pthread_join 似乎修改了我的循环索引

Question

我的代码（见下文）产生了奇怪的行为。输出为：

Testing whether there are problems with concurrency ...rc is 0. i is 0
.rc is 0. i is 0
.rc is 3. i is 1
.rc is 0. i is 0
.rc is 3. i is 1
.rc is 3. i is 2
.rc is 0. i is 0
.rc is 3. i is 1
.rc is 3. i is 2
.rc is 3. i is 3
.rc is 0. i is 0
.rc is 3. i is 1
.rc is 3. i is 2
.rc is 3. i is 3
.rc is 3. i is 4
.rc is 0. i is 0
Segmentation fault (core dumped)

我试图调试它，但只发现我在 pthread_join 之后立即重置为 0。这使我得出结论，修改必须发生在某个地方。但我找不到任何东西。我觉得有点傻，因为这并不是一段很难的代码。我没有注意到什么？

操作系统是 Ubuntu 14.04。 N_THREADS当前设置为10，N_RUNS为10000。

主线程：

pthread_t threads[N_THREADS];
pthread_attr_t attr;
int i;
int rc;
int status;

printf("Testing whether there are problems with concurrency ...");

pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

for (i = 0; i < N_THREADS; i++){
  if (i) {
    rc = pthread_create(&(threads[i]), &attr, addRemove, 0);
  } else {
    rc = pthread_create(&(threads[i]), &attr, readStuff, 0);
  }
  if (rc) return rc;
}

for(i = 0; i < N_THREADS; i++) {
  rc = pthread_join(threads[i], (void*) &status);
//  if(rc == 3) 
      printf("rc is %d. i is %d\n", rc, i);
//  if (rc) return rc;
  if (status) return status;
  printf(".");
}

pthread_attr_destroy(&attr);

return 0;

工作线程：

void* readStuff(void* a)
{
  int i;
  for (i = 0; i< N_RUNS; i++){
  ;
  }
  pthread_exit((void*)0);
}

void* addRemove(void* a)
{
   int i;
   for (i = 0; i< N_RUNS; i++){
   ;
   }
   pthread_exit((void*)0);
}

除了主线程和上面代码中创建的线程外，没有其他线程。

Compileable example

Answer 1

我认为您的问题出在 pthread_join 上。来自手册页：

       int pthread_join(pthread_t thread, void **retval);
       ...

       If  retval is not NULL, then pthread_join() copies the exit status of the tar‐
       get  thread  (i.e.,  the  value   that   the   target   thread   supplied   to
       pthread_exit(3))  into  the  location  pointed  to  by *retval.  If the target
       thread was canceled, then PTHREAD_CANCELED is placed in *retval.

请注意，它需要一个 void **，这意味着它会用 void *（64 位上的大小 8）覆盖 retval 指向的内容。您正在传递一个 int *（即 &status），它是一个指向大多数平台上大小为 4 的对象的指针。

因此，pthread_join 将覆盖内存。相反，根据函数原型将 status 声明为 void *。

您也在测试status；我不知道你想在这里实现什么。

一般来说，使用 -Wall 编译会显示这些错误。

pthread_join 似乎修改了我的循环索引

pthread_join seems to modify my loop index

c

multithreading

for-loop

pthreads

pthread-join