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<center><h1>Module <a href="type_Gc.html">Gc</a></h1></center>
<br>
<pre><span class="keyword">module</span> Gc: <code class="code"><span class="keyword">sig</span></code> <a href="Gc.html">..</a> <code class="code"><span class="keyword">end</span></code></pre>Memory management control and statistics; finalised values.<br>
<hr width="100%">
<br><code><span class="keyword">type</span> <a name="TYPEstat"></a><code class="type"></code>stat = {</code><table class="typetable">
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code>minor_words : <code class="type">float</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >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.</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>promoted_words : <code class="type">float</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >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.</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>major_words : <code class="type">float</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of words allocated in the major heap, including
the promoted words, since the program was started.</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>minor_collections : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of minor collections since the program was started.</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>major_collections : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of major collection cycles completed since the program
was started.</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>heap_words : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Total size of the major heap, in words.</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>heap_chunks : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of contiguous pieces of memory that make up the major heap.</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>live_words : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of words of live data in the major heap, including the header
words.</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>live_blocks : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of live blocks in the major heap.</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>free_words : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of words in the free list.</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>free_blocks : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of blocks in the free list.</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>largest_free : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Size (in words) of the largest block in the free list.</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>fragments : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of wasted words due to fragmentation. These are
1-words free blocks placed between two live blocks. They
are not available for allocation.</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>compactions : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Number of heap compactions since the program was started.</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>top_heap_words : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >Maximum size reached by the major heap, in words.</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
}
<div class="info">
The memory management counters are returned in a <code class="code">stat</code> record.
<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.<br>
</div>
<br><code><span class="keyword">type</span> <a name="TYPEcontrol"></a><code class="type"></code>control = {</code><table class="typetable">
<tr>
<td align="left" valign="top" >
<code> </code></td>
<td align="left" valign="top" >
<code><span class="keyword">mutable </span>minor_heap_size : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >The size (in words) of the minor heap. Changing
this parameter will trigger a minor collection. Default: 32k.</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>major_heap_increment : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >The minimum number of words to add to the
major heap when increasing it. Default: 62k.</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>space_overhead : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >The major GC speed is computed from this parameter.
This is the memory that will be "wasted" because the GC does not
immediatly 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.</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>verbose : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >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:<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 search at start-up.</li>
<li><code class="code">0x200</code> Computation of compaction triggering condition.
Default: 0.</li>
</ul>
</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>max_overhead : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >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.
Default: 500.</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>stack_limit : <code class="type">int</code>;</code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >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: 256k.</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
}
<div class="info">
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
ocamlrun.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALstat"></a>stat : <code class="type">unit -> <a href="Gc.html#TYPEstat">stat</a></code></pre><div class="info">
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.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALquick_stat"></a>quick_stat : <code class="type">unit -> <a href="Gc.html#TYPEstat">stat</a></code></pre><div class="info">
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.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALcounters"></a>counters : <code class="type">unit -> float * float * float</code></pre><div class="info">
Return <code class="code">(minor_words, promoted_words, major_words)</code>. This function
is as fast at <code class="code">quick_stat</code>.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALget"></a>get : <code class="type">unit -> <a href="Gc.html#TYPEcontrol">control</a></code></pre><div class="info">
Return the current values of the GC parameters in a <code class="code">control</code> record.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALset"></a>set : <code class="type"><a href="Gc.html#TYPEcontrol">control</a> -> unit</code></pre><div class="info">
<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><br>
</div>
<pre><span class="keyword">val</span> <a name="VALminor"></a>minor : <code class="type">unit -> unit</code></pre><div class="info">
Trigger a minor collection.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALmajor_slice"></a>major_slice : <code class="type">int -> int</code></pre><div class="info">
Do a minor collection and a slice of major collection. The argument
is the size of the slice, 0 to use the automatically-computed
slice size. In all cases, the result is the computed slice size.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALmajor"></a>major : <code class="type">unit -> unit</code></pre><div class="info">
Do a minor collection and finish the current major collection cycle.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALfull_major"></a>full_major : <code class="type">unit -> unit</code></pre><div class="info">
Do a minor collection, finish the current major collection cycle,
and perform a complete new cycle. This will collect all currently
unreachable blocks.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALcompact"></a>compact : <code class="type">unit -> unit</code></pre><div class="info">
Perform a full major collection and compact the heap. Note that heap
compaction is a lengthy operation.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALprint_stat"></a>print_stat : <code class="type"><a href="Pervasives.html#TYPEout_channel">out_channel</a> -> unit</code></pre><div class="info">
Print the current values of the memory management counters (in
human-readable form) into the channel argument.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALallocated_bytes"></a>allocated_bytes : <code class="type">unit -> float</code></pre><div class="info">
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.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALfinalise"></a>finalise : <code class="type">('a -> unit) -> 'a -> unit</code></pre><div class="info">
<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
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>
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>
Anything reachable from the closure of finalisation functions
is considered reachable, so the following code will not work
as expected:<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> x <span class="keywordsign">-></span> ...) v </code></li>
</ul>
Instead you should write:<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>
The <code class="code">f</code> function can use all features of O'Caml, including
assignments that make the value reachable again. It can also
loop forever (in this case, the other
finalisation functions will be called during the execution of f).
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>
<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
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.
You should also be aware that compiler optimisations may duplicate
some immutable values, for example floating-point numbers when
stored into arrays, so they can be finalised and collected while
another copy is still in use by the program.
<p>
The results of calling <a href="String.html#VALmake"><code class="code"><span class="constructor">String</span>.make</code></a>, <a href="String.html#VALcreate"><code class="code"><span class="constructor">String</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>.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALfinalise_release"></a>finalise_release : <code class="type">unit -> unit</code></pre><div class="info">
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.<br>
</div>
<pre><span class="keyword">type</span> <a name="TYPEalarm"></a><code class="type"></code>alarm </pre>
<div class="info">
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.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALcreate_alarm"></a>create_alarm : <code class="type">(unit -> unit) -> <a href="Gc.html#TYPEalarm">alarm</a></code></pre><div class="info">
<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>.<br>
</div>
<pre><span class="keyword">val</span> <a name="VALdelete_alarm"></a>delete_alarm : <code class="type"><a href="Gc.html#TYPEalarm">alarm</a> -> unit</code></pre><div class="info">
<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.<br>
</div>
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