#include <stdio.h>
#include <pthread.h>
#define MAX 10 /* maximum iterations */
int number; /* the resource */
pthread_mutex_t mu= PTHREAD_MUTEX_INITIALIZER; /* to protect the resource*/
/*
Condition variable to signal consumer that a new number is available for
consumption.
*/
pthread_cond_t sig_consumer= PTHREAD_COND_INITIALIZER;
/*
Condition variable to signal the producer that
(a) the new number has been consumed,
(b) generate another one.
*/
pthread_cond_t sig_producer= PTHREAD_COND_INITIALIZER;
void *consumer(void *dummy)
{
int printed= 0;
printf("Consumer : \"Hello I am consumer #%ld. Ready to consume numbers"
" now\"\n", pthread_self());
while (1)
{
pthread_mutex_lock(&mu);
/* Signal the producer that the consumer is ready. */
pthread_cond_signal(&sig_producer);
/* Wait for a new number. */
pthread_cond_wait(&sig_consumer, &mu);
/* Consume (print) the number. */
printf("Consumer : %d\n", number);
/* Unlock the mutex. */
pthread_mutex_unlock(&mu);
/*
If the MAX number was the last consumed number, the consumer should
stop.
*/
if (number == MAX)
{
printf("Consumer done.. !!\n");
break;
}
}
}
/**
@func producer
This function is responsible for incrementing the number and signalling the
consumer.
*/
void *producer(void *dummy)
{
printf("Producer : \"Hello I am producer #%ld. Ready to produce numbers"
" now\"\n", pthread_self());
while (1)
{
pthread_mutex_lock(&mu);
number ++;
printf("Producer : %d\n", number);
/*
Signal the consumer that a new number has been generated for its
consumption.
*/
pthread_cond_signal(&sig_consumer);
/*
Now wait for consumer to confirm. Note, expect no confirmation for
consumption of MAX from consumer.
*/
if (number != MAX)
pthread_cond_wait(&sig_producer, &mu);
/* Unlock the mutex. */
pthread_mutex_unlock(&mu);
/* Stop if MAX has been produced. */
if (number == MAX)
{
printf("Producer done.. !!\n");
break;
}
}
}
void main()
{
int rc, i;
pthread_t t[2];
number= 0;
/* Create consumer & producer threads. */
if ((rc= pthread_create(&t[0], NULL, consumer, NULL)))
printf("Error creating the consumer thread..\n");
if ((rc= pthread_create(&t[1], NULL, producer, NULL)))
printf("Error creating the producer thread..\n");
/* Wait for consumer/producer to exit. */
for (i= 0; i < 2; i ++)
pthread_join(t[i], NULL);
printf("Done..\n");
}
问题:如果消费者线程先于生产者线程启动,那么程序会提供预期的结果,但如果生产者先启动,那么消费者将从2号开始消费;消费者无法消费数字1。即使生产者线程首先启动,如何纠正程序,而不引入任何额外的变量或 sleep ?
最佳答案
pthread_cond_t 的问题在于它的名称。尽管名义上是一个“条件”,但它没有状态......特别是,它根本不记得它已被发出信号——如果你认为它可以计算它已经被发出了多少次发出信号,你会失望的(因为你需要一个信号量)。换句话说,如果在发出信号时没有 pthread 等待某个条件,则该信号无效并被遗忘。
“条件”最好被认为是“等待队列”,pthreads 在其中等待某些状态被更新。所以通常你有一些状态,受互斥锁的保护。如果状态不符合 pthread 继续所需的状态,则 pthread 会等待“条件”。当状态更新时,可以发出“条件”信号。当服务员醒来时,它必须检查状态,并决定一切现在是否准备好继续。
当有两个或多个 pthread 等待时,标准允许 pthread_cond_signal()
唤醒一个、两个或多个或全部等待者。互斥体确保等待者对状态的访问是串行的,但是等待者不能(一般来说,特别是由于这个原因)假设自发出信号以来状态没有改变。所以,服务员常见的写法是:
pthread_mutex_lock(&mutex) ;
....
while(...what we need to continue...)
pthread_cond_wait(&cond, &mutex) ;
....
pthread_mutex_unlock(&mutex) ;
这反射(reflect)了国家的重要性,以及“条件”的贡献有多么小。
关于c - 生产者消费者同步11,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/24971109/