What is read/write lock in C?

In many situations, data is read more often than it is modified or written. In these cases, you can allow threads to read concurrently while holding the lock and allow only one thread to hold the lock when data is modified. A multiple-reader single-writer lock (or read/write lock) does this.

What is a reader/writer lock and when is it useful?

A readers/writer lock regulates access to a set of data. The readers/writer lock is so called because many threads can hold the lock simultaneously for reading, but only one thread can hold the lock for writing. Most device drivers do not use readers/writer locks. These locks are slower than mutexes.

What is @synchronized Objective C?

@synchronized is a control construct in Objective-C. It takes an object pointer as a parameter and is followed by a block of code. The object pointer acts as a lock, and only one thread is permitted within a @synchronized block with that object pointer at any given time.

What is read write locks?

A ReadWriteLock maintains a pair of associated locks , one for read-only operations and one for writing. The read lock may be held simultaneously by multiple reader threads, so long as there are no writers. The write lock is exclusive.

What is a reader writer?

Your job as a reader/writer is to assist the student to overcome their learning difficulty during an exam. You may be reading the activity brief or questions for the student, or writing for the student, or reading and writing for the student.

How is reader/writer lock different from normal lock?

An RW lock allows concurrent access for read-only operations, while write operations require exclusive access. This means that multiple threads can read the data in parallel but an exclusive lock is needed for writing or modifying data.

What is Readlock?

When a transaction reads a row, the isolation level of the transaction determines if a read lock is acquired. Once a row is read locked, no other transaction can obtain a write lock on it. Acquiring a read lock ensures that a different transaction does not modify or delete a row while it is being read.

What are mutexes used for?

Mutex or Mutual Exclusion Object is used to give access to a resource to only one process at a time. The mutex object allows all the processes to use the same resource but at a time, only one process is allowed to use the resource. Mutex uses the lock-based technique to handle the critical section problem.

Is UIKit thread-safe?

As we can see, most of the components in UIKit is described as nonatomic `, this means there are not thread safe.

Is Nsdictionary thread-safe?

In general, the collection classes (for example, NSMutableArray , NSMutableDictionary ) are not thread-safe when mutations are concerned. That is, if one or more threads are changing the same array, problems can occur.

How do you solve a reader writer problem?

Let’s understand with an example – If two or more than two readers want to access the file at the same point in time there will be no problem….Code for Reader Process

1. static int readcount = 0;
2. wait (mutex);
3. readcount ++; // on each entry of reader increment readcount.
4. if (readcount == 1)
5. {
6. wait (write);
7. }
8. signal(mutex);

Is reader/writer a problem?

The readers-writers problem is used to manage synchronization so that there are no problems with the object data. For example – If two readers access the object at the same time there is no problem. However if two writers or a reader and writer access the object at the same time, there may be problems.

What is the relation between reader and writer?

The writer provides valid information (proposes an idea). The reader accepts that information (accesses and understands information) and responds with active participation.

How reader/writer lock differs from a normal lock?

Is read lock needed?

depends on how you use and read it. if your read is atomic (i.e, won’t be interrupted by write) and the read thread does not have dependency with the write threads, then you maybe able to skip read lock. But if your ‘read’ operation takes some time and takes heavy object interation, then you should lock it for read.

What is the difference between semaphore and lock?

Lock vs Semaphore Locks cannot be shared between more than one thread processes but semaphores can have multiple processes of the same thread. Only one thread works with the entire buffer at a given instance of time but semaphores can work on different buffers at a given time.

What are semaphores and mutexes?

A mutex object allows multiple process threads to access a single shared resource but only one at a time. On the other hand, semaphore allows multiple process threads to access the finite instance of the resource until available. In mutex, the lock can be acquired and released by the same process at a time.

What is UIKit framework?

Overview. The UIKit framework provides the core objects that you need to build apps for iOS and tvOS. You use these objects to display your content onscreen, to interact with that content, and to manage interactions with the system.

What is NSRecursiveLock?

NSRecursiveLock defines a lock that may be acquired multiple times by the same thread without causing a deadlock, a situation where a thread is permanently blocked waiting for itself to relinquish a lock.

Why do we need a reader-writer lock?

Reader-writer locks do not solve the fundamental problem – threads competing for access to a critical region. But reader-writer locks help a lot – to minimize the bottleneck.

How do I stop a read lock from waking up multiple writers?

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).

Is writer_unlock biased towards the writers?

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. Yes, this is C and not C++. Translation is an exercise left to the reader. Greg Rogers pointed out that the POSIX standard does specify pthread_rwlock_*.