1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<link rel="stylesheet" href="style.css" type="text/css">
<meta content="text/html; charset=iso-8859-1" http-equiv="Content-Type">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="Start" href="index.html">
<link rel="previous" href="Dynlink.html">
<link rel="next" href="Event.html">
<link rel="Up" href="index.html">
<link title="Index of types" rel=Appendix href="index_types.html">
<link title="Index of extensions" rel=Appendix href="index_extensions.html">
<link title="Index of exceptions" rel=Appendix href="index_exceptions.html">
<link title="Index of values" rel=Appendix href="index_values.html">
<link title="Index of modules" rel=Appendix href="index_modules.html">
<link title="Index of module types" rel=Appendix href="index_module_types.html">
<link title="Arg" rel="Chapter" href="Arg.html">
<link title="Array" rel="Chapter" href="Array.html">
<link title="ArrayLabels" rel="Chapter" href="ArrayLabels.html">
<link title="Bigarray" rel="Chapter" href="Bigarray.html">
<link title="Bool" rel="Chapter" href="Bool.html">
<link title="Buffer" rel="Chapter" href="Buffer.html">
<link title="Bytes" rel="Chapter" href="Bytes.html">
<link title="BytesLabels" rel="Chapter" href="BytesLabels.html">
<link title="Callback" rel="Chapter" href="Callback.html">
<link title="CamlinternalFormat" rel="Chapter" href="CamlinternalFormat.html">
<link title="CamlinternalFormatBasics" rel="Chapter" href="CamlinternalFormatBasics.html">
<link title="CamlinternalLazy" rel="Chapter" href="CamlinternalLazy.html">
<link title="CamlinternalMod" rel="Chapter" href="CamlinternalMod.html">
<link title="CamlinternalOO" rel="Chapter" href="CamlinternalOO.html">
<link title="Char" rel="Chapter" href="Char.html">
<link title="Complex" rel="Chapter" href="Complex.html">
<link title="Condition" rel="Chapter" href="Condition.html">
<link title="Digest" rel="Chapter" href="Digest.html">
<link title="Dynlink" rel="Chapter" href="Dynlink.html">
<link title="Ephemeron" rel="Chapter" href="Ephemeron.html">
<link title="Event" rel="Chapter" href="Event.html">
<link title="Filename" rel="Chapter" href="Filename.html">
<link title="Float" rel="Chapter" href="Float.html">
<link title="Format" rel="Chapter" href="Format.html">
<link title="Fun" rel="Chapter" href="Fun.html">
<link title="Gc" rel="Chapter" href="Gc.html">
<link title="Genlex" rel="Chapter" href="Genlex.html">
<link title="Hashtbl" rel="Chapter" href="Hashtbl.html">
<link title="Int" rel="Chapter" href="Int.html">
<link title="Int32" rel="Chapter" href="Int32.html">
<link title="Int64" rel="Chapter" href="Int64.html">
<link title="Lazy" rel="Chapter" href="Lazy.html">
<link title="Lexing" rel="Chapter" href="Lexing.html">
<link title="List" rel="Chapter" href="List.html">
<link title="ListLabels" rel="Chapter" href="ListLabels.html">
<link title="Map" rel="Chapter" href="Map.html">
<link title="Marshal" rel="Chapter" href="Marshal.html">
<link title="MoreLabels" rel="Chapter" href="MoreLabels.html">
<link title="Mutex" rel="Chapter" href="Mutex.html">
<link title="Nativeint" rel="Chapter" href="Nativeint.html">
<link title="Obj" rel="Chapter" href="Obj.html">
<link title="Ocaml_operators" rel="Chapter" href="Ocaml_operators.html">
<link title="Oo" rel="Chapter" href="Oo.html">
<link title="Option" rel="Chapter" href="Option.html">
<link title="Parsing" rel="Chapter" href="Parsing.html">
<link title="Printexc" rel="Chapter" href="Printexc.html">
<link title="Printf" rel="Chapter" href="Printf.html">
<link title="Queue" rel="Chapter" href="Queue.html">
<link title="Random" rel="Chapter" href="Random.html">
<link title="Result" rel="Chapter" href="Result.html">
<link title="Scanf" rel="Chapter" href="Scanf.html">
<link title="Seq" rel="Chapter" href="Seq.html">
<link title="Set" rel="Chapter" href="Set.html">
<link title="Spacetime" rel="Chapter" href="Spacetime.html">
<link title="Stack" rel="Chapter" href="Stack.html">
<link title="StdLabels" rel="Chapter" href="StdLabels.html">
<link title="Stdlib" rel="Chapter" href="Stdlib.html">
<link title="Str" rel="Chapter" href="Str.html">
<link title="Stream" rel="Chapter" href="Stream.html">
<link title="String" rel="Chapter" href="String.html">
<link title="StringLabels" rel="Chapter" href="StringLabels.html">
<link title="Sys" rel="Chapter" href="Sys.html">
<link title="Thread" rel="Chapter" href="Thread.html">
<link title="ThreadUnix" rel="Chapter" href="ThreadUnix.html">
<link title="Uchar" rel="Chapter" href="Uchar.html">
<link title="Unit" rel="Chapter" href="Unit.html">
<link title="Unix" rel="Chapter" href="Unix.html">
<link title="UnixLabels" rel="Chapter" href="UnixLabels.html">
<link title="Weak" rel="Chapter" href="Weak.html"><title>Ephemeron</title>
</head>
<body>
<div class="navbar"><a class="pre" href="Dynlink.html" title="Dynlink">Previous</a>
<a class="up" href="index.html" title="Index">Up</a>
<a class="post" href="Event.html" title="Event">Next</a>
</div>
<h1>Module <a href="type_Ephemeron.html">Ephemeron</a></h1>
<pre><span id="MODULEEphemeron"><span class="keyword">module</span> Ephemeron</span>: <code class="code"><span class="keyword">sig</span></code> <a href="Ephemeron.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info module top">
<div class="info-desc">
<p>Ephemerons and weak hash tables</p>
</div>
</div>
<hr width="100%">
<p>Ephemerons and weak hash tables are useful when one wants to cache
or memorize the computation of a function, as long as the
arguments and the function are used, without creating memory leaks
by continuously keeping old computation results that are not
useful anymore because one argument or the function is freed. An
implementation using <a href="Hashtbl.html#TYPEt"><code class="code"><span class="constructor">Hashtbl</span>.t</code></a> is not suitable because all
associations would keep the arguments and the result in memory.</p>
<p>Ephemerons can also be used for "adding" a field to an arbitrary
boxed OCaml value: you can attach some information to a value
created by an external library without memory leaks.</p>
<p>Ephemerons hold some keys and one or no data. They are all boxed
OCaml values. The keys of an ephemeron have the same behavior
as weak pointers according to the garbage collector. In fact
OCaml weak pointers are implemented as ephemerons without data.</p>
<p>The keys and data of an ephemeron are said to be full if they
point to a value, or empty if the value has never been set, has
been unset, or was erased by the GC. In the function that accesses
the keys or data these two states are represented by the <code class="code">option</code>
type.</p>
<p>The data is considered by the garbage collector alive if all the
full keys are alive and if the ephemeron is alive. When one of the
keys is not considered alive anymore by the GC, the data is
emptied from the ephemeron. The data could be alive for another
reason and in that case the GC will not free it, but the ephemeron
will not hold the data anymore.</p>
<p>The ephemerons complicate the notion of liveness of values, because
it is not anymore an equivalence with the reachability from root
value by usual pointers (not weak and not ephemerons). With ephemerons
the notion of liveness is constructed by the least fixpoint of:
A value is alive if:</p>
<ul>
<li>it is a root value</li>
<li>it is reachable from alive value by usual pointers</li>
<li>it is the data of an alive ephemeron with all its full keys alive</li>
</ul>
<p>Notes:</p>
<ul>
<li>All the types defined in this module cannot be marshaled
using <a href="Stdlib.html#VALoutput_value"><code class="code">output_value</code></a> or the functions of the
<a href="Marshal.html"><code class="code"><span class="constructor">Marshal</span></code></a> module.</li>
</ul>
<p>Ephemerons are defined in a language agnostic way in this paper:
B. Hayes, Ephemerons: A New Finalization Mechanism, OOPSLA'97</p>
<pre><span id="MODULETYPES"><span class="keyword">module type</span> <a href="Ephemeron.S.html">S</a></span> = <code class="code"><span class="keyword">sig</span></code> <a href="Ephemeron.S.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info">
<p>The output signature of the functor <a href="Ephemeron.K1.Make.html"><code class="code"><span class="constructor">Ephemeron</span>.<span class="constructor">K1</span>.<span class="constructor">Make</span></code></a> and <a href="Ephemeron.K2.Make.html"><code class="code"><span class="constructor">Ephemeron</span>.<span class="constructor">K2</span>.<span class="constructor">Make</span></code></a>.</p>
</div>
<pre><span id="MODULETYPESeededS"><span class="keyword">module type</span> <a href="Ephemeron.SeededS.html">SeededS</a></span> = <code class="code"><span class="keyword">sig</span></code> <a href="Ephemeron.SeededS.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info">
<p>The output signature of the functor <a href="Ephemeron.K1.MakeSeeded.html"><code class="code"><span class="constructor">Ephemeron</span>.<span class="constructor">K1</span>.<span class="constructor">MakeSeeded</span></code></a> and <a href="Ephemeron.K2.MakeSeeded.html"><code class="code"><span class="constructor">Ephemeron</span>.<span class="constructor">K2</span>.<span class="constructor">MakeSeeded</span></code></a>.</p>
</div>
<pre><span id="MODULEK1"><span class="keyword">module</span> <a href="Ephemeron.K1.html">K1</a></span>: <code class="code"><span class="keyword">sig</span></code> <a href="Ephemeron.K1.html">..</a> <code class="code"><span class="keyword">end</span></code></pre>
<pre><span id="MODULEK2"><span class="keyword">module</span> <a href="Ephemeron.K2.html">K2</a></span>: <code class="code"><span class="keyword">sig</span></code> <a href="Ephemeron.K2.html">..</a> <code class="code"><span class="keyword">end</span></code></pre>
<pre><span id="MODULEKn"><span class="keyword">module</span> <a href="Ephemeron.Kn.html">Kn</a></span>: <code class="code"><span class="keyword">sig</span></code> <a href="Ephemeron.Kn.html">..</a> <code class="code"><span class="keyword">end</span></code></pre>
<pre><span id="MODULEGenHashTable"><span class="keyword">module</span> <a href="Ephemeron.GenHashTable.html">GenHashTable</a></span>: <code class="code"><span class="keyword">sig</span></code> <a href="Ephemeron.GenHashTable.html">..</a> <code class="code"><span class="keyword">end</span></code></pre></body></html>
|
