C multithreading: What is the advantage of a Read Lock (pthread_rwlock_rdlock) if all threads can access it simultaneously?

Question

I'm noob on this thread issue :\\

I imagine that Write Lock behaves like a while that waits for the confirmation of a global variable. Like:

/* pthread_rwlock_wrlock */

    while (is_blocked) {
        /* waiting */
    }
    is_blocked = true;

    (writing code...)

/* pthread_rwlock_unlock */
    is_blocked = false;

Is that correct? And how Read Lock works? If it is shared, why use?

information:

pthread_rwlock_rdlock() – get a shared read lock

pthread_rwlock_wrlock() – get an exclusive write lock

Answer 1

The purpose of a read lock, is to block writers.

• A reader can get a lock if there are no write locks .
• A writer can get a lock if there are no locks at all .

Neither has a trivial implementation, the one you suggested for
example have a race-condition, and would sometimes fail.

Answer 2

When you read a protected resource you don't want it to change while you read it.

A read locks blocks write locks!

That means that while you have a read lock, someone trying to get a write lock will have to wait until you, and anyone else who has a read lock, are finished reading. Since reading doesn't change the resource it is OK that several readers read concurrently.

Note that the definition of a reading operation in this context is any operation that doesn't change the state of the protected resource. So for the purposes of read/write locks, if the protected resource is a stream (like stdin or a socket), reading from it will change its state and should require a write lock.