c++ - C++问题将信息传递给pthreads

我遇到的问题是showData（）中的printf调试语句会给我毫无意义的数字。即：thread_id为-1781505888。
如果我在设置thread_id，startIndex和stopIndex的值后立即在createThreads（）中插入printf语句，则这些值将正确打印。当我将threadData作为pthread_create的参数传递时，或者从showData（）中的threadArg读取数据时，数据以某种方式被破坏。同样，可以将N和k的值假定为整数，而N / k的余数为0。感谢所有帮助。

编辑：同样，如果它提供任何额外的信息，则当我在同一输入上运行此命令时，每次运行都会得到不同的输出。有时是无意义的数字，有时所有值都显示为0，有时会出现故障。

void createThreads(int k){
struct threadData threadData;
int numThreads = k;
int i = 0;
int err = 0;

pthread_t *threads = static_cast<pthread_t*>(malloc(sizeof(pthread_t) * numThreads));
for(i = 0;i<numThreads;i++){
    struct threadData threadData;
    threadData.thread_id = i;
    threadData.startIndex = ((N/k)*i);
    if(i == numThreads -1){
        threadData.stopIndex = ((N/k)*(i+1))-1;
    }
    else{
        threadData.stopIndex = ((N/k)*(i+1));
    }



    err = pthread_create(&threads[i], NULL, showData, (void *)&threadData);


    if(err != 0){
        printf("error creating thread\n");
    }
}
}

void *showData(void *threadArg){
    struct threadData *threadData;
    threadData = (struct threadData *) threadArg;

    printf("thread id : %d\n", threadData->thread_id);
    printf("start: %d\n", threadData->startIndex);
    printf("stop : %d\n", threadData->stopIndex);
}

最佳答案

threadData对您的for循环来说是本地的...在每次迭代中它都超出范围，因此指向它的指针在您的showData()例程中无效。您可以改为动态分配它，并在delete的末尾showData分配它。

您可能也想将threads数据返回给createThreads的调用者，因此它可以join线程等待showData“工作”的完成。

例：

...
for(i = 0; i < numThreads; ++i)
{
    struct threadData* threadData = new struct threadData;
    threadData->thread_id = i;
    threadData->startIndex = ((N/k)*i);
    if(i == numThreads -1){
        threadData->stopIndex = ((N/k)*(i+1))-1;
    }
    else{
        threadData->stopIndex = ((N/k)*(i+1));
    }

    err = pthread_create(&threads[i], NULL, showData, (void*)threadData);

    if(err != 0){
        printf("error creating thread\n");
        exit(1); // probably not worth trying to continue...
    }
    return threads;
}

void *showData(void *threadArg){
    struct threadData* threadData = (struct threadData*)threadArg;

    printf("thread id : %d\n", threadData->thread_id);
    printf("start: %d\n", threadData->startIndex);
    printf("stop : %d\n", threadData->stopIndex);

    delete threadData;
}