wait syscall

wait 和 exit 都在 kernel/exit.c 下

阅读 man

The wait() system call suspends execution of the calling thread until one of its children terminates. The call wait(&wstatus) is equivalent to:

waitpid(-1, &wstatus, 0);

waitid()

The waitid() system call (available since Linux 2.6.9) provides more precise control over which child state changes to wait for.

The wait3() and wait4() system calls are similar to waitpid(2), but additionally re‐ turn resource usage information about the child in the structure pointed to by rusage.

Other than the use of the rusage argument, the following wait3() call:

wait3(wstatus, options, rusage);

is equivalent to:

waitpid(-1, wstatus, options);

Similarly, the following wait4() call:

wait4(pid, wstatus, options, rusage);

is equivalent to:

waitpid(pid, wstatus, options);

和 fork 系列类似，三个是逐级递增的，wait / waitpid / wait

/* Wait for a child matching PID to die.
   If PID is greater than 0, match any process whose process ID is PID.
   If PID is (pid_t) -1, match any process.
   If PID is (pid_t) 0, match any process with the
   same process group as the current process.
   If PID is less than -1, match any process whose
   process group is the absolute value of PID.
   If the WNOHANG bit is set in OPTIONS, and that child
   is not already dead, return (pid_t) 0.  If successful,
   return PID and store the dead child's status in STAT_LOC.
   Return (pid_t) -1 for errors.  If the WUNTRACED bit is
   set in OPTIONS, return status for stopped children; otherwise don't.

   This function is a cancellation point and therefore not marked with
   __THROW.  */
extern __pid_t waitpid (__pid_t __pid, int *__stat_loc, int __options);

• If a child has already changed state, then these calls return immediately.
  • 只要发生过状态转变, 而不是在调用函数之后进行这些转变
• WNOWAIT Leave the child in a waitable state; a later wait call can be used to again retrieve the child status information.
  • WNOWAIT 的作用应该是, 之后表示还需要获取信息的，不要清空信息，否则默认清空(tlpi 找到佐证)

试图读读代码

• waitid
  • kernel_waitid : 根据 idtype 获取正确的 struct pid, 组装 struct wait_opts
    • do_wait : 将当前 current 加入 wait_queue, 等待 child 调用 __wake_up_parent
      • add_wait_queue(&current->signal->wait_chldexit, &wo->child_wait);
        • 难道说，current->signal->wait_chldexit 上可以放置多个 wo->child_wait（因为不会，只是为了利用 wait queue 的这个机制)
      • do_wait_thread : 首先检查一次有没有符合要求的，如果没有，那么就利用 wait_queue 睡眠，直到 child 通知自己
        • wait_consider_task
          • eligible_child : 检查这个 child 是不是应该被 wait 的
          • wait_task_zombie : 用于 WNOHANG 和 EXITED
          • wait_task_stopped
          • wait_task_continued
• 一个 child exit 了，怎么知道那些 parent 都在等待, 让后去 wake up 他们?
  • 就是通过 do_notify_parent : Let a parent know about the death of a child. For a stopped/continued status change, use do_notify_parent_cldstop instead.
• do_notify_parent_cldstop

void __wake_up_parent(struct task_struct *p, struct task_struct *parent)
{
	__wake_up_sync_key(&parent->signal->wait_chldexit,
			   TASK_INTERRUPTIBLE, p);
}

waitpid vs pthread_join

• waitpid 的操作对象是 process ，换言之，waitid 的第一个参数是 pid 而不是 tid 而 pthread_join 是等待 thread

参考: https://stackoverflow.com/questions/11295298/waiting-for-threads-of-another-process-using-waitpid

等待整理

kernel/sys.c

1. pid
2. rlimit
3. prctl

A process can set and reset the keep_capabilities flag by means of the Linux-specific prctl() system call.

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