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Read-write locks are another way to achieve synchronization between threads. Similar to mutexes, read-write locks divide operations into read and write modes, and multiple threads can occupy read-write locks at the same time, which makes read-write locks have higher parallelism.

The characteristics of read-write lock are: write monopoly, read sharing, and high priority of write lock. For read-write locks, mastering these 12 words is enough.

In a Linux environment, read-write locks have the following three states:

Locked state in read mode (read lock)

Locked state in write mode (write lock)

Unlocked state

Although the read-write lock has three states: read lock, write lock and unlocked state, it actually has only one lock, not three.

As mentioned earlier, the characteristics of read-write lock are: write monopoly, read sharing, and high priority of write lock. Specifically:

When a read-write lock is locked in write mode, all threads that attempt to lock the lock (whether read or write) will be blocked before unlocking;-- > write exclusive

When a read-write lock is locked in read mode, the thread will succeed if it is locked in read mode; if it is locked in write mode, it will block. -- > read sharing

When a read-write lock is locked in read mode, there are both threads trying to lock in write mode and threads trying to lock in read mode. Then the read-write lock blocks subsequent read-mode lock requests and gives priority to the write-mode lock. -- > write lock has high priority

Read-write locks are also called shared-exclusive locks. When a read-write lock is locked in read mode, it is locked in shared mode; when it is locked in write mode, it is locked in exclusive mode. Write monopoly, read sharing.

Read-write locks are ideal for situations where data structures are read more often than they are written. Because the read lock is shared, this can improve parallelism.

Main application functions:

Pthread_rwlock_init function

Pthread_rwlock_destroy function

Pthread_rwlock_rdlock function

Pthread_rwlock_wrlock function

Pthread_rwlock_tryrdlock function

Pthread_rwlock_trywrlock function

Pthread_rwlock_unlock function

The return values of the above seven functions are: 0 for success, and error number for failure.

Pthread_rwlock_t type: used to define a read-write lock variable, such as: pthread_rwlock_t rwlock

# # pthread_rwlock_init function

Function prototype:

Int pthread_rwlock_init (pthread_rwlock_t restrict rwlock, const pthread_rwlockattr_t restrict attr)

Function: initialize a read-write lock

Parameter description:

Rwlock: outgoing parameter, which should be passed & rwlock to the function when called

Attr: indicates the read-write lock attribute. NULL is usually passed, which means the default attribute is used.

# # pthread_rwlock_destroy function

Function prototype:

Int pthread_rwlock_destroy (pthread_rwlock_t * rwlock)

Function: destroy a read-write lock

# # pthread_rwlock_rdlock function

Function prototype:

Int pthread_rwlock_rdlock (pthread_rwlock_t * rwlock)

Function: request a read-write lock by reading. (often abbreviated as: request read lock)

# # pthread_rwlock_wrlock function

Function prototype:

Int pthread_rwlock_wrlock (pthread_rwlock_t * rwlock)

Function: request a read-write lock in a write manner. (often abbreviated as: request write lock)

# # pthread_rwlock_unlock function

Function prototype:

Int pthread_rwlock_unlock (pthread_rwlock_t * rwlock)

Function function: unlock.

# # pthread_rwlock_tryrdlock function

Function prototype:

Int pthread_rwlock_tryrdlock (pthread_rwlock_t * rwlock)

Function function: non-blocking request read-write lock (non-blocking request read lock)

# # pthread_rwlock_trywrlock function

Function prototype:

Int pthread_rwlock_trywrlock (pthread_rwlock_t * rwlock)

Function function: non-blocking request read-write lock (non-blocking request write lock)

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