在 main 函数中调用 join child pthread答案

【问题标题】：Call join child pthread in main function在 main 函数中调用 join child pthread
【发布时间】：2015-12-07 11:07:22
【问题描述】：

我有测试代码：

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

pthread_t th_worker, th_worker2;



void * worker2(void *data) {
    for(int i = 0; i< 1000000; i++){
            printf("thread for worker2----%d\n", i);
            usleep(500);
    }
}
void * worker(void *data){
    pthread_create(&th_worker2, NULL, worker2, data);

    for(int i = 0; i< 100; i++){
            printf("thread for worker-----%d\n", i);
            usleep(500);
    }
}
void join(pthread_t _th){
    pthread_join(_th, NULL);
}

在 main() 函数中，如果我调用 join(the_worker2)：

int main() {
    char* str = "hello thread";

    pthread_create(&th_worker, NULL, worker, (void*) str);
    /* problem in here */
    join(th_worker2);

    return 1;
}

--> 段错误错误

否则，我打电话：

join(the_worker);
join(th_worker2);

--->好的

为什么在上述情况下会出现段错误？感谢帮助！！！

【问题讨论】：

标签： c++ c main pthread-join pthread-exit

【解决方案1】：

如果您发布了所有您的代码，则您有一个竞态条件。

main 与worker 的开头同步，但不 worker2。

也就是说，main 正在尝试加入th_worker2 之前 worker 有机会调用pthread_create 并设置th_worker2 有效 em> [非空] 值。

所以，th_worker2 将在第二个 pthread_create 完成之前无效，但对于 main 来说已经太迟了。它已经获取了th_worker2，它的值为NULL，main 将出现段错误。

当您为th_worker 添加联接时，它会起作用，因为它保证同步和无竞争条件。

要在没有连接的情况下实现此保证，请执行以下操作：

int
main()
{
    char *str = "hello thread";

    pthread_create(&th_worker, NULL, worker, (void *) str);

    // give worker enough time to properly start worker2
    while (! th_worker2)
        usleep(100);

    /* problem in here */
    join(th_worker2);

    return 1;
}

一个更好的方法是添加一个额外的变量。有了这个，第一个循环就不需要了[但我把它留在了]：

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

int worker_running;
pthread_t th_worker;

int worker2_running;
pthread_t th_worker2;

void *
worker2(void *data)
{

    // tell main we're fully functional
    worker2_running = 1;

    for (int i = 0; i < 1000000; i++) {
        printf("thread for worker2----%d\n", i);
        usleep(500);
    }

    return NULL;
}

void *
worker(void *data)
{

    // tell main we're fully functional
    worker_running = 1;

    pthread_create(&th_worker2, NULL, worker2, data);

    for (int i = 0; i < 100; i++) {
        printf("thread for worker-----%d\n", i);
        usleep(500);
    }

    return NULL;
}

void
join(pthread_t _th)
{
    pthread_join(_th, NULL);
}

int
main()
{
    char *str = "hello thread";

    pthread_create(&th_worker, NULL, worker, (void *) str);

    // give worker enough time to properly start worker2
    // NOTE: this not necessarily needed as loop below is better
    while (! th_worker2)
        usleep(100);

    // give worker2 enough time to completely start
    while (! worker2_running)
        usleep(100);

    /* problem in here (not anymore!) */
    join(th_worker2);

    return 1;
}

【讨论】：