Question

我是C语言中的多线程新手，我有这个问题。我写了以下代码：

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

pthread_mutex_t m=PTHREAD_MUTEX_INITIALIZER;
pthread_attr_t attr;

void* test(void *a)
{
    int i=*((int *)a);
    printf("The thread %d has started.\n",i);
    pthread_mutex_lock(&m);
    sleep(1);
    printf("The thread %d has finished.\n",i);
    pthread_mutex_unlock(&m);
    pthread_exit(NULL);

}

int main()
{
    int i=0;
    pthread_attr_setdetachstate(&attr,PTHREAD_CREATE_JOINABLE);
    pthread_t thread[5];

    for (i=0;i<5;i++)
        pthread_create(&thread[i],&attr,test,&i);

    for (i=0;i<5;i++)
        pthread_join(thread[i],NULL);
    return 0;
}

为什么我会得到以下值：

The thread 0 has started.
The thread 0 has started.
The thread 5 has started.
The thread 5 has started.
The thread 0 has started.
The thread 0 has finished.
The thread 0 has finished.
The thread 5 has finished.
The thread 5 has finished.
The thread 0 has finished.

或

The thread 1 has started.
The thread 2 has started.
The thread 5 has started.
The thread 4 has started.
The thread 0 has started.
The thread 1 has finished.
The thread 2 has finished.
The thread 5 has finished.
The thread 4 has finished.
The thread 0 has finished.

甚至：

The thread 0 has started.
The thread 0 has started.
The thread 0 has started.
The thread 0 has started.
The thread 0 has started.
The thread 0 has finished.
The thread 0 has finished.
The thread 0 has finished.
The thread 0 has finished.
The thread 0 has finished.

等，当我期望得到：

The thread 0 has started.
The thread 1 has started.
The thread 2 has started.
The thread 3 has started.
The thread 4 has started.
The thread 0 has finished.
The thread 1 has finished.
The thread 2 has finished.
The thread 3 has finished.
The thread 4 has finished.

只有当我在usleep(10)之后添加thread_create时才会获得一些“正常”值。

我在Unix上的Code :: Blocks中编译并运行了这段代码。

Answer 1

您正在传递for正在变化的变量的地址（i），因此您受调度程序的支配。你应该传递一份副本。作为一种廉价而非完全犹太的方式：

pthread_create(&thread[i],&attr,test, (void*)i);

/* ... */

int i = (int)a;

Answer 2

请注意，您将i的地址作为参数传递给您的主题：

pthread_create(&thread[i],&attr,test,&i);

这意味着您的所有线程都将读取相同的变量i以确定它们是哪个线程。也就是说，所有五个线程都将查看相同的变量以确定其线程号。因此，当i循环中for的值递增时，所有线程都会感知其线程号更改为使用新值i。这就是为什么你有时会看到5出现作为主题号码，并且还解释了你经常跳过数字或看到太多重复的事实。

要解决此问题，您需要为每个帖子提供自己的i副本。例如，你可以这样做：

int* myI = malloc(sizeof(int));
*myI = i;
pthread_create(&thread[i], &attr, test, myI);

然后让线程在终止之前释放指针：

void* test(void *a)
{
    int i=*((int *)a);
    printf("The thread %d has started.\n",i);
    pthread_mutex_lock(&m);
    sleep(1);
    printf("The thread %d has finished.\n",i);
    pthread_mutex_unlock(&m);
    pthread_exit(NULL);
    free(a);
}

或者，您可以将i投射到void*并将其传递到：

pthread_create(&thread[i],&attr,test, (void*)i);

如果你这样做，你就会让线程直接将他们的参数反馈回int，而不是int*：

void* test(void *a)
{
    int i = (int)a;
    printf("The thread %d has started.\n",i);
    pthread_mutex_lock(&m);
    sleep(1);
    printf("The thread %d has finished.\n",i);
    pthread_mutex_unlock(&m);
    pthread_exit(NULL);
}

希望这有帮助！

多线程C程序中的线程ID错误？

2 个答案: