IEG 4180 Tutorial 5

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IEG 4180 Tutorial 5

• Prepared by Zero
• (Note Content adopt from LiKins tutorial notes)

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Threads SynchronizationIntroduction

• Sharing data between threads may pose
  synchronization problems
• Data corruption
• Synchronize threads by
• Critical Section
• Kernel Objects
• Mutex
• Semaphore

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Thread SynchronizationCritical Section

• A region of code that can only be accessed by one
  thread only

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Thread SynchronizationCritical Section An
Example

• Critical section can only synchronize threads
  within the same process
• Use kernel objects to synchronize between
  processes
• A critical section object will not be released
  automatically if the holding thread is terminated
  before calling LeaveCriticalSection()

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Thread SynchronizationKernel Objects

• Kernel objects
• Data structures maintained internally by the OS
  kernel
• Can be used for inter-process synchronization
• e.g. Mutex, Semaphore, Socket, Timer,
• Difference between Mutex and Semaphore
• Mutex max. counter value 1
• At most one thread can own it
• Semaphore max. counter value N
• At most N threads can own it

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Thread SynchronizationKernel Objects Object
State

• The counter
• Decrease by one when a thread claims it
• Increase by one when a thread releases it
• Always non-negative and not larger than the max.
  value
• Object state
• A kernel object is in signaled state when its
  counter is larger than zero, otherwise it is in
  non-signaled state
• A thread can only claims signaled kernel objects

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Thread SynchronizationCreating Mutex Semaphore

• Set bInitialOwner to true to claims the object as
  soon as it is being created
• lpName specify the name of Mutex / Semaphore,
  which identify the kernel object across
  difference processes
• Semaphore
• lMaximumCount specify the max. counter value of
  created semaphore
• lInitialCount specify the initial counter value
  of created semaphore

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Thread SynchronizationClaiming Mutex Semaphore

• WaitForSingleObject
• Block until hHandle becomes signaled
• WaitForMultipleObjects
• bWaitAll true
• Block until all objects in lpHandles become
  signaled
• bWaitAll false
• Block until one object in lpHandles becomes
  signaled

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Thread SynchronizationCalling Sequence
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Thread SynchronizationMutex Semaphore -
Examples
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Thread SynchronizationExamples

• void main(void)
• HANDLE handlesnumThreads
• DWORD threadID
• INT i
• for (i0 iltnumThreads i)
• // create the threads
• handlesiCreateThread(0, 0, CountThread,
• NULL, 0, threadID)
• // wait for all threads to finish execution
• WaitForMultipleObjects(numThreads, handles, TRUE,
  INFINITE)
• cout ltlt "Global count " ltlt count ltlt endl