Multi-thread----Condition Variables

Wait conditions are available in two ways: unconditional wait pthread_cond_wait () and timed wait pthread_cond_timedwait ()

There are two types of excitation conditions: pthread_cond_signal () activates a thread waiting for the condition, one is activated in the queued order when there are multiple waiting threads, and pthread_cond_broadcast () activates all waiting threads

Before calling Pthread_cond_wait (), the mutex must be Cheng (Pthread_mutex_lock ()), and before the update condition waits for the queue, the mutexes remain locked, and the thread hangs into the wait before unlocking.

The mutex will be re-locked to correspond to the lock action before entering Pthread_cond_wait () before the condition satisfies thereby leaving pthread_cond_wait ()

Lock the condition with a mutex to prevent the same condition from being contested by multiple threads.

The following example creates three threads, sets the global variable I, wakes thread 1 when i==1, wakes thread 2 when i==2, and wakes up thread 3 when i==3.

#include <stdio.h>#include <unistd.h>#include <pthread.h>int i =-1;p thread_mutex_t Mutex =pthread_mutex_initializer;pthread_cond_t condition =Pthread_cond_initializer;voidThread1 () {printf ("Therad1 begin\n"); Pthread_mutex_lock (&Mutex);while (i! =1) {printf ("Therad1 waiting\n"); Pthread_cond_wait (&condition, &Mutex); } printf ("Thread1 doing\n"); Pthread_mutex_unlock (&Mutex); printf"Thread1 end\n");}voidThread2 () {printf ("Therad2 begin\n"); Pthread_mutex_lock (&Mutex);while (i! =2) {printf ("Therad2 waiting\n"); Pthread_cond_wait (&condition, &Mutex); } printf ("Thread2 doing\n"); Pthread_mutex_unlock (&Mutex); printf"Thread2 end\n");}voidThread3 () {printf ("Therad3 begin\n"); Pthread_mutex_lock (&Mutex);while (i! =3) {printf ("Therad3 waiting\n"); Pthread_cond_wait (&condition, &Mutex); } printf ("Thread3 doing\n"); Pthread_mutex_unlock (&Mutex); printf"Thread3 end\n");}int main (int argc,Constchar *Argv[]) {pthread_t tid1; pthread_t tid2; pthread_t tid3;void *t1 =Thread1;void *t2 =Thread2;void *t3 = thread3; pthread_create (&tid1, NULL, T1, NULL); Pthread_create (&tid2, NULL, t2, NULL); Pthread_create (&tid3, NULL, T3, NULL); While (i! = 3) {i++; printf ("i =%d\n", i); Sleep (3); Pthread_cond_broadcast (&condition);} return 0;}                 

Output Result:

0123Thread3 doingThread3 End   

Summary: When the condition is not met, the thread hangs and waits for the condition to be met.

Because of the pthread_cond_broadcast used, the while loop is used to determine if the condition is true.

When the condition is not locked with a mutex:

0123
or/////////////.
i = 0
Therad1 Begin
Therad3 Begin
Therad2 Begin
Therad1 Waiting
Therad3 Waiting
Therad2 Waiting
i = 1
Thread1 doing
Thread1 End
i = 2
i = 3

Multi-thread----Condition Variables