pthread_cond_wait为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场景:

• 线程B等待条件
• 线程A打印"Hello"五次
• 线程A表示线程B
• 线程A等待
• 线程B打印"再见"
• 线程B表示线程A.
• 循环开始(x5)

到目前为止,代码打印"Hello"五次然后卡住了.从我看到的例子来看,我似乎在正确的轨道上,"锁定互斥,等待,通过其他线程发出信号,解锁互斥,做东西,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);
}

非常感谢正确方向的指针,谢谢!(使用"gcc timeTest.c -o timeTest -lpthread"在Linux上编译的代码)

Answer 1

你有两个问题.首先是你没有while()正确使用循环 - 例如,在这里:

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

i++;

while循环的主体是语句i++- 这将执行pthread_cond_wait(),i++直到pthread_cond_wait()返回错误,所以这实际上是一个无限循环.

第二个是你不能单独使用pthreads条件变量 - 它需要与一些实际的共享状态配对(最简单的,这个共享状态可能只是一个由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,并在pthread_cond_signal()线程更改共享状态时使用该函数.重写您的示例以使用此类变量:

//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-如果使用的条件只有一个变量,而不是代码将是一样正确.