---

Read/Write Locks

---

• Read/Write Locks: Theory
  • A read/write lock variable has 3 values (states)
    • Unlocked
    • Read Locked
    • Write Locked

  • A read/write lock is a synchronization object with 3 operations:
    • Read Lock
      • If the read/write lock is in the unlocked state, the read lock will complete (and the thread continues with the next instruction following the read lock command).

        The value (state) of the read/write lock is changed to read locked

      • If the read/write lock is in the read locked state, the thread that executes the read lock command complete (and the thread continues with the next instruction following the read lock command).

        The value (state) of the read/write lock remains read locked, but a count is increased (so we know how many times a read lock operation has been performed)

      • If the read/write lock is in the write locked state, the thread that executes the read lock command will block (it stops execution) until the value (state) of the read/write lock becomes unlocked

        (When the state of the read/write lock does become unlocked, the read lock command will complete and change the state of the read/write lock to read locked)

    • Write Lock
      • If the read/write lock is in the unlocked state, the write lock will complete (and the thread continues with the next instruction following the write lock command).

        The value (state) of the read/write lock is changed to write locked

      • If the read/write lock is in the read locked state, the thread that executes the write lock command will block (it stops execution) until the value (state) of the read/write lock becomes unlocked

        (When the state of the read/write lock does become unlocked, the write lock command will complete and change the state of the read/write lock to write locked)

      • If the read/write lock is in the write locked state, the thread that executes the write lock command will block (it stops execution) until the value (state) of the read/write lock becomes unlocked

        (When the state of the read/write lock does become unlocked, the write lock command will complete and change the state of the read/write lock to write locked)

  • Difference between mutex and read/write locks:
    • Read/write locks is more distinguishing than mutex locks, read/write locks can allow 2 read operations to a shared variable to proceed concurrently

• Read/Write Locks in PThreads
  • Defining a read/write lock variable in Pthreads:

    pthread_rwlock_t x;

  • Initializing a read/write lock variable:

    After defining the read/write lock variable, you must initialized it using the following function:

    int pthread_rwlock_init(pthread_rwlock_t *rwlock, pthread_rwlockattr_t *attr );

    • rwlock: is the read/write lock that you want to initialize (pass the address !)

    • attr: is the set of initial property of the read/write lock.

    The most common read/write lock is one where the lock is initially in the unlock.

    This kind of mutex lock is created using the (default) attribute null:

    Example: a read/write lock with an initial unlock state pthread_rwlock_init(&x, NULL); /* Default initialization */

  • Read lock a read/write lock:

    int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);

    Example: pthread_rwlock_t x; pthread_rwlock_init(&x, NULL); ... pthread_rwlock_rdlock(&x);

    • NOTE: if a thread already hold a write lock on a read/write lock, and performs a pthread_rwlock_rdlock() on that lock, then the outcome if undefined (in order words: do NOT try !)

    • NOTE: A thread may hold multiple concurrent read locks on a read/write lock (that is, successfully call the pthread_rwlock_rdlock() function n times). If so, the thread must perform matching unlocks (that is, it must call the pthread_rwlock_unlock() function n times).

  • Write lock a read/write lock:

    int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);

    Example: pthread_rwlock_t x; pthread_rwlock_init(&x, NULL); ... pthread_rwlock_wrlock(&x);

    • NOTE: if a thread already hold a read lock or write lock on a read/write lock, and performs a pthread_rwlock_wrlock() on that lock, then the outcome if undefined (in order words: do NOT try !)