Question

编辑1：

FILE            *fp;
pthread_mutex_t demoMutex;
unsigned short globalThreadIndex = 0;

struct serverInfo
{
    unsigned int                 serverId;
    pthread_t                        threadId;
    std :: vector <pthread_t> queue;
};

std :: vector <serverInfo> serverInfoVector;


void * printHello (void* threadId)
{
    pthread_t *my_tid = (pthread_t *)&threadId;
    printf ("\nIn `printHello ()`: thread id %ld\n", pthread_self ());

    ***pthread_mutex_lock (&demoMutex);***

    unsigned int i = 0;
    char               found = false;

    if (serverInfoVector.size () > 0)
    {
        // The following code should be executed when and only when the vector isn't empty.
        ***pthread_cond_wait (&demoConditionVar, &demoMutex);***

        while ((i <= serverInfoVector.size ()) && (found == false))
        {
            if (*my_tid == serverInfoVector [i].threadId)
            {
                found = true;
                break;
            }
            else
                i++;
        }

        if (found == true)
        {
            pthread_t            writeToFile = pthread_self ();
            unsigned short iterate;
            for (iterate = 0; iterate < 10000; iterate++)
            {
                fprintf (fp, " %d %d",  iterate,         4);
                fprintf (fp, " %lu %lu", writeToFile, sizeof (pthread_t));

                if (!serverInfoVector [i].queue.empty ())
                {
                    fprintf (fp, " %c %u", 'A', 1);
                    fprintf (fp, " %lu %lu", serverInfoVector [i].queue.front (), sizeof (pthread_t));
                    serverInfoVector [i].queue.pop_back ();
                }
                fprintf (fp, "\n %lu %u", writeToFile, 1);
            }
        }
        ***pthread_mutex_unlock (&demoMutex);***
        }
    ***pthread_exit (NULL);***
}

void checkServerExists (unsigned int serverNumber, std :: string message)
{
    unsigned int i         = 0;
    char               found = false;

    if (serverInfoVector.size () > 0)
    {
        while ((i <= serverInfoVector.size ()) && (found == false))
        {
            if (serverNumber == serverInfoVector [i].threadId)
            {
                found = true;
                break;
            }
            else
                i++;
        }
    }

    if (found == false)
    {
        pthread_t newThread [2];

        int           returnValue;
        if ((returnValue = pthread_create (&newThread [globalThreadIndex], NULL, printHello, (void*) &newThread [globalThreadIndex])) != 0)
        {
          printf ("\nerror: pthread_create failed with error number %d", returnValue);
        }
        printf ("\nIn checkServerExists ()`: thread id %ld\n", newThread [globalThreadIndex]);

        serverInfo obj;
        obj.serverId  = serverNumber;
        obj.threadId = newThread [globalThreadIndex];
        obj.queue.push_back (newThread [globalThreadIndex]);
        serverInfoVector.push_back (obj);

        // Now, since something has been pushed in the vector, it makes sense to wake up the sleeping thread.
        ***pthread_mutex_lock (&demoMutex)***; 
        // Now, since something has been pushed in the vector, it makes sense to wake up the sleeping thread.
        if (serverInfoVector.size () > 0)
            ***pthread_cond_signal (&demoConditionVar);***

        ***pthread_mutex_unlock(&demoMutex);*** 

        pthread_join (newThread [globalThreadIndex], NULL);
    }
    else
    {
    }
}


int main ()
{
    fp = fopen ("xyz", "w");
    ***pthread_mutex_init (&demoMutex, NULL);
    pthread_cond_t demoConditionVar = PTHREAD_COND_INITIALIZER;***

    checkServerExists (1, "anisha");
    globalThreadIndex++;
    checkServerExists (2, "anisha");

    return 0;
}

此代码已得到改进，问题仍然存在（程序挂起，第二个线程未显示）。

checkServerExists函数（在当前情况下）导致创建一个新线程，并将其存储在数组newThread中。

checkServerExists函数启动一个新线程，

当创建线程时，它将立即调用其函数printHello并在条件变量上被阻塞。

checkServerExists函数然后在全局结构的队列中输入值，设置线程唤醒的信号。

现在，我错过了什么意思？

编辑2：

FILE            *fp;
pthread_mutex_t demoMutex;
unsigned short globalThreadIndex = 0;

struct serverInfo
{
    unsigned int                 serverId;
    pthread_t                        threadId;
    std :: vector <pthread_t> queue;
};

std :: vector <serverInfo> serverInfoVector;


void * printHello (void* threadId)
{
    pthread_t *my_tid = (pthread_t *)&threadId;
    printf ("\nIn `printHello ()`: thread id %ld\n", pthread_self ());

    ***pthread_mutex_lock (&demoMutex);***

    unsigned int i = 0;
    char               found = false;

    if (serverInfoVector.size () > 0)
    {
        // The following code should be executed when and only when the vector isn't empty.
        ***pthread_cond_wait (&demoConditionVar, &demoMutex);***

        while ((i <= serverInfoVector.size ()) && (found == false))
        {
            if (*my_tid == serverInfoVector [i].threadId)
            {
                found = true;
                break;
            }
            else
                i++;
        }

        if (found == true)
        {
            pthread_t            writeToFile = pthread_self ();
            unsigned short iterate;
            for (iterate = 0; iterate < 10000; iterate++)
            {
                fprintf (fp, " %d %d",  iterate,         4);
                fprintf (fp, " %lu %lu", writeToFile, sizeof (pthread_t));

                if (!serverInfoVector [i].queue.empty ())
                {
                    fprintf (fp, " %c %u", 'A', 1);
                    fprintf (fp, " %lu %lu", serverInfoVector [i].queue.front (), sizeof (pthread_t));
                    serverInfoVector [i].queue.pop_back ();
                }
                fprintf (fp, "\n %lu %u", writeToFile, 1);
            }
        }
        ***pthread_mutex_unlock (&demoMutex);***
        }
    ***pthread_exit (NULL);***
}

void checkServerExists (unsigned int serverNumber, std :: string message)
{
    unsigned int i         = 0;
    char               found = false;

    ***pthread_mutex_lock (&demoMutex);*** 

    if (serverInfoVector.size () > 0)
    {
        while ((i <= serverInfoVector.size ()) && (found == false))
        {
            if (serverNumber == serverInfoVector [i].threadId)
            {
                found = true;
                break;
            }
            else
                i++;
        }
    }

    if (found == false)
    {
        pthread_t newThread [2];

        int           returnValue;
        if ((returnValue = pthread_create (&newThread [globalThreadIndex], NULL, printHello, (void*) &newThread [globalThreadIndex])) != 0)
        {
              printf ("\nerror: pthread_create failed with error number %d", returnValue);
        }
        printf ("\nIn checkServerExists ()`: thread id %ld\n", newThread [globalThreadIndex]);

        serverInfo obj;
        obj.serverId  = serverNumber;
        obj.threadId = newThread [globalThreadIndex];
        obj.queue.push_back (newThread [globalThreadIndex]);
        serverInfoVector.push_back (obj);

        // Now, since something has been pushed in the vector, it makes sense to wake up the sleeping thread.

        // Now, since something has been pushed in the vector, it makes sense to wake up the sleeping thread.
        if (serverInfoVector.size () > 0)
            ***pthread_cond_signal (&demoConditionVar);***


        pthread_join (newThread [globalThreadIndex], NULL);
    }
    else
    {
    }
        ***pthread_mutex_unlock(&demoMutex);*** 
}


int main ()
{
    fp = fopen ("xyz", "w");
    ***pthread_mutex_init (&demoMutex, NULL);
    pthread_cond_t demoConditionVar = PTHREAD_COND_INITIALIZER;***

    checkServerExists (1, "anisha");
    globalThreadIndex++;
    checkServerExists (2, "anisha");

    return 0;
}

在此编辑中，我将锁定放在checkServerExists函数的顶部（此函数处理全局结构serverInfoVector）

还挂着。：DOH：

Answer 1

此代码已损坏。

首先，您要修改checkServerExists内的变量而不锁定互斥锁。这是未定义的行为。

如果你解决了这个问题，那么你也没有在printHello函数之外发信号通知你的条件变量。因此，一旦线程在pthread_cond_wait调用中被阻塞，它将仅由于虚假唤醒而被唤醒，并且当另一个printHello线程发出信号时它将被唤醒。您应该在设置pthread_cond_signal标记的位置调用condition，而不是printHello。

条件变量只是一种通知机制。您需要将谓词与它相关联，这是等待的条件（在您的情况下，condition!=0）。您必须确保在设置和测试条件时访问的变量受互斥锁保护，并且此互斥锁是传递给pthread_cond_wait的互斥锁，以避免潜在的竞争条件。当您设置变量以指示睡眠线程应该唤醒时，您调用pthread_cond_signal。

我已稍微修改了您的代码以使其正常工作。特别是，我将循环置于pthread_cond_wait调用之前，并在调用pthread_join之前解锁了互斥锁，以便printHello线程可以获取互斥锁并继续。永远不应该在线程连接中持有互斥锁。这段代码仍然可以大大改进 - 除此之外，它不是例外安全。

#include <pthread.h>
#include <stdio.h>
#include <vector>
#include <string>
FILE            *fp;
pthread_mutex_t demoMutex;
pthread_cond_t demoConditionVar;

unsigned short globalThreadIndex = 0;

struct serverInfo
{
    unsigned int                 serverId;
    pthread_t                        threadId;
    std :: vector <pthread_t> queue;
};

std :: vector <serverInfo> serverInfoVector;

void * printHello (void* threadId)
{
    pthread_t *my_tid = (pthread_t *)threadId;
    printf ("\nIn `printHello ()`: thread id %ld\n", pthread_self ());

    pthread_mutex_lock (&demoMutex);

    unsigned int i = 0;
    bool found = false;

    while (serverInfoVector.empty())
        pthread_cond_wait (&demoConditionVar, &demoMutex);

    while ((i < serverInfoVector.size ()) && !found)
    {
        if (*my_tid == serverInfoVector [i].threadId)
        {
            found = true;
            break;
        }
        else
            i++;
    }


    if (found)
    {
        pthread_t            writeToFile = pthread_self ();
        unsigned short iterate;
        for (iterate = 0; iterate < 10000; iterate++)
        {
            fprintf (fp, " %d %d",  iterate,         4);
            fprintf (fp, " %lu %lu", writeToFile, sizeof (pthread_t));

            if (!serverInfoVector [i].queue.empty ())
            {
                fprintf (fp, " %c %u", 'A', 1);
                fprintf (fp, " %lu %lu", serverInfoVector [i].queue.front (), sizeof (pthread_t));
                serverInfoVector [i].queue.pop_back ();
            }
            fprintf (fp, "\n %lu %u", writeToFile, 1);
        }
    }
    pthread_mutex_unlock (&demoMutex);
    pthread_exit (NULL);
}

void checkServerExists (unsigned int serverNumber, std :: string message)
{
    unsigned int i         = 0;
    bool found = false;

    pthread_mutex_lock (&demoMutex);

    if (serverInfoVector.size () > 0)
    {
        while ((i <= serverInfoVector.size ()) && (found == false))
        {
            if (serverNumber == serverInfoVector [i].threadId)
            {
                found = true;
                break;
            }
            else
                i++;
        }
    }

    if (!found)
    {
        pthread_t newThread [2];

        int           returnValue;
        if ((returnValue = pthread_create (&newThread [globalThreadIndex], NULL, printHello, (void*) &newThread [globalThreadIndex])) != 0)
        {
            printf ("\nerror: pthread_create failed with error number %d", returnValue);
        }
        printf ("\nIn checkServerExists ()`: thread id %ld\n", newThread [globalThreadIndex]);

        serverInfo obj;
        obj.serverId  = serverNumber;
        obj.threadId = newThread [globalThreadIndex];
        obj.queue.push_back (newThread [globalThreadIndex]);
        serverInfoVector.push_back (obj);

        // Now, since something has been pushed in the vector, it makes sense to wake up the sleeping thread.

        // Now, since something has been pushed in the vector, it makes sense to wake up the sleeping thread.
        pthread_cond_signal (&demoConditionVar);

        pthread_mutex_unlock(&demoMutex);

        pthread_join (newThread [globalThreadIndex], NULL);
    }
    else
    {
        pthread_mutex_unlock(&demoMutex);
    }
}


int main ()
{
    fp = fopen ("xyz", "w");
    pthread_mutex_init (&demoMutex, NULL);
    pthread_cond_init (&demoConditionVar, NULL);

    checkServerExists (1, "anisha");
    globalThreadIndex++;
    checkServerExists (2, "anisha");

    return 0;
}

Answer 2

我猜第二个帖子没有从等待中醒来。第一个线程尝试发信号通知条件变量，但秒尚未启动。我想知道为什么它甚至超过了第一个线程中的等待，但它可能永远不会等待因为线程在条件已经为1之后开始执行。

如何设置pthread_cond_signal以便程序不挂起？

编辑1：

编辑2：

2 个答案: