Condition Variables

Question 1

What are

pthread Condition Variables?

Answer 1

• Not really a variable
  
  • Does not contain your program’s data
• Works basically as a list of threads
• A means of allowing threads to:
  
  • Wait to be signaled(woken up) by another thread
  • Send a signal to other waiting threads
• Specified by the type:

pthread_cond_t

Question 2

Pthread Conditional Variables:

Important Types

Important Functions

Answer 2

Important Types

• pthread_cond_t
• pthread_mutex_t

Important Functions (parameters omitted)

• Initialize: pthread_cond_init()
• Signal: pthread_cond_signal()
• Wait : pthread_cond_wait()

Question 3

pthread Conditional Variables:

Signaling a condition

Answer 3

pthread_cond_signal( pthread_cond_t *myCond);

• This call “wakes up” threads that are suspended and waiting for myCond
• If multiple threads are waiting, only one will be able to unlock it’s mutex and execute
• The selection of which thread gets the lock is arbitrary
• No mutex variable is passed, but the calling thread must have exclusive access to myCond when calling
  
  • Lock the associated mutex variable before calling
  • Unlock mutex variable after calling
• The calling thread is not suspended

Question 4

Problems with

Basic Threads

Answer 4

• The process of creating a thread to perform a task, then joining with it(to synchronize completion) is ok for some problems, but too restrictive for others
• Creating a thread is too expensive

Question 5

How pthread is

different from basic threads

Answer 5

Basic threads are created as needed.

• With pthread, we create a set of threads all at once.
• The threads are kept in a pool, ready to be allocated to tasks.
• This minimizes overhead.
• This is similar to the way the OS allocates processors to processes

Question 6

pthread

Conditional Variable Type:

Overview and Use

Answer 6

pthread_cond_t

• Basically a list of threads waiting to be signaled(woken up)
• Declare it as a global variable, or
  
  • allocate it on the heap
• Must be shareable by all threads
• Must initialize before use:

pthread_cond_init(

pthread_cond_t* someCondVar);

Question 7

Relationship between

Conditional Variables

and

Mutexes

Answer 7

• In most cases, you should protect access to a conditional variable, and the shared data associated with it
• This is accomplished by the use of a

pthread_mutex_t

• A thread should not be able to view or modify your shared data or condition variable unless it has exclusive access to it.
• The mutex protects access to the conditional variable

Question 8

pthread Conditional Variables:

Waiting for a Condition

Answer 8

pthread_cond_wait( pthread_cond_t* myCond,

pthread_mutex_t* mutex);

• Adds the calling thread to the list of threads waiting for myCond
• Suspends calling thread until “myCond” signaled
• Associated mutex should already be locked
  
  • a function unlocks it after adding thread to myCond and suspending thread
• When the thread returns from waiting, the mutex will be locked for the thread again.