“解铸造” from（void *）和解引用到char数组 [英] "Un-casting" from (void *) and de-referencing to char array

问题描述

我几乎完成了一项家庭作业，我需要使用pthreads。我已经找出了pthreads。我剩下的唯一的问题是弄清楚如何通过pthread_create（）传递多个参数到线程。

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().

我需要传递两个字符到线程。我必须将它们转换（* void）以与pthread_create（）一起使用。我可以通过它们，但我不知道如何从函数中的*参数获取值。

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);
}

我可以传递一个值，没有问题。任何建议？

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

我可以找到的最接近的现有问题是，但我还没有运气：在C中将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！ / p>

Thank you!

推荐答案

请注意，在此循环中：

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，在这种情况下，存储这些参数的内存可能会在线程访问它之前释放，这将导致未定义的行为。

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体，你可以用来传递数据到这个线程：

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;

然后你的代码看起来像这样：

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<$ c $因为使用 return ，可以保证线程例程中的变量将会被毁坏，堆叠将被解开。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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