# FBB::Semaphore(3bobcat)

## Dijkstra's Semaphore
(libbobcat-dev_5.07.03)

## 2005-2021

## NAME

FBB::Semaphore - Implements the Semaphore type designed by Dijkstra

## SYNOPSIS

#include <bobcat/semaphore>

Linking option: -lpthread -lbobcat

## DESCRIPTION

According to http://en.wikipedia.org/wiki/Semaphore_(programming) a semaphore is a variable or abstract data type that is used for controlling access, by multiple processes, to a common resource in a parallel programming or a multi user environment. The Semaphore as a data type was designed around 1962 by Edsger Dijkstra.

A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to safely (i.e., without race conditions) adjust that record as units are required or become free, and, if necessary, wait until a unit of the resource becomes available.

Semaphores are a useful tool in the prevention of race conditions. Semaphores allowing an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores. Both types are supported by Bobcat's implementation.

## NAMESPACE

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

## INHERITS FROM

-

## CONSTRUCTORS

- Semaphore(size_t nAvailable):
  The constructor defines the semaphore's initial state. With a counting semaphore nAvailable defines, e.g., the number of available locations in a storage area. Locking/unlocking, supporting facilities to notify other waiting threads is implemented via binary semaphores, which are initialized to 1 or 0. A semaphore containing the value 0 blocks. I.e., its wait member waits until its value is incremented by another thread, calling one of the semaphore's notify members.

Copy and move constructors (and assignment operators) are not available.

## MEMBER FUNCTIONS

- void notify():
  The internally maintained available value is incremented and one waiting thread (cf. the wait members below) is notified, reactivating that thread.
- void notify_all():
  The internally maintained available value is incremented and all waiting threads (cf. the wait members below) are notified. Only one waiting thread will be able to obtain the semaphore's lock and to reduce available, and that particular thread is thereupon reactivated.
- void set(size_t available):
  This member blocks until it has obtained the lock of the std::mutex which is maintained internally by the Semaphore object. Next the Semaphore's available value receives the value of the member's argument, and the lock is released.
- size_t size() const:
  Without trying to lock the Semaphore object's mutex the current value of the Semaphore's available value is returned.
- void wait():
  This member blocks for as long as the internally stored value (available) equals zero. When returning from wait the current thread holds the lock of the std::mutex which is maintained internally by the Semaphore object. Notify members are used to increment available and to inform wait that it may return. When multiple threads are waiting only one thread will stop waiting, while the remaining threads will continue to wait for another notification.
- bool wait(Fun fun, Params &&...args):
  This is a member template, where Fun is a function (object) receiving the argument passed to wait, and returning a bool. This member blocks until it has obtained the Semaphore's mutex lock, then, while its available value equals 0, waits until being notified. Once it has reacquired the lock after being notified fun is called, receiving wait's perfectly forwarded remaining arguments. This member returns false if fun returns false. It returns true if fun returns true and available was unequal zero following fun returning true. The function may therefore perform tasks outside of the Semaphore local environment, which might even involve updating the Semaphore's available value.
- std::cv_status wait_for(std::chrono::duration<Rep, Period> const &relTime):
  This member blocks for as long as the internally stored value (available) equals zero and the amount of time specified by relTime hasn't passed. If the latter happens, std::cv_status::timeout is returned, otherwise std::cv_status::no_timeout is returned, in which case the current thread holds the lock of the std::mutex which is maintained internally by the Semaphore object. Notify members are used to increment available and to inform wait that it may return. When multiple threads are waiting only one thread will stop waiting, while the remaining threads will continue to wait for another notification.
- std::cv_status wait_until(std::chrono::time_point<Clock, Duration> const &absTime):
  This member blocks for as long as the internally stored value (available) equals zero and the time specified by absTime hasn't been reached. If the latter happens (or if absTime lies in the past) std::cv_status::timeout is returned, otherwise std::cv_status::no_timeout is returned, in which case the current thread holds the lock of the std::mutex which is maintained internally by the Semaphore object. Notify members are used to increment available and to inform wait that it may return. When multiple threads are waiting only one thread will stop waiting, while the remaining threads will continue to wait for another notification.

## EXAMPLE

        #include <bobcat/semaphore>

        using namespace FBB;

        Semaphore produce(10);          // storage area size
        Semaphore consume(0);           // # items in store
        std::queue itemQueue;           // storage queue

        void consumer()
        {
            while (true)
            {
                consume.wait();          // wait until there's an item in store

                    // mutex lock the queue with multiple consumers
                size_t item = itemQueue.front();
                itemQueue.pop();

                produce.notify();   // notify the producer

                process(item);      // not implemented
            }
        }

        void producer()
        {
            size_t item = 0;
            while (true)
            {
                ++item;
                produce.wait();     // wait for room in the storage

                    // mutex lock the queue with multiple consumers
                itemQueue.push(item);

                consume.notify();   // notify the consumer
            }
        }
        int main()
        {
            thread cons(consumer);
            thread prod(producer);

            cons.join();            // ending the threads not implemented
            prod.join();
        }

## FILES

bobcat/semaphore - defines the class interface

## SEE ALSO

bobcat(7)

## 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).