pthread_cond_wait用于2个线程

Question

我正在尝试实现pthread_cond_wait2个线程。我的测试代码试图使用两个线程来执行以下方案：

• 线程B等待条件
• 线程A打印“ Hello”五次
• 线程A通知线程B
• 线程A等待
• 线程B打印“再见”
• 线程B向线程A发出信号
• 循环开始（x5）

到目前为止，代码将打印“ Hello”五次，然后卡住。从我看过的示例中，我似乎走对了，“锁定互斥锁，等待，由其他线程发出信号，解锁互斥锁，执行操作，循环”

测试代码：

//Import 
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

//global variables
pthread_cond_t      condA  = PTHREAD_COND_INITIALIZER;
pthread_cond_t      condB  = PTHREAD_COND_INITIALIZER;
pthread_mutex_t     mutex = PTHREAD_MUTEX_INITIALIZER;




void *threadA()
{
    int i = 0, rValue, loopNum;

    while(i<5)
    {
        //unlock mutex
        rValue = pthread_mutex_unlock(&mutex);

        //do stuff
        for(loopNum = 1; loopNum <= 5; loopNum++)
            printf("Hello %d\n", loopNum);

        //signal condition of thread b
        rValue = pthread_cond_signal(&condB);

        //lock mutex
        rValue = pthread_mutex_lock(&mutex);

        //wait for turn
        while( pthread_cond_wait(&condA, &mutex) != 0 )

        i++;
    }

}



void *threadB()
{
    int n = 0, rValue;

    while(n<5)
    {
        //lock mutex
        rValue = pthread_mutex_lock(&mutex);

        //wait for turn
        while( pthread_cond_wait(&condB, &mutex) != 0 )

        //unlock mutex
        rValue = pthread_mutex_unlock(&mutex);

        //do stuff
        printf("Goodbye");

        //signal condition a
        rValue = pthread_cond_signal(&condA);

        n++;        
    }
}




int main(int argc, char *argv[])
{
    //create our threads
    pthread_t a, b;

    pthread_create(&a, NULL, threadA, NULL);
    pthread_create(&b, NULL, threadB, NULL);

    pthread_join(a, NULL);
    pthread_join(b,NULL);
}

朝着正确方向的指针将不胜感激，谢谢！（在Linux上使用“ gcc timeTest.c -o timeTest -lpthread”编译的代码）

Answer 1

你有两个问题。首先是您没有while()正确使用循环-例如，在这里：

//wait for turn
while( pthread_cond_wait(&condA, &mutex) != 0 )

i++;

在体内while循环是声明i++-这将执行pthread_cond_wait()并i++直到pthread_cond_wait()返回一个错误，所以这基本上是一个死循环。

第二个问题是您不能单独使用pthreads条件变量-
它需要与一些实际的共享状态配对（最简单的说，此共享状态可能只是受互斥锁保护的标志变量）。该pthread_cond_wait()函数用于等待共享状态达到某个值，并且pthread_cond_signal()当线程更改共享状态时使用该函数。重新编写示例以使用这样的变量：

//global variables
/* STATE_A = THREAD A runs next, STATE_B = THREAD B runs next */
enum { STATE_A, STATE_B } state = STATE_A;
pthread_cond_t      condA  = PTHREAD_COND_INITIALIZER;
pthread_cond_t      condB  = PTHREAD_COND_INITIALIZER;
pthread_mutex_t     mutex = PTHREAD_MUTEX_INITIALIZER;

void *threadA()
{
    int i = 0, rValue, loopNum;

    while(i<5)
    {
        /* Wait for state A */
        pthread_mutex_lock(&mutex);
        while (state != STATE_A)
            pthread_cond_wait(&condA, &mutex);
        pthread_mutex_unlock(&mutex);

        //do stuff
        for(loopNum = 1; loopNum <= 5; loopNum++)
            printf("Hello %d\n", loopNum);

        /* Set state to B and wake up thread B */
        pthread_mutex_lock(&mutex);
        state = STATE_B;
        pthread_cond_signal(&condB);
        pthread_mutex_unlock(&mutex);

        i++;
    }

    return 0;
}

void *threadB()
{
    int n = 0, rValue;

    while(n<5)
    {
        /* Wait for state B */
        pthread_mutex_lock(&mutex);
        while (state != STATE_B)
            pthread_cond_wait(&condB, &mutex);
        pthread_mutex_unlock(&mutex);

        //do stuff
        printf("Goodbye\n");

        /* Set state to A and wake up thread A */
        pthread_mutex_lock(&mutex);
        state = STATE_A;
        pthread_cond_signal(&condA);
        pthread_mutex_unlock(&mutex);

        n++;
    }

    return 0;
}

需要注意的是使用两个条件变量的condA和condB不必要在这里-代码将是一样，如果正确使用的条件只有一个变量，而不是。