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<link title="Get" rel="Chapter" href="Get.html"><title>Ocamlnet 4 Reference Manual : Netplex_semaphore</title>
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<h1>Module <a href="type_Netplex_semaphore.html">Netplex_semaphore</a></h1>
<pre><span id="MODULENetplex_semaphore"><span class="keyword">module</span> Netplex_semaphore</span>: <code class="code">sig</code> <a href="Netplex_semaphore.html">..</a> <code class="code">end</code></pre><div class="info module top">
<div class="info-desc">
<p>Netplex-wide semaphores</p>
</div>
</div>
<hr width="100%">
<p>Semaphores are counters with atomic increment and decrement operations.
They are very useful for counting the number of uses of a shared
resource, and allow the identification of the first use (so the resource
must be made available at all), and the last use (the resource can be
released).</p>
<p>This implementation works in both multi-processing and
multi-threading netplex environments. It is, however, not very
fast, because the counters live in the controller, and the
increment/decrement operations are realized by RPC's. It is good
enough when these operations are only infrequently called, e.g. in
the post-start and pre-finish processor callbacks.</p>
<p>This interface is designed so that a later re-implementation with
POSIX semaphores is relatively straight-forward.</p>
<p><b>Thread safety:</b> Full. The functions can be called from any thread.</p>
<pre><span id="VALplugin"><span class="keyword">val</span> plugin</span> : <code class="type"><a href="Netplex_types.plugin-c.html">Netplex_types.plugin</a></code></pre><div class="info ">
<div class="info-desc">
<p>To enable semaphores, call the controller's <code class="code">add_plugin</code> method
with this object as argument. This can e.g. be done in the
<code class="code">post_add_hook</code> of the processor.</p>
</div>
</div>
<p>The following functions can <b>only</b> be invoked in container
contexts. Outside of such a context the exception
<a href="Netplex_cenv.html#EXCEPTIONNot_in_container_thread"><code class="code">Netplex_cenv.Not_in_container_thread</code></a> is raised.</p>
<pre><span id="VALincrement"><span class="keyword">val</span> increment</span> : <code class="type">string -> int64</code></pre><div class="info ">
<div class="info-desc">
<p>Increment the named semaphore by 1, and return the new value.
If the semaphore does not exist yet, it is created with an initial
value of 0, which is then incremented.</p>
<p>Semaphore names are global to the whole netplex system. By convention,
these names are formed like <code class="code">"service_name.local_name"</code>, i.e. they
are prefixed by the socket service to which they refer.</p>
</div>
</div>
<pre><span id="VALdecrement"><span class="keyword">val</span> decrement</span> : <code class="type">?wait:bool -> string -> int64</code></pre><div class="info ">
<div class="info-desc">
<p>Decrement the named semaphore by 1, and return the new value.
Semaphore values cannot become negative. If the value is already 0,
it is not decremented anymore if <code class="code">wait = false</code>. However, (-1)
is then returned nevertheless.</p>
<p>If the value is already 0 and <code class="code">wait=true</code>, the operation waits until
the value exceeds 0, and when this happens, the semaphore is then
decremented again. If several waiters exist, only one waiter gets
the chance to decrement.</p>
</div>
</div>
<pre><span id="VALget"><span class="keyword">val</span> get</span> : <code class="type">string -> int64</code></pre><div class="info ">
<div class="info-desc">
<p>Get the value of the named semaphore. Useful e.g. for monitoring
the semaphore. If the semaphore does not exist, a value of 0 is
returned.</p>
<p><code class="code">get</code> can also be invoked from the controller process.</p>
</div>
</div>
<pre><span id="VALcreate"><span class="keyword">val</span> create</span> : <code class="type">?protected:bool -> string -> int64 -> bool</code></pre><div class="info ">
<div class="info-desc">
<p>Create the semaphore with this initial value. Returns <code class="code">true</code> if the
creation is successful, and <code class="code">false</code> if the semaphore already existed.</p>
<p>If <code class="code">protected</code>, the semaphore
is automatically decremented by some value when the container
calling this function terminates. This value is <code class="code">pi - d</code> where
<code class="code">pi</code> is the number of increments and <code class="code">d</code> is the number
of (successful) decrements requested by the container.</p>
<p>A semaphore needs not to be explicitly created by calling <code class="code">create</code>.
It is automatically created at the first use time with a value of 0
and <code class="code">protected=true</code>.</p>
<p><code class="code">create</code> can also be invoked from the controller process.</p>
</div>
</div>
<pre><span id="VALdestroy"><span class="keyword">val</span> destroy</span> : <code class="type">string -> unit</code></pre><div class="info ">
<div class="info-desc">
<p>Destroy this semaphore. Any waiting <code class="code">decrement</code> will immediately
get (-1L).</p>
<p>Note that there is no protection against unintended re-creation
after <code class="code">destroy</code>.</p>
<p><code class="code">destroy</code> can also be invoked from the controller process.</p>
</div>
</div>
<pre><span id="VALctrl_increment"><span class="keyword">val</span> ctrl_increment</span> : <code class="type">string -> <a href="Netplex_types.html#TYPEcontainer_id">Netplex_types.container_id</a> -> int64</code></pre><div class="info ">
<div class="info-desc">
<p>Increment the named semaphore from controller context, substituting
an increment from a container (e.g. a container that terminated
or is otherwise no longer able to do so). In this
case the ID of the container needs to be passed</p>
</div>
</div>
<p>Example (code fragment):</p>
<p>Override the processor callbacks as follows to count the number of
containers for the service:</p>
<pre class="codepre"><code class="code">
method post_add_hook sockserv ctrl =
ctrl # add_plugin Netplex_semaphore.plugin
method post_start_hook container =
let sem_name = container#socket_service#name ^ ".counter" in
let n =
Netplex_semaphore.increment sem_name in
if n=1 then
prerr_endline "First container"
method pre_finish_hook container =
let sem_name = container#socket_service#name ^ ".counter" in
let n =
Netplex_semaphore.decrement sem_name in
if n=0 then
prerr_endline "Last container"
</code></pre></body></html>
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