父进程或子进程的终止。
重要的是父进程必须以某种方式确认子进程的终止。
What is important is that the parent must somehow acknowledge the death of the child.
由于该进程已经退出，这相当于向内核确认了子进程的终止。
Because the process has exited already, this serves as the acknowledgement of the child's death to the kernel.
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