/usr/share/doc/libbobcat4-dev/man/sharedcondition.3.html

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<h1 id="title">FBB::SharedCondition(3bobcat)</h1>
<h2 id="author">Shared Memory Cond. Var.<br/>(libbobcat-dev_4.08.02-x.tar.gz)</h2>
<h2 id="date">2005-2017</h2>


<p>
<h2 >NAME</h2>FBB::SharedCondition - Shared Memory Condition Variable
<p>
<h2 >SYNOPSIS</h2>
    <strong >#include &lt;bobcat/sharedcondition&gt;</strong><br/>
    Linking option: <em >-lpthread, -lbobcat </em> 
<p>
<h2 >DESCRIPTION</h2>
<p>
Condition variables are used to synchronize threads based on the values of
data. Condition variables allow threads to wait until a certain condition has
occurred, after which the threads continue their actions. Thus waiting threads
don't continuously have to poll the state of a variable (requiring the threads
to gain access to the variable before they can inspect its value). Using
condition variables waiting threads simply wait until they are notified.
<p>
<strong >SharedCondition</strong> objects can be used in combination with shared
memory. <strong >SharedCondition</strong> objects interface to objects (called <em >Condition</em>
objects in this man-page) which are defined in shared memory and contain a
<em >SharedMutex</em> and a shared condition object. These <em >Condition</em> objects may
be accessed by threads running in different processes. These different
processes might run a single <em >main</em> thread, or they themselves can be
multi-threaded.
<p>
Condition variables are used in situations like these:
    <ul>
    <li> There exists a thread which should be suspended until a certain
        condition has been met. 
    <li> This thread locks a mutex (or waits until the lock has been obtained)
    <li> While the condition hasn't been met, the thread is suspended (i.e.,
        waits), automatically releasing the mutex's lock. 
    <li> Somehow (see below) the thread is resumed, at which point the thread
        has automatically reacquired the lock.
    <li> Once the condition has been met, the while loop ends, and the mutex's
        lock is released.
    <li> There exists a second thread, which influences the variables that are
        elements of the condition, and which may notify the waiting thread,
        once the required condition has been met.
    <li> This second thread locks the same mutex as used by the first thread.
    <li> The second thread modifies the variables that are involved, and if
        the required condition has been met, it notifies the first thread.
    <li> The second thread releases the mutex's lock, allowing the first
        thread to obtain the mutex's lock. 
    </ul>
<p>
While the first thread is waiting, it is suspended. It may be resumed when it
receives a notification from another thread, but also for spurious
reasons. Therefore the first thread must verify that the condition has been
met after resuming its actions.
<p>
As condition variables are always used in combination with a mutex,
<strong >SharedMutex</strong> encapsulates the mutex-handling. The software using
<strong >SharedCondition</strong> objects doesn't have to handle the mutex itself.
<p>
<strong >SharedCondition</strong> objects are used to synchronize actions by different
processes, using shared memory as their vehicle of
synchronization/communication. The actual condition variable that is used by a
<strong >SharedCondition</strong> object is defined in shared memory.  <strong >SharedCondition</strong>
objects themselves are small objects, containing the necessary information to
access the actual shared memory condition variable.
<p>
<h2 >NAMESPACE</h2>
    <strong >FBB</strong><br/>
    All constructors, members, operators and manipulators, mentioned in this
man-page, are defined in the namespace <strong >FBB</strong>.
<p>
<h2 >INHERITS FROM</h2>
    <strong >SharedMutex</strong>(3bobcat)
<p>
<h2 >CONSTRUCTORS, DESTRUCTOR</h2>
    <ul>
    <li> <strong >SharedCondition()</strong>:<br/>
       The default constructor creates an empty stub which cannot yet be used
        (or an <em >FBB::Exception</em> is thrown). As the <strong >SharedCondition</strong> class
        supports assignment operators, empty stubs can easily be
        (re)configured at any time after their construction.
<p>
<li> <strong >~SharedCondition()</strong>:<br/>
       The class's destructor releases (if applicable) its lock on the shared
        condition variables mutex lock. The destructor takes no action if its
        object is an empty stub.
    </ul>
       (Default) copy and move constructors are available. 
<p>
<h2 >OVERLOADED OPERATORS</h2>
<p>
The (default) overloaded move and copy assignment operators are available.
<p>
<h2 >MEMBER FUNCTIONS</h2>
<p>
Returning from <strong >SharedCondition</strong> member functions the offset of the
<em >SharedMemory</em> object in which the condition variable has been defined has
not changed. Internally, the current offset is saved; the requested function
is performed; and the original offset is restored. Consequently,
<strong >SharedCondition</strong> member functions can be used disregarding the
<em >SharedMemory</em>'s current offset.
<p>
<ul>
    <li> <strong >void lock() const</strong>:<br/>
       When returning from this member, the current process has locked the
        <strong >SharedCondition</strong> object. Be careful not to call <em >lock</em> twice
        during the same thread of execution (cf. <strong >sharedmutex</strong>(3bobcat) for
        details).
<p>
<li> <strong >void notify() noexept</strong>:<br/>
       One of the threads waiting on the <strong >SharedCondition</strong> object wakes
        up. The thread calling <em >notify</em> should release its mutex lock
        shortly after calling <em >notify</em>, allowing the notified thread to
        obtain the lock. A prototypical piece of pseudo code illustrating
        the use of <em >notify</em> looks like this:
       <pre>

    sharedCondition.lock();     // lock the mutex
    ...                         // operate on the condition's variables
    if (conditionWasMet)        // ready to notify
        sharedCondition.notify(); 
    sharedCondition.unlock();   // release the lock
       
</pre>

       As the <em >sharedCondition.lock ... sharedCondition.unlock</em> sequence
        itself may be executed at different flow of control sections, the
        <em >unlock</em> member cannot be called from within <em >notify</em>.
<p>
<li> <strong >void notifyAll() noexept</strong>:<br/>
       Different from the plain <em >notify</em> member, this member wakes up all of
        the threads waiting on the <strong >SharedCondition</strong> object. However, after
        the current thread has released its mutex lock only one of these
        signaled threads will actually obtain the lock. The pseudo code for
        using <em >notifyAll</em> is identical to the pseudo code for using
        <em >notify</em> (i.e., calling <em >notifyAll</em>, of course).
<p>
<li> <strong >std::streamsize offset() const</strong>:<br/>
       The location of the shared condition variable (within the
        <em >SharedMemory</em> object) is returned. The shared condition object ends
        at <em >offset() + SharedCondition::width()</em>, see below.
<p>
<li> <strong >void unlock() const</strong>:<br/>
       The object's lock is released (nothing happens if called when the
        current object does not have the object's  lock).
<p>
<li> <strong >void wait()</strong>:<br/>
       Before calling <em >wait</em> the current thread should have obtained a lock
        on the <strong >SharedCondition</strong> object. 
<p>
When calling <em >wait</em> the running thread suspends its activities and
        waits until being notified. Once notified, it reacquires the lock and
        continues. Shortly after this the process should again release its
        lock on the <strong >SharedCondition</strong> object.  lock. A prototypical piece of
        pseudo code illustrating how to use <em >wait</em> looks like this:
       <pre>

    sharedCondition.lock();         // lock the mutex
    while (conditionWasNotYetMet)   // waiting required
        sharedCondition.wait(); 
    ...                             // do something: we have the lock
    sharedCondition.unlock();       // release the lock
       
</pre>

<p>
<li> <strong >void wait(Predicate pred)</strong>:<br/>
       This member was implemented as a member template. <em >Predicate</em> either
        is a predicate function or a predicate function object. The predicate
        function or the predicate function object's function call operators
        may not require arguments. As long as <em >pred</em> is returning false,
        <em >wait()</em> (no arguments) is called. The function returns once
        <em >pred</em> has returned <em >true</em>.
<p>
The running thread should have obtained a lock on the
        <strong >SharedCondition</strong> condition variable prior to calling this member,
        and should release the lock after this member has returned.
<p>
The pseudo code for using <em >wait(pred)</em> is identical to the pseudo
        code for using <em >wait</em> (albeit that <em >pred</em> has to be passed to
        <em >wait</em>, of course).
<p>
<li> <strong >std::cv_status wait_for(std::chrono::duration&lt;Type, Unit&gt; 
        const &amp;relTime)</strong>:<br/>
       This member was implemented as a member template. <em >Type</em> defines the
        type of the variable holding the amount of time (usually <em >int64_t</em>),
        specified in time unit <em >Unit</em>. Predefined <em >duration</em> types are
        available from the <em >std::chrono</em> namespace, like
        <em >std::chrono::seconds(4)</em>, representing 4 seconds, or
        <em >std::chrono::milliseconds(30)</em>, representing 30 milliseconds.
<p>
The running thread should have obtained a lock on <strong >SharedCondition</strong>
        prior to calling this member, and should release the lock after this
        member has returned.
<p>
This member acts like <em >wait</em>, returning
        <em >std::cv_status::no_timeout</em> if a notification was received before
        <em >relTime</em> has passed. Otherwise <em >std::cv_status::timeout</em> is
        returned.
<p>
A prototypical piece of pseudo code illustrating how to use
        <em >wait_for</em> looks like this:
       <pre>

    sharedCondition.lock();         // lock the mutex
    while (conditionWasNotYetMet)   // waiting required
    {
        while (sharedCondition.wait_for(someTime) 
               == std::cv_status::timeout)
            handle_timeout 

        do_something
    }
    sharedCondition.unlock();       // release the lock
       
</pre>

       When returning from <em >wait_for</em> the current thread has obtained the
        shared condition's lock, but maybe due to a timeout: this can be
        verified by inspecting <em >wait_for's</em> return value, and an appropriate
        action can be selected.
<p>
<li> <strong >bool wait_for(std::chrono::duration&lt;Type, Unit&gt; 
        const &amp;relTime, Predicate pred)</strong>:<br/>
       This member was implemented as a member template. <em >Type</em> defines the
        type of the variable holding the amount of time (usually <em >int64_t</em>),
        specified in time unit <em >Unit</em>. <em >Predicate</em> either is a predicate
        function or a predicate function object.  The predicate function or
        the predicate function object's function call operators may not
        require arguments. 
<p>
The running thread should have obtained a lock on <strong >SharedCondition</strong>
        prior to calling this member, and should release the lock after this
        member has returned.
<p>
As long as <em >pred</em> returns false, <em >wait_for(relTime)</em> is called. If
        the latter function returns <em >std::cv_status::timeout</em>, then
        <em >pred</em> is called, and its return value is returned. Otherwise
        <em >true</em> is returned.
<p>
The pseudo code for using this member is identical to the pseudo code
        for using the abovementioned <em >wait_for</em> member (albeit that <em >pred</em>
        must also be passed to <em >wait_for</em>, of course).
<p>
<li> <strong >std::cv_status wait_until(std::chrono::time_point&lt;Clock, Duration&gt; 
        const &amp;absTime)</strong>:<br/>
       This member has been implemented as a member template. <em >Clock</em>
        defines the clock-type to use (usually <em >std::chrono::system_clock</em>),
        <em >Duration</em> is the type name of a duration type (as used with
        <em >wait_for</em>). E.g., to specify 5 seconds after the current time this
        member could be called like this:
       <pre>

    std::chrono::system_clock::now() + std::chrono::seconds(5)
        
</pre>

<p>
The running thread should have obtained a lock on <strong >SharedCondition</strong>
        prior to calling this member, and should release the lock after this
        member has returned.
<p>
This member acts like <em >wait_for(relative-time)</em>, returning
        <em >std::cv_status::no_timeout</em> if a notification was received before
        <em >absTime</em> has passed. Otherwise <em >std::cv_status::timeout</em> is
        returned.
<p>
The pseudo code for using this member is identical to the pseudo code
        for using the abovementioned <em >wait_for(relative-time)</em> member
        (albeit that absolute time must be specified).
<p>
<li> <strong >bool wait_until(std::chrono::time_point&lt;Clock, Duration&gt; 
        const &amp;absTime, Predicate pred)</strong>:<br/>
       This member was implemented as a member template. <em >Clock</em> and
        <em >Duration</em> define identical types as mentioned at the previous
        member.  <em >Predicate</em> either is a predicate function or a predicate
        function object (not expecting arguments).
<p>
The running thread should have obtained a lock on <strong >SharedCondition</strong>
        prior to calling this member, and should release the lock after this
        member has returned.
<p>
As long as <em >pred</em> returns false, <em >wait_until(absTime)</em>
        is called. If the latter function returns <em >std::cv_status::timeout</em>,
        then <em >pred</em> is called, and its return value is returned. Otherwise
        <em >true</em> is returned.
<p>
The pseudo code for using this member is identical to the pseudo code
        for using the abovementioned <em >wait_until</em> member (albeit that
        <em >pred</em> must also be passed to <em >wait_until</em>, of course).
    </ul>
<p>
<h2 >STATIC MEMBER FUNCTIONS</h2>
<p>
<ul>
    <li> <strong >SharedCondition &amp;attach(SharedMemory &amp;shmem,
                        std::ios::off_type offset = 0,
                        std::ios::seekdir origin = std::ios::beg)</strong>:<br/>
       The <em >SharedCondition</em> object interfacing to the shared condition
        variable located at <em >offset</em> (relative to <em >origin</em>) in <em >shmem</em>
        is returned. 
<p>
An <em >FBB::Exception</em> is thrown if the requested offset is invalid
        (i.e., smaller than 0 or exceeding <em >shmem.maxOffset()</em>).
<p>
<li> <strong >FBB::SharedCondition create(SharedMemory &amp;shmem)</strong>:<br/> 
       A shared condition variable is initialized at the current offset of
        the <em >SharedMemory</em> object referred to by <em >shmem</em>, or at the first
        offset of the next physical shared data segment. 
<p>
A <strong >SharedCondition</strong> object interfacing to the initialized shared
        condition variable is returned.
<p>
An <em >FBB::Exception</em> is thrown if there isn't enough memory available
        in the <em >SharedMemory</em> object to define a shared condition variable.
<p>
<li> <strong >size_t size() const</strong>:<br/>
       Returns the size in bytes of the shared condition variables stored in
        <em >SharedMemory</em> objects.
    </ul>
<p>
<h2 >EXAMPLE</h2>
<p>
<pre >
#include &lt;iostream&gt;

#include &lt;bobcat/sharedcondition&gt;
#include &lt;bobcat/sharedmemory&gt;

using namespace std;
using namespace FBB;

int main(int argc, char **argv)
try
{
    if (argc == 1)
    {
        cout &lt;&lt; 
            "Argument:\n"
            "   c: create a shared memory segment + SharedCondition "
                                                    ", display ID\n" 
            "   k &lt;id&gt;: kill shared memory segment &lt;id&gt;\n"
            "   m &lt;id&gt;: show a message every 5 secs, otherwise wait until\n"
            "           being notified in segment &lt;id&gt;\n"
            "   n &lt;id&gt;: notify the SharedCondition in segment ID &lt;id&gt;\n"
        ;
        return 0;
    }

    switch (argv[1][0])
    {
        case 'c':
        {
            SharedMemory shmem(1, SharedMemory::kB);

            SharedCondition cond = SharedCondition::create(shmem);   

            void *ptr = shmem.ptr();

            cout &lt;&lt; "ID = " &lt;&lt; shmem.id() &lt;&lt; ", SharedCondition at " &lt;&lt; 
                    cond.offset() &lt;&lt; endl;
            break;
        }

        case 'k':
        {
            SharedMemory shmem(stoll(argv[2]));
            shmem.kill();
            break;
        }

        case 'm':
        {
            SharedMemory shmem(stoll(argv[2]));
            SharedCondition cond = SharedCondition::attach(shmem);

            cond.lock();
            cout &lt;&lt; "Obtained the lock. Now waiting for a notification\n";
        
            while (true)
            {
                switch (cond.wait_for(chrono::seconds(5)))
                {
                    case cv_status::timeout:
                        cout &lt;&lt; "Waited for 5 seconds\n\n";
                    break;

                    case cv_status::no_timeout:
                        cond.unlock();
                        cout &lt;&lt; "Received the notification. Unlocked.\n";
                    return 0;
                }
            }
        }
            
        case 'w':
        {
            SharedMemory shmem(stoll(argv[2]));
            SharedCondition cond = SharedCondition::attach(shmem);

            cond.lock();
            cout &lt;&lt; "Obtained the lock. Now waiting for a notification\n";
        
            cond.wait();
            cout &lt;&lt; "Received the notification. Unlocking.\n";

            cond.unlock();
            break;
        }
            
        case 'n':
        {
            SharedMemory shmem(stoll(argv[2]));

            SharedCondition cond = SharedCondition::attach(shmem);

            cout &lt;&lt; "Notifying the other after Enter ";
            cin.ignore(1000, '\n');

            cond.lock();
            cout &lt;&lt; "Obtained the lock. Now notifying the other\n";
            cond.notify();
            cout &lt;&lt; "Sent the notification. Now unlocking.\n";
            cond.unlock();
            break;
        }

    }
}
catch (exception const &amp;exc)
{
    cout &lt;&lt; "Exception: " &lt;&lt; exc.what() &lt;&lt; endl;
}

</pre>

<p>
<h2 >FILES</h2>
    <em >bobcat/sharedcondition</em> - defines the class interface
<p>
<h2 >SEE ALSO</h2>
    <strong >bobcat</strong>(7)
        <strong >isharedstream</strong>(3bobcat),
        <strong >osharedstream</strong>(3bobcat),
        <strong >sharedblock</strong>(3bobcat), 
        <strong >sharedmemory</strong>(3bobcat),
        <strong >sharedpos</strong>(3bobcat), 
        <strong >sharedreadme</strong>(7bobcat), 
        <strong >sharedsegment</strong>(3bobcat), 
        <strong >sharedstream</strong>(3bobcat), 
        <strong >sharedstreambuf</strong>(3bobcat)
<p>
<h2 >BUGS</h2>
    None Reported.
<p>

<h2 >DISTRIBUTION FILES</h2>
    <ul>
    <li> <em >bobcat_4.08.02-x.dsc</em>: detached signature;
    <li> <em >bobcat_4.08.02-x.tar.gz</em>: source archive;
    <li> <em >bobcat_4.08.02-x_i386.changes</em>: change log;
    <li> <em >libbobcat1_4.08.02-x_*.deb</em>: debian package holding the
            libraries;
    <li> <em >libbobcat1-dev_4.08.02-x_*.deb</em>: debian package holding the
            libraries, headers and manual pages;
    <li> <em >http://sourceforge.net/projects/bobcat</em>: public archive location;
    </ul>
<p>
<h2 >BOBCAT</h2>
    Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.
<p>
<h2 >COPYRIGHT</h2>
    This is free software, distributed under the terms of the 
    GNU General Public License (GPL).
<p>
<h2 >AUTHOR</h2>
    Frank B. Brokken (<strong >f.b.brokken@rug.nl</strong>).
<p>
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libbobcat-dev 4.08.02-2build1 / usr / share / doc / libbobcat4-dev / man / sharedcondition.3.html
