Synchronization techniques among threads

When you create code that is threadsafe but still benefits from sharing data or resources between threads, the most important aspect of programming becomes the ability to synchronize threads.

Synchronization is the cooperative act of two or more threads that ensures that each thread reaches a known point of operation in relationship to other threads before continuing. Attempting to share resources without correctly using synchronization is the most common cause of damage to application data.

Typically, synchronizing two threads involves the use of one or more synchronization primitives. Synchronization primitives are low-level functions or application objects (not i5/OS™ objects) that your application used or created to provide the synchronization behavior the application requires.

These concepts pertain to all programming languages. To determine how each language enables these concepts, refer to the Language topic in the information center for the specific language.

• Mutexes and threads
  A mutual exclusion (mutex) is used cooperatively between threads to ensure that only one of the cooperating threads is allowed to access the data or run certain application code at a time.
• Semaphores and threads
  Semaphores (sometimes referred to as counting semaphores) can be used to control access to shared resources. A semaphore can be thought of as an intelligent counter. Every semaphore has a current count, which is greater than or equal to 0.
• Condition variables and threads
  Condition variables allow threads to wait for certain events or conditions to occur and they notify other threads that are also waiting for the same events or conditions.
• Threads as synchronization primitives
  Threads themselves can be used as synchronization primitives when one thread specifically waits for another thread to complete.
• Space location locks
  A space location lock puts a logical lock on any single byte of storage. The lock does not change the storage or effect your application's access to the storage. The lock is a piece of information recorded by the system.
• Object locks
  Object locks provide ways to acquire locks on specific system or application objects.
• Compare and Swap
  You can use the Machine Interface's (MI) Compare and Swap (CMPSWP) instruction to access data in a multithreaded program.