c - 互斥线程 - 代码似乎无法正确退出

Question

所以我这里有代码可以创建两种线程。一种“生产”数据，另一种“消费”数据。任何一次只能存在一定数量的数据，因此一旦创建了一定数量的数据(即当sharedData = BUFFER时)，生产者就会暂停生产，而当sharedData = 0时，消费者将暂停生产. 也只能制作这么多数据(存储在 dataleft 中的数量)，一旦制作并消耗了所有数据，程序就应该结束。

出于某种原因，代码末尾的 printf() 行似乎从未触发。我无法判断线程是否因此正确关闭。感觉好像我做了一件非常愚蠢的事情，但我看不到问题所在。

开头的几个定义:

#define                 NUMCONSUMERS    4
#define                 NUMPRODUCERS    4
#define                 PACKETS         10
#define                 tryMainlock     pthread_mutex_trylock(&dataMutex)
#define                 openMainlock    pthread_mutex_lock(&dataMutex)
#define                 closeMainlock   pthread_mutex_unlock(&dataMutex)
#define                 waitMainlock    pthread_cond_wait(&dataPresentCondition, &dataMutex);
#define                 signalMainlock  pthread_cond_signal(&dataPresentCondition);

#define                 trydatalock     pthread_mutex_trylock(&IsthereDataleft)
#define                 opendatalock    pthread_mutex_lock(&IsthereDataleft)
#define                 closedatalock   pthread_mutex_unlock(&IsthereDataleft)

pthread_mutex_t         dataMutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t          dataPresentCondition = PTHREAD_COND_INITIALIZER;

pthread_mutex_t         IsthereDataleft = PTHREAD_MUTEX_INITIALIZER;

int                     sharedData=0;   //amount of data present
int                     BUFFER = 5;
int                     dataleft=PACKETS;

主要功能:

int main(int argc, char **argv)
{
int rc;                                 
int i;
pthread_t  consumer[NUMCONSUMERS];
pthread_t  producer[NUMPRODUCERS];

rc = opendatalock;                      //lock to determine whether there's any point waiting for data

for (i=0; i <NUMPRODUCERS; i++) {       //Build up the producers
    rc = pthread_create(&producer[i], NULL, Producer, (void *)i);
    if (rc)
        printf("Error building Producer Thread: %x\n", i);
}

for (i=0; i <NUMCONSUMERS; i++) {       //Build up the consumers
    rc = pthread_create(&consumer[i], NULL, Consumer, (void *)i);
    if (rc)
        printf("Error building Consumer Thread: %x\n", i);
}

printf("All Producers and Consumers created\n");

for (i=0; i <NUMPRODUCERS; i++) {       //Join up the producers
    rc = pthread_join(producer[i], NULL);
    if (rc)
        printf("Error: Producer %x: Failed to join\n", i);
}

rc = closedatalock;                     //producers finished, no data left to make

printf("datalock closed, consumers finishing...\n");
for (i=0; i <NUMCONSUMERS; i++) {       //Join up the consumers
    rc = pthread_join(consumer[i], NULL);
    if (rc)
        printf("Error: Consumer %x: Failed to join\n", i);
}
rc = pthread_mutex_destroy(&dataMutex);
rc = pthread_cond_destroy(&dataPresentCondition);
rc = pthread_mutex_destroy(&IsthereDataleft);

printf("All Threads finished. Exiting....\n");
return 0;
}

消费者线程:

void *Consumer(void *threadid){
int rc;
printf("Consumer Thread %x: Created\n", (int)threadid);
while (1)
{
    printf("Consumer %x: Entering Loop\n", (int)threadid);
    rc = openMainlock;      //take hold of main lock
    if (rc)
    {
        printf("Consumer %x: Waiting...\n", (int)threadid);
        rc = waitMainlock;  //if main lock is taken, wait
        if (rc)             //if wait fails, exit the thread.
        {
            printf("Consumer Thread %x: wait for Main Lock failed\n", threadid);
            exit(0);
        }
    }

    while (sharedData == 0) //if the buffer is empty
    {
        rc = trydatalock;
        if (!rc)
        {
            printf("Consumer %x: Completed. Exiting...\n");
            exit(0);
        }
        rc = closeMainlock;
        if (rc)
        {
            printf("code.\n");
        }
        rc = waitMainlock;
        if (rc)
        {
            printf("code.\n");
        }
    }
    sharedData--;
    rc = closeMainlock;
    rc = signalMainlock;
    if (rc)
        {
            printf("code.\n");
        }
    printf("Consumer %x: Releasing Lock\n", (int)threadid);         
}
}

和生产者线程:

void *Producer(void *threadid){
int rc;
printf("Producer Thread %x: Created\n", (int)threadid);
while (1)
{
    printf("Producer %x: Entering Loop\n", (int)threadid);
    rc = openMainlock;          //take hold of the lock
    if (rc)                     //if lock is currently being used by a consumer or a producer
    {   
        printf("Producer %x: Waiting...\n", (int)threadid);
        rc = waitMainlock;      //wait here until lock is released
        if (rc)
        {
            printf("Producer Thread %x: wait for Main Lock failed\n", threadid);
            exit(0);
        }
    }
    if (!dataleft)              //If there's no data left to add to the stream, close the thread
    {
        printf("Producer Thread %x: Completed, exiting...\n", (int)threadid);
        exit(0);
    }
    while (sharedData >=BUFFER)
    {
        rc = closeMainlock;
        if (rc)
        {
            printf("code.\n");
        }
        rc = waitMainlock;
        if (rc)
        {
            printf("code.\n");
        }
    }
    printf("Producer %x: Lock Acquired\n", (int)threadid);
    sharedData++;
    dataleft--;
    rc = closeMainlock;
    rc = signalMainlock;
    if (rc)
        {
            printf("code.\n");
        }
    printf("Producer %x: Releasing Lock\n", (int)threadid);
}
}

Answer 1

查看这段代码:

    if (!rc)
    {
        printf("Consumer %x: Completed. Exiting...\n");
        exit(0);
    }

如果消费者已完成，则进程(!)将终止。您需要使用 pthread_exit() 来代替，或者只是从线程函数返回。

然后还有

../nptl/pthread_mutex_lock.c:80:
    __pthread_mutex_lock: Assertion `mutex->__data.__owner == 0' failed.

我运行了几次代码。这可能是由于例如双重解锁或其他一些无效使用。我将从清理奇怪的宏开始，以便您可以自由地了解程序本身的逻辑。

此外，关于互斥锁的一个重要建议:始终准确记录哪些数据应受互斥锁保护。关键是，它并不总是很清楚，如果犯了这个错误，意味着您意外地访问了没有同步的数据。为了使这一点非常清楚，请使用如下结构:

 struct {
     pthread_mutex_t mutex;
     pthread_cond_t cond;
     int data;
 } synced_data = {
     PTHREAD_MUTEX_INITIALIZER,
     PTHREAD_COND_INITIALIZER,
     0
 };

实际上，文档的重要性不仅仅与共享数据有关。考虑一下 IsthereDataleft 的例子:这是一个互斥体，但它不保护任何东西，对吧？相反，它用于向启动的线程发出信号，表明没有什么可做的，对吧？记录下来不仅可以帮助其他人理解您的代码，还可以确保您了解自己的意图是什么。有时，当你试图解释它时，你会发现自己有些事情没有道理。