从 (void *) 中“取消转换”并取消对 char 数组的引用

Question

我几乎完成了一项要求我使用 pthread 的家庭作业。我已经弄清楚了pthreads。我剩下的唯一问题是弄清楚如何通过 pthread_create() 将多个参数传递给线程。

我需要将两个字符传递给线程。我必须将它们转换为 (*void) 才能与 pthread_create() 一起使用。我可以传递它们，但我不知道如何从函数中的 *parameter 获取值。

void *my_function(void *parameter) {

    /* ATTEMPT 1 - DOESN'T WORK */
    //char* arguments[2];
    //arguments = (char * [2]) parameter;
    /*Error message: 
    error: ISO C++ forbids casting to an array type char* [2] [-fpermissive]
    error: incompatible types in assignment of char** to char*[2]
    */

    /* ATTEMPT 2 - DOESN'T WORK */
    //char *my_data = (char *)parameter;
    //my_data is blank when I try to use cout to check it's values

    /* What I need to do is get those two chars from the array and print them out as part of this thread function */

    pthread_exit(NULL);
}

int main(int argc, char **argv) {

    char duration = '5'; //in reality, this value is taken from argv but I am leaving that out for brevity

    pthread_t threads[3];

    for(int i=0; i < 3; i++){

         char thread_args[2] = {i, duration};

         //create thread with arguments passed in
         int results = pthread_create(&threads[i], NULL, my_function, (void *) &thread_args);

         //testing for pthread error
         if (results){
             printf("ERROR; return code from pthread_create() is %d\n", results);
             exit(-1);
         }

     }

    /* Wait for all threads to complete */
    for (int j=0; j < num_threads; j++) { // https://computing.llnl.gov/tutorials/pthreads/
        pthread_join(threads[j], NULL);
    }


    /* some information prints here that is unrelated to my problem (the date, time, etc) */

    pthread_exit(NULL);
}

我能够毫无问题地传递一个值。有什么建议吗？

我能找到的最接近的现有问题是这个，但我仍然没有运气：Converting from void* to char ** in C

谢谢！

Answer 1

注意在这个循环中：

for(int i=0; i < 3; i++){
    char thread_args[2] = {i, duration};
    int results = pthread_create(&threads[i], NULL, my_function, (void *) 
    ...
}

thread_args 是一个具有自动存储持续时间的本地数组，其生命周期与每次迭代相关，因此您存储这些参数的内存可能会在线程访问它之前被释放，这将导致未定义的行为在这种情况下。

更好的方法是创建一个结构，用于将数据传递给该线程：

typedef struct {
    char duration;
    int num;
} ThreadData;

那么您的代码可能如下所示：

void *my_function(void *parameter) {
    // retrieve and print the thread data:
    ThreadData* td = (ThreadData*) parameter;
    printf("num = %d, duration = %c\n", td->num, td->duration);
    delete td;
    return NULL;
}

int main(int argc, char **argv) {
    char duration = '5';
    pthread_t threads[3];

    for (int i = 0; i < 3; i++) {
        // create structure that will be passed to thread:
        ThreadData* td = new ThreadData;
        td->duration = duration;
        td->num = i;

        //create thread with arguments passed in:
        int ret = pthread_create(&threads[i], NULL, my_function, (void *) td);

        //testing for pthread error:
        if (ret) {
            printf("ERROR; return code from pthread_create() is %d\n", ret);
            exit(-1);
        }
    }
    // wait for all threads to complete:
    for (int i = 0; i < 3; i++) {
        pthread_join(threads[i], NULL);
    }
    exit(0);
}

还要注意，要结束线程的执行，最好使用return 而不是pthread_exit，因为使用return 可以保证线程例程中的变量将被销毁并且堆栈将被展开。欲了解更多信息，请参阅return() versus pthread_exit() in pthread start functions