the corresponding events:</p>
<ul>
<li><code class="code">0x001</code> Start of major GC cycle.</li>
<li><code class="code">0x002</code> Minor collection and major GC slice.</li>
<li><code class="code">0x004</code> Growing and shrinking of the heap.</li>
<li><code class="code">0x008</code> Resizing of stacks and memory manager tables.</li>
<li><code class="code">0x010</code> Heap compaction.</li>
<li><code class="code">0x020</code> Change of GC parameters.</li>
<li><code class="code">0x040</code> Computation of major GC slice size.</li>
<li><code class="code">0x080</code> Calling of finalisation functions.</li>
<li><code class="code">0x100</code> Bytecode executable and shared library search at start-up.</li>
<li><code class="code">0x200</code> Computation of compaction-triggering condition.</li>
<li><code class="code">0x400</code> Output GC statistics at program exit.
Default: 0.</li>
</ul>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span><span id="TYPEELTcontrol.max_overhead">max_overhead</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>Heap compaction is triggered when the estimated amount
of "wasted" memory is more than <code class="code">max_overhead</code> percent of the
amount of live data. If <code class="code">max_overhead</code> is set to 0, heap
compaction is triggered at the end of each major GC cycle
(this setting is intended for testing purposes only).
If <code class="code">max_overhead >= 1000000</code>, compaction is never triggered.
If compaction is permanently disabled, it is strongly suggested
to set <code class="code">allocation_policy</code> to 1.
Default: 500.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span><span id="TYPEELTcontrol.stack_limit">stack_limit</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The maximum size of the stack (in words). This is only
relevant to the byte-code runtime, as the native code runtime
uses the operating system's stack. Default: 1024k.</p>
</div>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span><span id="TYPEELTcontrol.allocation_policy">allocation_policy</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The policy used for allocating in the heap. Possible
values are 0 and 1. 0 is the next-fit policy, which is
quite fast but can result in fragmentation. 1 is the
first-fit policy, which can be slower in some cases but
can be better for programs with fragmentation problems.
Default: 0.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 3.11.0</li>
</ul>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span id="TYPEELTcontrol.window_size">window_size</span> : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" ><div class="info ">
<div class="info-desc">
<p>The size of the window used by the major GC for smoothing
out variations in its workload. This is an integer between
1 and 50.
Default: 1.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.03.0</li>
</ul>
</div>
</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
}
<div class="info ">
<div class="info-desc">
<p>The GC parameters are given as a <code class="code">control</code> record. Note that
these parameters can also be initialised by setting the
OCAMLRUNPARAM environment variable. See the documentation of
<code class="code">ocamlrun</code>.</p>
</div>
</div>
<pre><span id="VALstat"><span class="keyword">val</span> stat</span> : <code class="type">unit -> <a href="Gc.html#TYPEstat">stat</a></code></pre><div class="info ">
<div class="info-desc">
<p>Return the current values of the memory management counters in a
<code class="code">stat</code> record. This function examines every heap block to get the
statistics.</p>
</div>
</div>
<pre><span id="VALquick_stat"><span class="keyword">val</span> quick_stat</span> : <code class="type">unit -> <a href="Gc.html#TYPEstat">stat</a></code></pre><div class="info ">
<div class="info-desc">
<p>Same as <code class="code">stat</code> except that <code class="code">live_words</code>, <code class="code">live_blocks</code>, <code class="code">free_words</code>,
<code class="code">free_blocks</code>, <code class="code">largest_free</code>, and <code class="code">fragments</code> are set to 0. This
function is much faster than <code class="code">stat</code> because it does not need to go
through the heap.</p>
</div>
</div>
<pre><span id="VALcounters"><span class="keyword">val</span> counters</span> : <code class="type">unit -> float * float * float</code></pre><div class="info ">
<div class="info-desc">
<p>Return <code class="code">(minor_words, promoted_words, major_words)</code>. This function
is as fast as <code class="code">quick_stat</code>.</p>
</div>
</div>
<pre><span id="VALminor_words"><span class="keyword">val</span> minor_words</span> : <code class="type">unit -> float</code></pre><div class="info ">
<div class="info-desc">
<p>Number of words allocated in the minor heap since the program was
started. This number is accurate in byte-code programs, but only an
approximation in programs compiled to native code.</p>
<p>In native code this function does not allocate.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.04</li>
</ul>
</div>
<pre><span id="VALget"><span class="keyword">val</span> get</span> : <code class="type">unit -> <a href="Gc.html#TYPEcontrol">control</a></code></pre><div class="info ">
<div class="info-desc">
<p>Return the current values of the GC parameters in a <code class="code">control</code> record.</p>
</div>
</div>
<pre><span id="VALset"><span class="keyword">val</span> set</span> : <code class="type"><a href="Gc.html#TYPEcontrol">control</a> -> unit</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">set r</code> changes the GC parameters according to the <code class="code">control</code> record <code class="code">r</code>.
The normal usage is: <code class="code"><span class="constructor">Gc</span>.set { (<span class="constructor">Gc</span>.get()) <span class="keyword">with</span> <span class="constructor">Gc</span>.verbose = 0x00d }</code></p>
</div>
</div>
<pre><span id="VALminor"><span class="keyword">val</span> minor</span> : <code class="type">unit -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Trigger a minor collection.</p>
</div>
</div>
<pre><span id="VALmajor_slice"><span class="keyword">val</span> major_slice</span> : <code class="type">int -> int</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">major_slice n</code>
Do a minor collection and a slice of major collection. <code class="code">n</code> is the
size of the slice: the GC will do enough work to free (on average)
<code class="code">n</code> words of memory. If <code class="code">n</code> = 0, the GC will try to do enough work
to ensure that the next automatic slice has no work to do.
This function returns an unspecified integer (currently: 0).</p>
</div>
</div>
<pre><span id="VALmajor"><span class="keyword">val</span> major</span> : <code class="type">unit -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Do a minor collection and finish the current major collection cycle.</p>
</div>
</div>
<pre><span id="VALfull_major"><span class="keyword">val</span> full_major</span> : <code class="type">unit -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Do a minor collection, finish the current major collection cycle,
and perform a complete new cycle. This will collect all currently
unreachable blocks.</p>
</div>
</div>
<pre><span id="VALcompact"><span class="keyword">val</span> compact</span> : <code class="type">unit -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Perform a full major collection and compact the heap. Note that heap
compaction is a lengthy operation.</p>
</div>
</div>
<pre><span id="VALprint_stat"><span class="keyword">val</span> print_stat</span> : <code class="type"><a href="Pervasives.html#TYPEout_channel">out_channel</a> -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Print the current values of the memory management counters (in
human-readable form) into the channel argument.</p>
</div>
</div>
<pre><span id="VALallocated_bytes"><span class="keyword">val</span> allocated_bytes</span> : <code class="type">unit -> float</code></pre><div class="info ">
<div class="info-desc">
<p>Return the total number of bytes allocated since the program was
started. It is returned as a <code class="code">float</code> to avoid overflow problems
with <code class="code">int</code> on 32-bit machines.</p>
</div>
</div>
<pre><span id="VALget_minor_free"><span class="keyword">val</span> get_minor_free</span> : <code class="type">unit -> int</code></pre><div class="info ">
<div class="info-desc">
<p>Return the current size of the free space inside the minor heap.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.03.0</li>
</ul>
</div>
<pre><span id="VALget_bucket"><span class="keyword">val</span> get_bucket</span> : <code class="type">int -> int</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">get_bucket n</code> returns the current size of the <code class="code">n</code>-th future bucket
of the GC smoothing system. The unit is one millionth of a full GC.
Raise <code class="code"><span class="constructor">Invalid_argument</span></code> if <code class="code">n</code> is negative, return 0 if n is larger
than the smoothing window.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.03.0</li>
</ul>
</div>
<pre><span id="VALget_credit"><span class="keyword">val</span> get_credit</span> : <code class="type">unit -> int</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">get_credit ()</code> returns the current size of the "work done in advance"
counter of the GC smoothing system. The unit is one millionth of a
full GC.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.03.0</li>
</ul>
</div>
<pre><span id="VALhuge_fallback_count"><span class="keyword">val</span> huge_fallback_count</span> : <code class="type">unit -> int</code></pre><div class="info ">
<div class="info-desc">
<p>Return the number of times we tried to map huge pages and had to fall
back to small pages. This is always 0 if <code class="code"><span class="constructor">OCAMLRUNPARAM</span></code> contains <code class="code"><span class="constructor">H</span>=1</code>.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.03.0</li>
</ul>
</div>
<pre><span id="VALfinalise"><span class="keyword">val</span> finalise</span> : <code class="type">('a -> unit) -> 'a -> unit</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">finalise f v</code> registers <code class="code">f</code> as a finalisation function for <code class="code">v</code>.
<code class="code">v</code> must be heap-allocated. <code class="code">f</code> will be called with <code class="code">v</code> as
argument at some point between the first time <code class="code">v</code> becomes unreachable
(including through weak pointers) and the time <code class="code">v</code> is collected by
the GC. Several functions can
be registered for the same value, or even several instances of the
same function. Each instance will be called once (or never,
if the program terminates before <code class="code">v</code> becomes unreachable).</p>
<p>The GC will call the finalisation functions in the order of
deallocation. When several values become unreachable at the
same time (i.e. during the same GC cycle), the finalisation
functions will be called in the reverse order of the corresponding
calls to <code class="code">finalise</code>. If <code class="code">finalise</code> is called in the same order
as the values are allocated, that means each value is finalised
before the values it depends upon. Of course, this becomes
false if additional dependencies are introduced by assignments.</p>
<p>In the presence of multiple OCaml threads it should be assumed that
any particular finaliser may be executed in any of the threads.</p>
<p>Anything reachable from the closure of finalisation functions
is considered reachable, so the following code will not work
as expected:</p>
<ul>
<li><code class="code"> <span class="keyword">let</span> v = ... <span class="keyword">in</span> <span class="constructor">Gc</span>.finalise (<span class="keyword">fun</span> _ <span class="keywordsign">-></span> ...v...) v </code></li>
</ul>
<p>Instead you should make sure that <code class="code">v</code> is not in the closure of
the finalisation function by writing:</p>
<ul>
<li><code class="code"> <span class="keyword">let</span> f = <span class="keyword">fun</span> x <span class="keywordsign">-></span> ... <span class="keyword">let</span> v = ... <span class="keyword">in</span> <span class="constructor">Gc</span>.finalise f v </code></li>
</ul>
<p>The <code class="code">f</code> function can use all features of OCaml, including
assignments that make the value reachable again. It can also
loop forever (in this case, the other
finalisation functions will not be called during the execution of f,
unless it calls <code class="code">finalise_release</code>).
It can call <code class="code">finalise</code> on <code class="code">v</code> or other values to register other
functions or even itself. It can raise an exception; in this case
the exception will interrupt whatever the program was doing when
the function was called.</p>
<p><code class="code">finalise</code> will raise <code class="code"><span class="constructor">Invalid_argument</span></code> if <code class="code">v</code> is not
guaranteed to be heap-allocated. Some examples of values that are not
heap-allocated are integers, constant constructors, booleans,
the empty array, the empty list, the unit value. The exact list
of what is heap-allocated or not is implementation-dependent.
Some constant values can be heap-allocated but never deallocated
during the lifetime of the program, for example a list of integer
constants; this is also implementation-dependent.
Note that values of types <code class="code">float</code> are sometimes allocated and
sometimes not, so finalising them is unsafe, and <code class="code">finalise</code> will
also raise <code class="code"><span class="constructor">Invalid_argument</span></code> for them. Values of type <code class="code"><span class="keywordsign">'</span>a <span class="constructor">Lazy</span>.t</code>
(for any <code class="code"><span class="keywordsign">'</span>a</code>) are like <code class="code">float</code> in this respect, except that the
compiler sometimes optimizes them in a way that prevents <code class="code">finalise</code>
from detecting them. In this case, it will not raise
<code class="code"><span class="constructor">Invalid_argument</span></code>, but you should still avoid calling <code class="code">finalise</code>
on lazy values.</p>
<p>The results of calling <a href="String.html#VALmake"><code class="code"><span class="constructor">String</span>.make</code></a>, <a href="Bytes.html#VALmake"><code class="code"><span class="constructor">Bytes</span>.make</code></a>, <a href="Bytes.html#VALcreate"><code class="code"><span class="constructor">Bytes</span>.create</code></a>,
<a href="Array.html#VALmake"><code class="code"><span class="constructor">Array</span>.make</code></a>, and <a href="Pervasives.html#VALref"><code class="code">ref</code></a> are guaranteed to be
heap-allocated and non-constant except when the length argument is <code class="code">0</code>.</p>
</div>
</div>
<pre><span id="VALfinalise_last"><span class="keyword">val</span> finalise_last</span> : <code class="type">(unit -> unit) -> 'a -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>same as <a href="Gc.html#VALfinalise"><code class="code"><span class="constructor">Gc</span>.finalise</code></a> except the value is not given as argument. So
you can't use the given value for the computation of the
finalisation function. The benefit is that the function is called
after the value is unreachable for the last time instead of the
first time. So contrary to <a href="Gc.html#VALfinalise"><code class="code"><span class="constructor">Gc</span>.finalise</code></a> the value will never be
reachable again or used again. In particular every weak pointer
and ephemeron that contained this value as key or data is unset
before running the finalisation function. Moreover the
finalisation function attached with `GC.finalise` are always
called before the finalisation function attached with `GC.finalise_last`.</p>
</div>
<ul class="info-attributes">
<li><b>Since</b> 4.04</li>
</ul>
</div>
<pre><span id="VALfinalise_release"><span class="keyword">val</span> finalise_release</span> : <code class="type">unit -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>A finalisation function may call <code class="code">finalise_release</code> to tell the
GC that it can launch the next finalisation function without waiting
for the current one to return.</p>
</div>
</div>
<pre><span id="TYPEalarm"><span class="keyword">type</span> <code class="type"></code>alarm</span> </pre>
<div class="info ">
<div class="info-desc">
<p>An alarm is a piece of data that calls a user function at the end of
each major GC cycle. The following functions are provided to create
and delete alarms.</p>
</div>
</div>
<pre><span id="VALcreate_alarm"><span class="keyword">val</span> create_alarm</span> : <code class="type">(unit -> unit) -> <a href="Gc.html#TYPEalarm">alarm</a></code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">create_alarm f</code> will arrange for <code class="code">f</code> to be called at the end of each
major GC cycle, starting with the current cycle or the next one.
A value of type <code class="code">alarm</code> is returned that you can
use to call <code class="code">delete_alarm</code>.</p>
</div>
</div>
<pre><span id="VALdelete_alarm"><span class="keyword">val</span> delete_alarm</span> : <code class="type"><a href="Gc.html#TYPEalarm">alarm</a> -> unit</code></pre><div class="info ">
<div class="info-desc">
<p><code class="code">delete_alarm a</code> will stop the calls to the function associated
to <code class="code">a</code>. Calling <code class="code">delete_alarm a</code> again has no effect.</p>
</div>
</div>
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