Nfc Reader And Writer Apk

[Elva Stuller]

Im looking for a good reader/writer lock in C++. We have a use case of a single infrequent writer and many frequent readers and would like to optimize for this. Preferable I would like a cross-platform solution, however a Windows only one would be acceptable.

Using standard pre-tested, pre-built stuff is always good (for example, Boost as another answer suggested), but this is something that's not too hard to build yourself. Here's a dumb little implementation pulled out from a project of mine:

pthreads not really being Windows-native, but the general idea is here. This implementation is slightly biased towards writers (a horde of writers can starve readers indefinitely); just modify writer_unlock if you'd rather the balance be the other way around.

Greg Rogers pointed out that the POSIX standard does specify pthread_rwlock_*. This doesn't help if you don't have pthreads, but it stirred my mind into remembering: Pthreads-w32 should work! Instead of porting this code to non-pthreads for your own use, just use Pthreads-w32 on Windows, and native pthreads everywhere else.

Edit: The MSDN Magazine link isn't available anymore. The CodeProject article is now available on -reader-writer-locks and sums it up pretty nicely. Also I found a new MSDN link about Compound Synchronisation Objects.

There is an article about reader-writer locks on MSDN that presents some implementations of them. It also introduces the Slim reader/writer lock, a kernel synchronisation primitive introduced with Vista. There's also a CodeProject article about comparing different implementations (including the MSDN article's ones).

They have a spin_rw_mutex for very short periods of contention and a queueing_rw_mutex for longer periods of contention. The former can be used in particularly performance sensitive code. The latter is more comparable in performance to that provided by Boost.Thread or directly using pthreads. But profile to make sure which one is a win for your access patterns.

Boost.Thread has since release 1.35.0 already supports reader-writer locks. The good thing about this is that the implementation is greatly cross-platform, peer-reviewed, and is actually a reference implementation for the upcoming C++0x standard.

Yes it's in Java, but you can easily read and transpose it to C++, even if you don't know any Java. The documentation I linked to contains all the behavioral properties of this implementation so you can make sure it does what you want.

I have a set of data structures I need to protect with a readers/writer lock. I am aware of boost::shared_lock, but I would like to have a custom implementation using std::mutex, std::condition_variable and/or std::atomic so that I can better understand how it works (and tweak it later).

I'm writing scientific code and can generally avoid data races; this lock is mostly a safeguard against the mistakes I'll probably make later. Thus my priority is low read overhead so I don't hamper a correctly-running program too much. Each thread will probably run on its own CPU core.

Could you please show me (pseudocode is ok) a readers/writer lock? What I have now is supposed to be the variant that prevents writer starvation. My main problem so far has been the gap in read_lock between checking if a read is safe to actually incrementing a reader count, after which write_lock knows to wait.

Here's pseudo-code for a ver simply reader/writer lock using a mutex and a condition variable. The mutex API should be self-explanatory. Condition variables are assumed to have a member wait(Mutex&) which (atomically!) drops the mutex and waits for the condition to be signaled. The condition is signaled with either signal() which wakes up one waiter, or signal_all() which wakes up all waiters.

If most of the waiters are waiting for a write lock, this is wastefull - most waiters will fail to acquire the lock, after all, and resume waiting. Simply using signal() doesn't work, because you do want to wake up everyone waiting for a read lock unlocking. So to fix that, you need separate condition variables for readability and writability.

You can fix that by tracking the number of pending read and write locks, and either stop acquiring read locks once there a pending write locks (though you'll then starve readers!), or randomly waking up either all readers or one writer (assuming you use separate condition variable, see section above).

To guarantee this, you'll need a real wait queue. You could e.g. create one condition variable for each waiter, and signal all readers or a single writer, both at the head of the queue, after releasing the lock.

This one is hard to fix. One way is to use atomic instructions to acquire read or write locks (usually compare-and-exchange). If the acquisition fails because the lock is taken, you'll have to fall back to the mutex. Doing that correctly is quite hard, though. Plus, there'll still be contention - atomic instructions are far from free, especially on machines with lots of cores.

Implementing synchronization primitives correctly is hard. Implementing efficient and fair synchronization primitives is even harder. And it hardly ever pays off. pthreads on linux, e.g. contains a reader/writer lock which uses a combination of futexes and atomic instructions, and which thus probably outperforms anything you can come up with in a few days of work.

What unites both groups is a sense that the promises made at the beginning of the show have not yet been fulfilled. Whether that leads to disappointment or a desire for more so that the promises can be fulfilled (a Jane Austen heroine always gets her man!), it teaches an important principle of writing:

Audiences have expectations about what makes a satisfying conclusion or resolution for any given story, and these expectations are based on implied promises made by the writers of stories.

If you went to the movie theater to watch a romantic comedy and half-way through it transformed into a zombie film, you would probably walk out, unless there had been clues planted at the beginning of the film that a zombie film was coming.

Read or watch the rest of the story, and then evaluate how it met your expectations. Was the reader-writer contract kept? Was the story effective for you as a reader/viewer? Did anything defy what was established in the reader-writer contract, and if so, was it effective?

Exercise 2: Create a template Reader-Writer Contract that works for an entire category, genre, or subgenre of writing. Make sure that this category, genre, or subgenre is one that you write or would like to write.

Slim reader/writer (SRW) locks enable the threads of a single process to access shared resources; they are optimized for speed and occupy very little memory. Slim reader-writer locks cannot be shared across processes.

Reader threads read data from a shared resource whereas writer threads write data to a shared resource. When multiple threads are reading and writing using a shared resource, exclusive locks such as a critical section or mutex can become a bottleneck if the reader threads run continuously but write operations are rare.

Shared mode, which grants shared read-only access to multiple reader threads, which enables them to read data from the shared resource concurrently. If read operations exceed write operations, this concurrency increases performance and throughput compared to critical sections.

Exclusive mode, which grants read/write access to one writer thread at a time. When the lock has been acquired in exclusive mode, no other thread can access the shared resource until the writer releases the lock.

Exclusive mode SRW locks cannot be acquired recursively. If a thread tries to acquire a lock that it already holds, that attempt will fail (for TryAcquireSRWLockExclusive) or deadlock (for AcquireSRWLockExclusive)

A single SRW lock can be acquired in either mode; reader threads can acquire it in shared mode whereas writer threads can acquire it in exclusive mode. There is no guarantee about the order in which threads that request ownership will be granted ownership; SRW locks are neither fair nor FIFO.

An SRW lock is the size of a pointer. The advantage is that it is fast to update the lock state. The disadvantage is that very little state information can be stored, so SRW locks do not detect incorrect recursive use in shared mode. In addition, a thread that owns an SRW lock in shared mode cannot upgrade its ownership of the lock to exclusive mode.

The caller must allocate an SRWLOCK structure and initialize it by either calling InitializeSRWLock (to initialize the structure dynamically) or assign the constant SRWLOCK_INIT to the structure variable (to initialize the structure statically).

The idea is that the connection_loop function is the only one that'll read from the stream, and the hasher futures are the only ones that'll write to the stream (though there may be several hasher future instances). I assumed the lifetimes are broken (the BufReader/Writer objects may outlive stream), and indeed I get an error:

I tried to use the pattern used in TODO: Collected Small Patterns - Async programming in Rust with async-std which essentially does that, but uses one extra level of indirection. When I did that I ended up running into lifetime issues; details are fuzzy since it was a while back -- but I recall thinking that the split() function seems to "consume" stream so that only the reader and writers remained, which solved the lifetime issues.

I haven't actually looked at the implementation, but somehow I imagine that when split() is used in tokio it stores the underlying socket handle reference counted in the reader and writer, and once both of them are closed the socket handle is actually released.

One option is to convert the TcpStream into the std-lib version, call try_clone, then convert both back. You will get unpredictable results if you try writing or reading to both at the same time though.

3a8082e126