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It's possible to simulate a reader/writer lock using a Mutex and a Semaphore. I wouldn't do it if I had to access it thousands of times per second, but for dozens or perhaps hundreds of times per second, it should work just fine.

I appreciate Jim Mischel's fine answer, but I see opportunity for performance improvement by avoiding Thread.Sleep() and avoiding lock contention when multiple readers try to acquire at the same time!

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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.

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.

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.

You can use futures::io::AsyncReadExt::split with async-std's IO types. That provides owned reader/writer halves which share the underlying IO via a bi-lock (which should be marginally faster than a mutex, though I don't know if it's been benchmarked).

I am relatively new to KNIME and therefore ask you for help in this.

I do want to change the path of every writer/reader node (in my case all excel) based on the user who is executing the workflow.

HI Iris

we have a workflow that uses the excel reader to read from the local file system, and currently, we are having a problem to read data through Webportal after Server update to Version: 4.10.1.0033 and client version 4.1.1.

showing workflow did not fully execute.

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.

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?

Hello! I would like to embed a NFC reader / writer into my desk, and for that purpose I'd like to find an affordable NFC reader/writer that would be fully natively compatible with Linux (and, if possible, for the sensor part to be either really small or easily movable so I can embed it into the wood).

Create an object which operates like a regular writer but maps dictionariesonto output rows. The fieldnames parameter is a sequence of keys that identify the order in which values in thedictionary passed to the writerow() method are written to filef. The optional restval parameter specifies the value to bewritten if the dictionary is missing a key in fieldnames. If thedictionary passed to the writerow() method contains a key not found infieldnames, the optional extrasaction parameter indicates what action totake.If it is set to 'raise', the default value, a ValueErroris raised.If it is set to 'ignore', extra values in the dictionary are ignored.Any other optional or keyword arguments are passed to the underlyingwriter instance.

The Dialect class is a container class whose attributes containinformation for how to handle doublequotes, whitespace, delimiters, etc.Due to the lack of a strict CSV specification, different applicationsproduce subtly different CSV data. Dialect instances define howreader and writer instances behave.

Instructs writer objects to never quote fields. When the currentdelimiter occurs in output data it is preceded by the current escapecharcharacter. If escapechar is not set, the writer will raise Error ifany characters that require escaping are encountered.

To make it easier to specify the format of input and output records, specificformatting parameters are grouped together into dialects. A dialect is asubclass of the Dialect class having a set of specific methods and asingle validate() method. When creating reader orwriter objects, the programmer can specify a string or a subclass ofthe Dialect class as the dialect parameter. In addition to, or insteadof, the dialect parameter, the programmer can also specify individualformatting parameters, which have the same names as the attributes defined belowfor the Dialect class.

A one-character string used by the writer to escape the delimiter if quotingis set to QUOTE_NONE and the quotechar if doublequote isFalse. On reading, the escapechar removes any special meaning fromthe following character. It defaults to None, which disables escaping.

Writer objects (DictWriter instances and objects returned bythe writer() function) have the following public methods. A row must bean iterable of strings or numbers for Writer objects and a dictionarymapping fieldnames to strings or numbers (by passing them through str()first) for DictWriter objects. Note that complex numbers are writtenout surrounded by parens. This may cause some problems for other programs whichread CSV files (assuming they support complex numbers at all).

Consider this another pitfall warning. If you are a frequent user of reader/writer locking (via the ReaderWriterLockSlim class) like I am, you will undoubtedly run into this situation. As more and more code we write these days are asynchronous with the use of async/await, it is easy to end up in the following situation (an oversimplification, but just imagine write locks in there as well):

This, of course, will not work. This is because reader/writer locks, at least the implementation in .NET, are thread-affine. This means the very same thread that acquired a lock must be the one to release it. As soon as you hit an await, you have dispatched the rest of the behavior to some other thread. So this cannot work.

In this solution of the readers/writers problem, the first reader must lock the resource (shared file) if such is available. Once the file is locked from writers, it may be used by many subsequent readers without having them to re-lock it again.

Before entering the critical section, every new reader must go through the entry section. However, there may only be a single reader in the entry section at a time. This is done to avoid race conditions on the readers (in this context, a race condition is a condition in which two or more threads are waking up simultaneously and trying to enter the critical section; without further constraint, the behavior is nondeterministic. E.g. two readers increment the readcount at the same time, and both try to lock the resource, causing one reader to block). To accomplish this, every reader which enters the will lock the for themselves until they are done with it. At this point the readers are not locking the resource. They are only locking the entry section so no other reader can enter it while they are in it. Once the reader is done executing the entry section, it will unlock it by signalling the mutex. Signalling it is equivalent to: mutex.V() in the above code. Same is valid for the . There can be no more than a single reader in the exit section at a time, therefore, every reader must claim and lock the Exit section for themselves before using it.

Once the first reader is in the entry section, it will lock the resource. Doing this will prevent any writers from accessing it. Subsequent readers can just utilize the locked (from writers) resource. The reader to finish last (indicated by the readcount variable) must unlock the resource, thus making it available to writers.

In this solution, every writer must claim the resource individually. This means that a stream of readers can subsequently lock all potential writers out and starve them. This is so, because after the first reader locks the resource, no writer can lock it, before it gets released. And it will only be released by the last reader. Hence, this solution does not satisfy fairness.

In this solution, preference is given to the writers. This is accomplished by forcing every reader to lock and release the readtry semaphore individually. The writers on the other hand don't need to lock it individually. Only the first writer will lock the readtry and then all subsequent writers can simply use the resource as it gets freed by the previous writer. The very last writer must release the readtry semaphore, thus opening the gate for readers to try reading.

No reader can engage in the entry section if the readtry semaphore has been set by a writer previously. The reader must wait for the last writer to unlock the resource and readtry semaphores. On the other hand, if a particular reader has locked the readtry semaphore, this will indicate to any potential concurrent writer that there is a reader in the entry section. So the writer will wait for the reader to release the readtry and then the writer will immediately lock it for itself and all subsequent writers. However, the writer will not be able to access the resource until the current reader has released the resource, which only occurs after the reader is finished with the resource in the critical section. ff782bc1db