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本文将介绍几种如何在 C 语言中使用 pthread_join 函数的方法。

使用 pthread_join 函数等待线程终止

程序使用 pthread_create 函数创建线程，通常，它等待它们通过 pthread_join 函数终止。pthread_join 仅接受两个参数：用于指定等待线程的线程 ID 和指向可以存储指定线程的退出状态的 void*的指针。如果用户不想检索等待的线程的退出代码，则应将 NULL 值作为第二个参数传递。在下面的示例中，我们演示了创建 8 个线程并在每个线程中执行 printHello 函数的程序。然后，调用线程在循环中等待带有 pthread_join 调用的每个线程。注意，我们还将线程的退出状态代码存储在 retval 变量中，并通过将其强制转换为 int 来打印其值。请注意，如果线程被取消，则将 PTHREAD_CANCELED 值放置在 retval 地址中。

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

#ifndef NUM_THREADS
#define NUM_THREADS 8
#endif

void *printHello(void *threadid) {
    long tid;
    tid = (long)threadid;
    printf("Hello from thread %ld, pthread ID - %lu\n", tid, pthread_self());
    return NULL;
}


int main(int argc, char const *argv[]) {
    pthread_t threads[NUM_THREADS];
    int rc;
    long t;

    for (t = 0; t < NUM_THREADS; t++) {
        rc = pthread_create(&threads[t], NULL, printHello, (void *)t);
        if (rc) {
            printf("ERORR; return code from pthread_create() is %d\n", rc);
            exit(EXIT_FAILURE);
        }
    }

    int ret;
    for (t = 0; t < NUM_THREADS; t++) {
        void *retval;
        ret = pthread_join(threads[t], &retval);
        if (retval == PTHREAD_CANCELED)
            printf("The thread was canceled - ");
        else
            printf("Returned value %d - ", (int)retval);
    }
    pthread_exit(NULL);
}

输出：

Hello from thread 0, pthread ID - 140716669929216
Hello from thread 1, pthread ID - 140716661536512
Hello from thread 2, pthread ID - 140716653143808
Hello from thread 3, pthread ID - 140716644751104
Hello from thread 5, pthread ID - 140716627965696
Hello from thread 4, pthread ID - 140716636358400
Hello from thread 6, pthread ID - 140716550387456
Hello from thread 7, pthread ID - 140716541994752

使用 pthread_join 函数返回值检查错误

与设置 errno 全局变量的函数相比，pthread_join 函数返回的整数值还指示不同的错误代码。如果调用成功，则返回值为 0，这可以确保给定线程已终止。如果返回的整数等于 EDEADLK，则报告检测到死锁。如果返回 EINVAL 值，则给定线程不可连接，如果该值等于 ESRCH，则表明找不到给定线程 ID。在这种情况下，我们实现 switch 语句以检查每种情况，并将相应的消息打印到 stdout。

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

#ifndef NUM_THREADS
#define NUM_THREADS 8
#endif

void *printHello(void *threadid) {
    long tid;
    tid = (long)threadid;
    printf("Hello from thread %ld, pthread ID - %lu\n", tid, pthread_self());
    return NULL;
}

int main(int argc, char const *argv[]) {
    pthread_t threads[NUM_THREADS];
    int rc;
    long t;

    for (t = 0; t < NUM_THREADS; t++) {
        rc = pthread_create(&threads[t], NULL, printHello, (void *)t);
        if (rc) {
            printf("ERORR; return code from pthread_create() is %d\n", rc);
            exit(EXIT_FAILURE);
        }
    }

    int ret;
    for (t = 0; t < NUM_THREADS; t++) {
        void *retval;
        ret = pthread_join(threads[t], &retval);
        if (retval == PTHREAD_CANCELED)
            printf("The thread was canceled - ");
        else
            printf("Returned value %d - ", (int)retval);


        switch (ret) {
            case 0:
                printf("The thread joined successfully\n");
                break;
            case EDEADLK:
                printf("Deadlock detected\n");
                break;
            case EINVAL:
                printf("The thread is not joinable\n");
                break;
            case ESRCH:
                printf("No thread with given ID is found\n");
                break;
            default:
                printf("Error occurred when joining the thread\n");
        }
    }
    pthread_exit(NULL);
}

输出：

Hello from thread 0, pthread ID - 140082577512192
Hello from thread 1, pthread ID - 140082569119488
Hello from thread 3, pthread ID - 140082552334080
Hello from thread 5, pthread ID - 140082535548672
Hello from thread 6, pthread ID - 140082527155968
Returned value 0 - The thread joined successfully
Hello from thread 4, pthread ID - 140082543941376
Hello from thread 2, pthread ID - 140082560726784
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Hello from thread 7, pthread ID - 140082518763264
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully
Returned value 0 - The thread joined successfully