Mutual%20exclusion%20--%20pthreads

1
Mutual exclusion -- pthreads

• Declaration
• pthread_mutex_t StructLock
• Initialization
• pthread_mutex_init(StructLock,NULL)
• Recursive and debugging locks may also be
  available

2
Using pthread mutex

• pthread_mutex_lock(StructLock)
• do whatever needs to be done
• but quickly
• pthread_mutex_unlock(StructLock)

3
Pthread conditionals

• Declaration
• pthread_mutex_t StructTstLck
• pthread_cond_t StructQueue
• Initialization
• pthread_mutex_init(StructTstLck, NULL)
• pthread_cond_init(StructQueue, NULL)
• No alternatives in most implementations

4
Typical conditional entry

• pthread_mutex_lock(StructTstLck)
• See if it is OK to go ahead
• if (not OK)
• pthread_cond_wait(StructQueue,
• StructTstLck)
• pthread_mutex_unlock(StructTstLck)

5
Paranoid conditional entry

• pthread_mutex_lock(StructTstLck)
• do
• See if it is OK to go ahead
• if (not OK)
• pthread_cond_wait(StructQueue,
• StructTstLck)
• while (not OK)
• pthread_mutex_unlock(StructTstLck)

6
Unlocking the cond

• pthread_mutex_lock(StructTstLock)
• Set up stuff for get another thread started
• This may not be required
• pthread_mutex_unlock(StructTstLock)
• pthread_cond_signal(StructQueue)

7
Java

• Mutex with synchronized methods
• Conditionals with wait/notify
• final methods in all objects
• But in Java 5.0,
• java.util.concurrent.locks

8
Atomic read/writes

• When is the following safe?
• X Y
• In Java 32-bits are guaranteed atomic,
• longer read/writes are not
• In 5.0, java.util.concurrent.atomic