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includefile(include/header)
COMMENT(manpage, section, releasedate, archive, short name)
manpage(FBB::Semaphore)(3bobcat)(_CurYrs_)(libbobcat-dev__CurVers_)
(Dijkstra's Semaphore)
manpagename(FBB::Semaphore)
(Implements the Semaphore type designed by Dijkstra)
manpagesynopsis()
bf(#include <bobcat/semaphore>)nl()
Linking option: tt(-lpthread -lbobcat)
manpagedescription()
According to tt(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 tt(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.
includefile(include/namespace)
manpagesection(INHERITS FROM)
-
manpagesection(CONSTRUCTORS)
itemization(
itb(Semaphore(size_t nAvailable))
The constructor defines the semaphore's initial state. With a counting
semaphore tt(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 tt(wait) member waits
until its value is incremented by another thread, calling one of the
semaphore's tt(notify) members.
)
Copy and move constructors (and assignment operators) are not available.
manpagesection(MEMBER FUNCTIONS)
itemization(
itb(void notify())
The internally maintained tt(available) value is incremented and one
waiting thread (cf. the tt(wait) members below) is notified,
reactivating that thread.
itb(void notify_all())
The internally maintained tt(available) value is incremented and all
waiting threads (cf. the tt(wait) members below) are notified. Only
one waiting thread will be able to obtain the semaphore's lock and to
reduce tt(available), and that particular thread is thereupon
reactivated.
itb(void set(size_t available))
This member blocks until it has obtained the lock of the tt(std::mutex)
which is maintained internally by the tt(Semaphore) object. Next the
tt(Semaphore's available) value receives the value of the member's
argument, and the lock is released.
itb(size_t size() const)
Without trying to lock the tt(Semaphore) object's tt(mutex) the current
value of the tt(Semaphore's available) value is returned.
itb(void wait())
This member blocks for as long as the internally stored value
(tt(available)) equals zero. When returning from tt(wait) the current
thread holds the lock of the tt(std::mutex) which is maintained
internally by the tt(Semaphore) object. tt(Notify) members are used to
increment tt(available) and to inform tt(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.
itb(bool wait(Fun fun, Params &&...args))
This is a member template, where tt(Fun) is a function (object)
receiving the argument passed to tt(wait), and returning a
tt(bool). This member blocks until it has obtained the tt(Semaphore's)
mutex lock, then, while its tt(available) value equals 0, waits until
being notified. Once it has reacquired the lock after being notified
tt(fun) is called, receiving tt(wait's) perfectly forwarded remaining
arguments. This member returns tt(false) if tt(fun) returns
tt(false). It returns tt(true) if tt(fun) returns tt(true) and
tt(available) was unequal zero following tt(fun) returning
tt(true). The function may therefore perform tasks outside of the
tt(Semaphore) local environment, which might even involve updating the
tt(Semaphore's) tt(available) value.
itb(std::cv_status wait_for(std::chrono::duration<Rep, Period> const
&relTime))
This member blocks for as long as the internally stored value
(tt(available)) equals zero and the amount of time specified by
tt(relTime) hasn't passed. If the latter happens,
tt(std::cv_status::timeout) is returned, otherwise
tt(std::cv_status::no_timeout) is returned, in which case the current
thread holds the lock of the tt(std::mutex) which is maintained
internally by the tt(Semaphore) object. tt(Notify) members are used to
increment tt(available) and to inform tt(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.
itb(std::cv_status wait_until(std::chrono::time_point<Clock, Duration>
const &absTime))
This member blocks for as long as the internally stored value
(tt(available)) equals zero and the time specified by tt(absTime)
hasn't been reached. If the latter happens (or if tt(absTime) lies in
the past) tt(std::cv_status::timeout) is returned, otherwise
tt(std::cv_status::no_timeout) is returned, in which case the current
thread holds the lock of the tt(std::mutex) which is maintained
internally by the tt(Semaphore) object. tt(Notify) members are used to
increment tt(available) and to inform tt(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.
)
manpagesection(EXAMPLE)
verb(
#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();
}
)
manpagefiles()
em(bobcat/semaphore) - defines the class interface
manpageseealso()
bf(bobcat)(7)
manpagebugs()
None Reported.
includefile(include/trailer)
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