为什么没有 volatile 声明此代码会失败？ [复制]

Question

我有以下代码，来自一本书，在 Ubuntu 18.4 上编译，使用 Eclipse -O0 -g3

#include <pthread.h>
#include <stdio.h>

void *myThread( void *arg )
{
  printf( "Thread %d started\n", (int)arg );

  pthread_exit( arg );
}

#define MAX_THREADS 5

int main()
{
  volatile int  i;
  int status;
  int ret;
  pthread_t threadIds[MAX_THREADS];

  for (i = 0 ; i < MAX_THREADS ; i++) {
    ret = pthread_create( &threadIds[i], NULL, myThread, (void *)i );
    if (ret != 0) {
      printf( "Error creating thread %d\n", (int)threadIds[i] );
    }
  }

  for (i = 0 ; i < MAX_THREADS ; i++) {
    printf("-----------  i = %d\n",i);
    ret = pthread_join( threadIds[i], (void **)&status );
    if (ret != 0) {
      printf( "Error joining thread %d\n", (int)threadIds[i] );
    } else {
      printf( "Status = %d\n", status );
    }
  }

  return 0;
}

这段代码正常工作并产生了输出

-----------  i = 0
Thread 3 started
Thread 0 started
Status = 0
-----------  i = 1
Thread 2 started
Thread 1 started
Status = 1
-----------  i = 2
Status = 2
-----------  i = 3
Status = 3
-----------  i = 4
Thread 4 started
Status = 4

但是，如果变量 i 声明处的 volatile 关键字被忽略，则会产生以下输出：

Thread 1 started
Thread 0 started
Thread 2 started
Thread 3 started
-----------  i = 0
Status = 0
-----------  i = 1
Status = 1
-----------  i = 1
Error joining thread -947054848
-----------  i = 2
Status = 2
-----------  i = 1
Error joining thread -947054848
-----------  i = 2
Error joining thread -955447552
-----------  i = 3
Status = 3
-----------  i = 1
Error joining thread -947054848
-----------  i = 2
Error joining thread -955447552
-----------  i = 3
Error joining thread -963840256
-----------  i = 4
Thread 4 started
Status = 4
-----------  i = 1
Error joining thread -947054848
-----------  i = 2
Error joining thread -955447552
-----------  i = 3
Error joining thread -963840256
-----------  i = 4
Error joining thread -1073744128

请帮助我确定导致此问题的可能原因。

Answer 1

在您的代码中：

int status;

// ...

pthread_join( threadIds[i], (void **)&status );

这是一个严格的别名违规。 pthread_join 将 void * 类型的值写入该位置，但是您提供了 int 的位置。这会导致未定义的行为，这意味着任何事情都可能发生。

您的系统上发生的可能是写入越界（int = 4 字节，void * = 8 字节），导致堆栈混乱。

您可以通过以下方式解决此问题：

void *status;
// ...
pthread_join( threadIds[i], &status );

以后，如果您想使用%d 打印状态，请使用(int)status 作为参数。