c - 输出值与两个 pthread 交错

Question

我想创建两个输出交错的线程，如下所示

 Thread1:1=>Ping!
 Thread2:2=>Pong!
 Thread1:3=>Ping!
 Thread1:4=>Ping!
 Thread2:5=>Pong!
 Thread2:6=>Pong!
 Thread1:7=>Ping!
 Thread2:8=>Pong!
 Thread1:9=>Ping!
 ..........
 until 50

我的代码在下面

#include <stdio.h>
#include <stdlib.h> 
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
void* increment1(void* arg);
void* increment2(void* arg);
int count = 0;
sem_t sem;
int main() {
    //variable initialize
    pthread_t thread1, thread2;
    int res1 = 0, res2 = 0;
    int number = 0;
    int i = 0;
    //create semaphore
    if (sem_init(&sem, 0, 1) == -1){
        printf("Semaphore creation failed!!\n");
        exit(EXIT_FAILURE);     
    }
    for (i = 0; i < 25; ++i){
        //create thread
        res1 = pthread_create(&thread1, NULL, increment1, NULL);
        if (res1 != 0) {
            printf("Thread1 creation failed!!\n");
            exit(EXIT_FAILURE);
        }
        //wait thread synchronization
        pthread_join( thread1, NULL);
        res2 = pthread_create(&thread2, NULL, increment2, NULL);
        if (res2 != 0) {
            printf("Thread2 creation failed!!\n");
            exit(EXIT_FAILURE);
        }
        //wait thread synchronization
        pthread_join( thread2, NULL);
    }
        exit(EXIT_SUCCESS);
}
void* increment1(void* arg) { 
    sem_wait(&sem);
    count ++;
    printf("Thread1:%d\nPing!\n",count);
    fsync(fileno(stdout));
    sem_post(&sem);
}
void* increment2(void* arg) { 
    sem_wait(&sem);
    count ++;
    printf("Thread2:%d\nPong!\n",count);
    fsync(fileno(stdout));
    sem_post(&sem);
}

但我认为我做的不是并行使用两个线程是错误的，我使用的是顺序替代执行两个线程而不是不并行。(通过使用 pthread_join，thread2 将在 thread1 之后执行完成)。

我尝试使用信号量，它似乎不能保证线程执行顺序。

我想问的是

1.如何使用信号量保证两个线程的顺序？

2.如何暂停和恢复线程？我认为我所做的是在循环中创建新的两个 pthread。

提前致谢。

Answer 1

添加第二个信号量并将其初始化为零，以便线程 1 被强制为第一个。然后 increment1 和 increment2 保持信号，它是其他线程转去。你在连接的地方遇到了一些小问题，你可以弄清楚。

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

void* increment1(void* arg);
void* increment2(void* arg);
int count = 0;
sem_t sem1, sem2;

int main()
{
    pthread_t thread[2];

    int res = 0;
    int number = 0;
    int i = 0;

    if (sem_init(&sem1, 0, 1) == -1)
    {
        printf("Semaphore creation failed!!\n");
        exit(EXIT_FAILURE);
    }

    if (sem_init(&sem2, 0, 0) == -1)
    {
        printf("Semaphore creation failed!!\n");
        exit(EXIT_FAILURE);
    }

    for (i = 0; i < 25; ++i)
    {
         res = pthread_create(&thread[0], NULL, increment1, NULL);
         if (res != 0)
         {
             printf("Thread creation failed!!\n");
             exit(EXIT_FAILURE);
         }

         res = pthread_create(&thread[1], NULL, increment2, NULL);
         if (res != 0)
         {
             printf("Thread creation failed!!\n");
             exit(EXIT_FAILURE);
         }

        for (int j = 0; j < 2; ++j)
        {
            pthread_join(thread[j], NULL);
        }
    }

    exit(EXIT_SUCCESS);
}

void* increment1(void* arg)
{
    sem_wait(&sem1);
    count ++;
    printf("Thread1:%d Ping!\n", count);
    fsync(fileno(stdout));
    sem_post(&sem2);
}

void* increment2(void* arg)
{
    sem_wait(&sem2);
    count ++;
    printf("Thread2:%d Pong!\n", count);
    fsync(fileno(stdout));
    sem_post(&sem1);
}