glibc 里的线程 id

news2024/11/29 0:40:55

这里讲的是通过 pthread_create() 函数返回的线程 id，其实就是 glibc 库里维护的线程id，它跟内核维护的线程 id 不一样，pthread_create() 返回的线程 id 并不一定是唯一的。我们看 pthread_create 函数的源码，它最后返回的线程 id 是什么样的？

      //...............................
      /* Check for valid priorities.  */
      int minprio = INTERNAL_SYSCALL (sched_get_priority_min, scerr, 1,
				      iattr->schedpolicy);
      int maxprio = INTERNAL_SYSCALL (sched_get_priority_max, scerr, 1,
				      iattr->schedpolicy);
      if (pd->schedparam.sched_priority < minprio
	  || pd->schedparam.sched_priority > maxprio)
	{
	  /* Perhaps a thread wants to change the IDs and if waiting
	     for this stillborn thread.  */
	  if (__builtin_expect (atomic_exchange_acq (&pd->setxid_futex, 0)
				== -2, 0))
	    lll_futex_wake (&pd->setxid_futex, 1, LLL_PRIVATE);

	  __deallocate_stack (pd);

	  return EINVAL;
	}
    }

  /* Pass the descriptor to the caller.  */
  *newthread = (pthread_t) pd;

  LIBC_PROBE (pthread_create, 4, newthread, attr, start_routine, arg);

  /* Start the thread.  */
  return create_thread (pd, iattr, STACK_VARIABLES_ARGS);
}

函数就是通过参数 newthread 返回的线程 id，实现里直接把 pd 强转成 pthread_t 类型赋值给了 newthread。这个 pd 是什么类型呢？

int
__pthread_create_2_1 (newthread, attr, start_routine, arg)
     pthread_t *newthread;
     const pthread_attr_t *attr;
     void *(*start_routine) (void *);
     void *arg;
{
  STACK_VARIABLES;

  const struct pthread_attr *iattr = (struct pthread_attr *) attr;
  if (iattr == NULL)
    /* Is this the best idea?  On NUMA machines this could mean
       accessing far-away memory.  */
    iattr = &default_attr;

  struct pthread *pd = NULL;
  int err = ALLOCATE_STACK (iattr, &pd);
  if (__builtin_expect (err != 0, 0))
    /* Something went wrong.  Maybe a parameter of the attributes is
       invalid or we could not allocate memory.  Note we have to
       translate error codes.  */
    return err == ENOMEM ? EAGAIN : err;

struct pthread 是一个结构体，记录线程信息的：

所以这个线程 id 就是一个地址（被强转成pthread_t，也就是 unsigned long int 类型值）值？用 GDB 看一下最清楚了：

这个 pd 指针的值就是 0x7ffff7fd8700，而这个值也是结构体里数据的首地址：

所以这个地址值转成 unsignd long int 是多少呢？强转后的十进制值就是 140737353975552。

就是执行程序里打印的线程 id。所以 pthread_create() 返回的线程 id，其实就是保存线程信息的结构体的首地址。因此这个线程 id 是很重要的，其他线程相关函数可以通过这个 id 访问线程信息，如 pthread_t 强转为 struct pthread* 类型，其实 thread_id 就是一个地址：

int
pthread_getattr_np (thread_id, attr)
     pthread_t thread_id;
     pthread_attr_t *attr;
{
  struct pthread *thread = (struct pthread *) thread_id;
  struct pthread_attr *iattr = (struct pthread_attr *) attr;
  int ret = 0;

  lll_lock (thread->lock, LLL_PRIVATE);

  /* The thread library is responsible for keeping the values in the
     thread desriptor up-to-date in case the user changes them.  */
  memcpy (&iattr->schedparam, &thread->schedparam,
	  sizeof (struct sched_param));
  iattr->schedpolicy = thread->schedpolicy;

  /* Clear the flags work.  */
  iattr->flags = thread->flags;

  /* The thread might be detached by now.  */
  if (IS_DETACHED (thread))
    iattr->flags |= ATTR_FLAG_DETACHSTATE;

  /* This is the guardsize after adjusting it.  */
  iattr->guardsize = thread->reported_guardsize;

这个函数就是获取指定线程的信息，其中当然包括用户设置的栈大小