c - 线程已创建但无法相应工作

Question

我正在编写的代码遇到问题，但我真的无法识别其原因，因此非常感谢您的帮助。
案例很简单
--> 从命令行给出线程数作为参数
--> 创建N个线程，带参数
--> 每个线程都打招呼及其参数并退出
这是我的代码:

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

void* client_func(void *arg) {
    int myid = *((int *) arg);
    printf("hello!\n my id is %d\n",myid);
    return NULL;
}

int main(int argc, char *argv[] ){
    int i,j = 0;
    int N_cust = atoi(argv[1]);

    //creation of client threads
    pthread_t tid[N_cust];
    int err;

    for (i = 0; i < N_cust; i++){
        int *id = malloc(sizeof(*id));
        err = pthread_create(&tid[i], NULL, &client_func,(void *) id);
        if(err !=0){
            perror("pthread_create() error");
            exit(1);
       }
       else{
           printf("\n Thread created successfully\n");
       }
    }

    return 0;
}

我正在等待获得带有线程 ID 的“hello”消息，但我得到的是:

$ ./proj1 3 5

Thread created successfully
Thread created successfully
Thread created successfully

我是线程新手，但根据我的理解，线程根本没有执行。对我做错的事情有什么帮助吗？谢谢

Answer 1

正如我在 comment 中指出的那样，您的代码中存在许多问题:

• 您为 id 指针分配了指向的空间，但从未将其设置为任何已知值。
• 您的线程函数还忘记释放为其分配的内存。
• 您的主程序可能还应该等待子线程使用 pthread_join() 完成，然后再退出(通过 return 0;)。

调用 exit() — 甚至通过从 main() 返回间接调用 — 意味着进程(以及其中的所有线程)立即终止。您可以在 main() 中使用 pthread_exit(0); 而不是 return 0;，这样您的线程就会运行完成。

以下是已修复这些问题的代码的两个变体。变体 1 (pthr41.c) 的主线程退出为 pthread_exit(0);。变体 2 (pthr43.c) 的主线程使用 pthread_join()。添加了一些错误检测。

pthr41.c

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

static void *client_func(void *arg)
{
    int myid = *((int *)arg);
    printf("\nhello! my id is %d\n", myid);
    free(arg);
    return NULL;
}

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s numthreads\n", argv[0]);
        return 1;
    }
    int N_cust = atoi(argv[1]);
    if (N_cust <= 0 || N_cust > 1000)
    {
        fprintf(stderr, "%s: number of threads %s out of range 1..1000\n", argv[0], argv[1]);
        return 1;
    }

    // creation of client threads
    pthread_t tid[N_cust];

    for (int i = 0; i < N_cust; i++)
    {
        int *id = malloc(sizeof(*id));
        *id = i + 1;
        int err = pthread_create(&tid[i], NULL, client_func, id);
        if (err != 0)
        {
            perror("pthread_create() error");
            exit(1);
        }
        else
        {
            printf("\nThread %d created successfully\n", i + 1);
        }
    }

    printf("\nMain thread exits\n");
    pthread_exit(0);
    //return 0;
}

示例输出

在获得此输出之前，需要运行多次，但这显示线程 4 在主线程之后完成:

$ pthr41 4

Thread 1 created successfully

Thread 2 created successfully

hello! my id is 1

hello! my id is 2

hello! my id is 3

Thread 3 created successfully

Thread 4 created successfully

Main thread exits

hello! my id is 4
$

pthr43.c

#include <assert.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

static void *client_func(void *arg)
{
    int myid = *((int *)arg);
    printf("\nhello! my id is %d\n", myid);
    free(arg);
    return NULL;
}

int main(int argc, char *argv[])
{
    if (argc != 2)
    {
        fprintf(stderr, "Usage: %s numthreads\n", argv[0]);
        return 1;
    }
    int N_cust = atoi(argv[1]);
    if (N_cust <= 0 || N_cust > 1000)
    {
        fprintf(stderr, "%s: number of threads %s out of range 1..1000\n", argv[0], argv[1]);
        return 1;
    }

    // creation of client threads
    pthread_t tid[N_cust];

    for (int i = 0; i < N_cust; i++)
    {
        int *id = malloc(sizeof(*id));
        *id = i + 1;
        int err = pthread_create(&tid[i], NULL, client_func, id);
        if (err != 0)
        {
            perror("pthread_create() error");
            exit(1);
        }
        else
        {
            printf("\nThread %d created successfully\n", i + 1);
        }
    }

    for (int i = 0; i < N_cust; i++)
    {
        void *vp;
        int err = pthread_join(tid[i], &vp);
        if (err != 0)
        {
            fprintf(stderr, "%s: error %d (%s) from joining thread %d\n",
                    argv[0], err, strerror(err), i + 1);
        }
        else
            assert(vp == NULL);
    }
    printf("All threads complete\n");

    //pthread_exit(0);
    return 0;
}

示例输出

$ pthr43 4

Thread 1 created successfully

hello! my id is 1

hello! my id is 2

Thread 2 created successfully

Thread 3 created successfully

hello! my id is 3

Thread 4 created successfully

hello! my id is 4
All threads complete
$