Condition Variables
Allow threads to wait for arbitrary conditions to become true
Each CV corresponds to a particular condition that is of interest to an application
Example
cond_t nonempty→count > 0(there are items in the buffer)cond_t nonfull→count < BUFFER_SIZE(there is free space in the buffer)
When condition is not true, thread can cond_wait until condition becomes true
- Unlock the mutex, so not busy waiting
When another thread detects that condition is true, usecond_signalorcond_broadcastto notify blocked threads
Always recheck conditions on wake up
while (count == 0)
cond_wait(&nonempty, &mutex)
Do not use if
- Condition may change
Why must
cond_wait both release mutex and sleep?Will Deadlock if sleep with locked mutex
Why not separate mutexes and CV?
Can end up stuck waiting when bad interleaving
- Another thread acquires the same lock and signals
PThread CV API
- Use Mutex to implement
- Each
cond_waittakes a mutex- To lock and check condition when awake
- Each