“非铸造"from (void *) 并取消对 char 数组的引用 [英] "Un-casting" from (void *) and de-referencing to char array

问题描述

我几乎完成了一项需要使用 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.

I have almost completed an homework assignment where I am required to use pthreads. I have figured out the pthreads. The only problem I have left is figuring out how to pass multiple arguments to threads through pthread_create().

我需要向线程传递两个字符.我必须将它们强制转换为 (*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.

I need to pass two chars to the thread. I have to cast them to (*void) to use with pthread_create(). I can pass them, but I can't figure out how to get the values from *parameter in the function.

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?

I was able to pass one value through with no problem. Any suggestions?

我能找到的最接近的现有问题是这个，但我仍然没有运气:在 C 中从 void* 转换为 char **

The closest existing question I could find was this but I still am having no luck: Converting from void* to char ** in C

谢谢！

推荐答案

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有可能在线程访问它之前被释放，在这种情况下会导致未定义的行为.

thread_args is a local array with automatic storage duration, lifetime of which is tied to each iteration so there is a chance that the memory where you store these arguments might be freed before the thread will access it, which would lead to undefined behavior in that case.

更好的方法是创建一个结构，用于将数据传递给该线程:

Much better approach would be to create a structure, that you would use to pass the data to this thread:

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

那么您的代码可能如下所示:

Then your code could look like this:

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，堆栈将被解开.有关详细信息，请参阅 pthread 启动函数中的 return() 与 pthread_exit()

Also note that to end thread's execution it is better to use return than pthread_exit since with return it is guaranteed that variables within thread's routine will be destroyed and stack will be unwound. For more information see return() versus pthread_exit() in pthread start functions

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