# FBB::SharedCondition(3bobcat)

## Shared Memory Cond. Var.
(libbobcat-dev_5.07.03)

## 2005-2021

## NAME

FBB::SharedCondition - Shared Memory Condition Variable

## SYNOPSIS

#include <bobcat/sharedcondition>
Linking option: -lpthread, -lbobcat

## DESCRIPTION

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.

SharedCondition objects can be used in combination with shared memory. SharedCondition objects interface to objects (called Condition objects in this man-page) which are defined in shared memory and contain a SharedMutex and a shared condition object. These Condition objects may be accessed by threads running in different processes. These different processes might run a single main thread, or they themselves can be multi-threaded.

Condition variables are used in situations like these:

- There exists a thread which should be suspended until a certain condition has been met.
- This thread locks a mutex (or waits until the lock has been obtained)
- While the condition hasn't been met, the thread is suspended (i.e., waits), automatically releasing the mutex's lock.
- Somehow (see below) the thread is resumed, at which point the thread has automatically reacquired the lock.
- Once the condition has been met, the while loop ends, and the mutex's lock is released.
- 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.
- This second thread locks the same mutex as used by the first thread.
- The second thread modifies the variables that are involved, and if the required condition has been met, it notifies the first thread.
- The second thread releases the mutex's lock, allowing the first thread to obtain the mutex's lock.

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.

As condition variables are always used in combination with a mutex, SharedMutex encapsulates the mutex-handling. The software using SharedCondition objects doesn't have to handle the mutex itself.

SharedCondition 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 SharedCondition object is defined in shared memory. SharedCondition objects themselves are small objects, containing the necessary information to access the actual shared memory condition variable.

## NAMESPACE

FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

## INHERITS FROM

SharedMutex(3bobcat)

## CONSTRUCTORS, DESTRUCTOR

- SharedCondition():
  The default constructor creates an empty stub which cannot yet be used (or an FBB::Exception is thrown). As the SharedCondition class supports assignment operators, empty stubs can easily be (re)configured at any time after their construction.

- ~SharedCondition():
  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.

Default, copy, and move constructors as well as the copy and move assignment operators are available.

## MEMBER FUNCTIONS

Returning from SharedCondition member functions the offset of the SharedMemory 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, SharedCondition member functions can be used disregarding the SharedMemory's current offset.

- void lock() const:
  When returning from this member, the current process has locked the SharedCondition object. Be careful not to call lock twice during the same thread of execution (cf. sharedmutex(3bobcat) for details).

- void notify() noexept:
  One of the threads waiting on the SharedCondition object wakes up. The thread calling notify should release its mutex lock shortly after calling notify, allowing the notified thread to obtain the lock. A prototypical piece of pseudo code illustrating the use of notify looks like this:

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

  As the sharedCondition.lock ... sharedCondition.unlock sequence itself may be executed at different flow of control sections, the unlock member cannot be called from within notify.

- void notifyAll() noexept:
  Different from the plain notify member, this member wakes up all of the threads waiting on the SharedCondition 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 notifyAll is identical to the pseudo code for using notify (i.e., calling notifyAll, of course).

- std::streamsize offset() const:
  The location of the shared condition variable (within the SharedMemory object) is returned. The shared condition object ends at offset() + SharedCondition::width(), see below.

- void unlock() const:
  The object's lock is released (nothing happens if called when the current object does not have the object's lock).

- void wait():
  Before calling wait the current thread should have obtained a lock on the SharedCondition object.

  When calling wait 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 SharedCondition object. lock. A prototypical piece of pseudo code illustrating how to use wait looks like this:

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

- void wait(Predicate pred):
  This member was implemented as a member template. Predicate 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 pred is returning false, wait() (no arguments) is called. The function returns once pred has returned true.

  The running thread should have obtained a lock on the SharedCondition condition variable prior to calling this member, and should release the lock after this member has returned.

  The pseudo code for using wait(pred) is identical to the pseudo code for using wait (albeit that pred has to be passed to wait, of course).

- std::cv_status wait_for(std::chrono::duration<Type, Unit> const &relTime):
  This member was implemented as a member template. Type defines the type of the variable holding the amount of time (usually int64_t), specified in time unit Unit. Predefined duration types are available from the std::chrono namespace, like std::chrono::seconds(4), representing 4 seconds, or std::chrono::milliseconds(30), representing 30 milliseconds.

  The running thread should have obtained a lock on SharedCondition prior to calling this member, and should release the lock after this member has returned.

  This member acts like wait, returning std::cv_status::no_timeout if a notification was received before relTime has passed. Otherwise std::cv_status::timeout is returned.

  A prototypical piece of pseudo code illustrating how to use wait_for looks like this:

          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

  When returning from wait_for the current thread has obtained the shared condition's lock, but maybe due to a timeout: this can be verified by inspecting wait_for's return value, and an appropriate action can be selected.

- bool wait_for(std::chrono::duration<Type, Unit> const &relTime, Predicate pred):
  This member was implemented as a member template. Type defines the type of the variable holding the amount of time (usually int64_t), specified in time unit Unit. Predicate 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.

  The running thread should have obtained a lock on SharedCondition prior to calling this member, and should release the lock after this member has returned.

  As long as pred returns false, wait_for(relTime) is called. If the latter function returns std::cv_status::timeout, then pred is called, and its return value is returned. Otherwise true is returned.

  The pseudo code for using this member is identical to the pseudo code for using the abovementioned wait_for member (albeit that pred must also be passed to wait_for, of course).

- std::cv_status wait_until(std::chrono::time_point<Clock, Duration> const &absTime):
  This member has been implemented as a member template. Clock defines the clock-type to use (usually std::chrono::system_clock), Duration is the type name of a duration type (as used with wait_for). E.g., to specify 5 seconds after the current time this member could be called like this:

          std::chrono::system_clock::now() + std::chrono::seconds(5)

  The running thread should have obtained a lock on SharedCondition prior to calling this member, and should release the lock after this member has returned.

  This member acts like wait_for(relative-time), returning std::cv_status::no_timeout if a notification was received before absTime has passed. Otherwise std::cv_status::timeout is returned.

  The pseudo code for using this member is identical to the pseudo code for using the abovementioned wait_for(relative-time) member (albeit that absolute time must be specified).

- bool wait_until(std::chrono::time_point<Clock, Duration> const &absTime, Predicate pred):
  This member was implemented as a member template. Clock and Duration define identical types as mentioned at the previous member. Predicate either is a predicate function or a predicate function object (not expecting arguments).

  The running thread should have obtained a lock on SharedCondition prior to calling this member, and should release the lock after this member has returned.

  As long as pred returns false, wait_until(absTime) is called. If the latter function returns std::cv_status::timeout, then pred is called, and its return value is returned. Otherwise true is returned.

  The pseudo code for using this member is identical to the pseudo code for using the abovementioned wait_until member (albeit that pred must also be passed to wait_until, of course).

## STATIC MEMBER FUNCTIONS

- SharedCondition &attach(SharedMemory &shmem, std::ios::off_type offset = 0, std::ios::seekdir origin = std::ios::beg):
  The SharedCondition object interfacing to the shared condition variable located at offset (relative to origin) in shmem is returned.

  An FBB::Exception is thrown if the requested offset is invalid (i.e., smaller than 0 or exceeding shmem.maxOffset()).

- FBB::SharedCondition create(SharedMemory &shmem):
  A shared condition variable is initialized at the current offset of the SharedMemory object referred to by shmem, or at the first offset of the next physical shared data segment.

  A SharedCondition object interfacing to the initialized shared condition variable is returned.

  An FBB::Exception is thrown if there isn't enough memory available in the SharedMemory object to define a shared condition variable.

- size_t size() const:
  Returns the size in bytes of the shared condition variables stored in SharedMemory objects.

## EXAMPLE

    #include <iostream>

    #include <bobcat/sharedcondition>
    #include <bobcat/sharedmemory>

    using namespace std;
    using namespace FBB;

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

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

                SharedCondition cond = SharedCondition::create(shmem);

                void *ptr = shmem.ptr();

                cout << "ID = " << shmem.id() << ", SharedCondition at " <<
                        cond.offset() << 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 << "Obtained the lock. Now waiting for a notification\n";

                while (true)
                {
                    switch (cond.wait_for(chrono::seconds(5)))
                    {
                        case cv_status::timeout:
                            cout << "Waited for 5 seconds\n\n";
                        break;

                        case cv_status::no_timeout:
                            cond.unlock();
                            cout << "Received the notification. Unlocked.\n";
                        return 0;
                    }
                }
            }

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

                cond.lock();
                cout << "Obtained the lock. Now waiting for a notification\n";

                cond.wait();
                cout << "Received the notification. Unlocking.\n";

                cond.unlock();
                break;
            }

            case 'n':
            {
                SharedMemory shmem(stoll(argv[2]));

                SharedCondition cond = SharedCondition::attach(shmem);

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

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

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

## FILES

bobcat/sharedcondition - defines the class interface

## SEE ALSO

bobcat(7) isharedstream(3bobcat), osharedstream(3bobcat), sharedblock(3bobcat), sharedmemory(3bobcat), sharedpos(3bobcat), sharedreadme(7bobcat), sharedsegment(3bobcat), sharedstream(3bobcat), sharedbuf(3bobcat)

## BUGS

None Reported.

## BOBCAT PROJECT FILES

- https://fbb-git.gitlab.io/bobcat/: gitlab project page;
- bobcat_5.07.03-x.dsc: detached signature;
- bobcat_5.07.03-x.tar.gz: source archive;
- bobcat_5.07.03-x_i386.changes: change log;
- libbobcat1_5.07.03-x_*.deb: debian package containing the libraries;
- libbobcat1-dev_5.07.03-x_*.deb: debian package containing the libraries, headers and manual pages;

## BOBCAT

Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

## COPYRIGHT

This is free software, distributed under the terms of the GNU General Public License (GPL).

## AUTHOR

Frank B. Brokken (f.b.brokken@rug.nl).