pthread_create

int pthread_create(pthread_t
 *restrict thread,
              const pthread_attr_t *restrict attr,
              void *(*start_routine)(void*), void *restrict arg);

以NULL attr, static linked为例子

1. 使用default_attr;

2. 分配线程栈，struct pthread结构放在栈底（添加或者删除元素的一端叫栈顶，另一端叫栈底）
    pthread代表新的线程,  pthread结构上面是TLS（thread local storage）空间，它的内容为程序中
   可执行文件中的（libc初始化时会查出来在哪）副本， 这样每个线程都有一份
   _______________
   |struct pthread|
   |      TLS            |

   STACK top

3.   pd->start_routine = start_routine;
      pd->arg = arg;

4.  调用clone函 数， 建立新线程，它的起始函数为start_thread, 该函数的参数为刚刚分配的pthread结构
     ！ 用户传的start_routine在pthread中有
     给clone传递的tls参数和初始化的时候用set_thread_area一样，除了base_addr指向自身的
     TLS之外
     
    

/* The "thread register" gets initialized from a segment descriptor.
   Initialize such a descriptor first.  */
#define PREPARE_CREATE \
  union user_desc_init desc;                              \
                                          \
  /* Describe the thread-local storage segment.  */                  \
                                          \
  /* The 'entry_number' field.  The first three bits of the segment          \
     register value select the GDT, ignore them.  We get the index          \
     from the value of the %gs register in the current thread.  */          \
  desc.vals[0] = TLS_GET_GS () >> 3;                          \
  /* The 'base_addr' field.  Pointer to the TCB.  */                  \
  desc.vals[1] = (unsigned long int) pd;                      \
  /* The 'limit' field.  We use 4GB which is 0xfffff pages.  */              \
  desc.vals[2] = 0xfffff;                              \
  /* Collapsed value of the bitfield:                          \
       .seg_32bit = 1                                  \
       .contents = 0                                  \
       .read_exec_only = 0                              \
       .limit_in_pages = 1                              \
       .seg_not_present = 0                              \
       .useable = 1 */                                  \
  desc.vals[3] = 0x51

 

  

5.  start_thread 调用 pd->start_routine,  线程的起始函数pd->start_routine的被调用和整个程序
      的main函数被调用的方式类似
    

static int
start_thread (void *arg)
{
  struct pthread *pd = (struct pthread *) arg;

  struct pthread_unwind_buf 
unwind_buf;

  int not_first_call;
  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
  会两次到这里来，一次是开始，一次是 _被_ 
结束或者主动调用pthread_exit
  否则只有一次，start_routine可能直接返回
  if (! 
not_first_call)
    {
      /* Run the code the user 
provided.  */
         pd->start_routine
 (pd->arg));

    }

  /* Run the destructor for the 
thread-local data.  */
  __nptl_deallocate_tsd ();


  /* If this is the last thread 
we terminate the process now.  We
     do not notify the debugger, it might just irritate it if there
     is no thread left.  */
  最后一个线程
  if (__builtin_expect (atomic_decrement_and_test (&__nptl_nthreads), 0))
    /* This was the last thread. 
 */
    exit (0);

  /* We cannot call '_exit' here.
  '_exit' will terminate the process.

     The 'exit' implementation in the kernel will signal when the
     process is really dead since 'clone' got passed the CLONE_CLEARTID
     flag.  The 'tid' field in the TCB will be set to zero.

     The exit code is zero since in case all threads exit by calling
     'pthread_exit' the exit status must be 0 (zero).  */
  __exit_thread_inline (0);

  /* NOTREACHED */
  return 0;
}