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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 waitingfor 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 meanaccessing 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 isinvalid or we could not allocate memory. Note we have totranslate 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 thethread 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
所以要获取线程栈大小，需要使用两个函数调用：
pthread_getattr_np(pthread_self(), &attr)
pthread_attr_getstacksize(&attr, &stack_size)