C pthread_cond_wait() 在 pthread_cond_signal() 之后不恢复程序

Question

我想测试 pthread_cond_wait 来暂停一个线程，但它只是在 printf("thread2");声明。

我的代码：

#include <stdio.h>
#include <pthread.h>
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
volatile int done = 1;


void *testf(void* no) {
    while (1) {
        printf("thread1\n");
        pthread_mutex_lock(&mut);
        pthread_cond_wait(&cond, &mut);
        pthread_mutex_unlock(&mut);
        printf("thread2\n");
        done = 0;
    }
}




int main() {
    printf("mthread1\n");
    pthread_t pt;
    pthread_create(&pt, NULL, &testf, NULL);
    printf("mthread2\n");
    
    pthread_mutex_lock(&mut);
    printf("mthread3\n");
    pthread_cond_signal(&cond);
    printf("mthread4\n");
    pthread_mutex_unlock(&mut);
    printf("mthread5\n");

    while (done);
    return 0;
}

为什么在 cond_signal() 之后没有恢复？它应该恢复。

Answer 1

由于条件变量用于设置/检查变量，我可以提出以下增强代码来测试机制。 “完成”是要检查的变量。我在主线程中添加了一个 sleep() ，否则在大多数情况下，它有时间在辅助线程有机会检查它并至少循环一次之前设置“完成”变量。

补充说明：

与请求中提供的代码相比：即使pthread_cond_signal()的信号丢失，我们仍然无法确保在主线程中pthread_create()返回时启动辅助线程，辅助线程首先检查受互斥体保护的“完成”变量的值。

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>

pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
volatile int done = 0;


void *testf(void* no) {

        printf("thread 2 looping\n");

        // Lock the mutex to check the variable
        pthread_mutex_lock(&mut);

    do {

        if (done) {
          pthread_mutex_unlock(&mut);
          printf("thread 2 exiting\n");
          break;
        }

        // The mutex is locked and will be unlocked inside
        // pthread_cond_wait()

        pthread_cond_wait(&cond, &mut);
        printf("thread 2 signaled\n");

        // The mutex is locked

    } while (1);
}




int main() {

    printf("thread1 starting...\n");
    pthread_t pt;
    pthread_create(&pt, NULL, &testf, NULL);
    printf("thread 1 created thread 2\n");

    printf("thread 1 is doing some dummy work\n");
    sleep(1);

    // Lock the mutex to check/set the variable
    pthread_mutex_lock(&mut);
    done = 1;
    printf("thread 1 signaling thread 2\n");
    pthread_cond_signal(&cond);
    printf("thread 1 releases the mutex\n");
    pthread_mutex_unlock(&mut);

    printf("thread 1 waiting end of thread 2...\n");
    pthread_join(pt, NULL);

    return 0;
}