_exit():直接使進程終止運行，清除其使用的記憶體空間，並銷毀其在核心中的各種數據結構。
NAME
_exit, _Exit - terminate the current process
SYNOPSIS
#include <unistd.h>
void _exit(int status);
#include <stdlib.h>
void _Exit(int status);
DESCRIPTION
The function _exit() terminates the calling process "immediately". Any
open file descriptors belonging to the process are closed; any children
of the process are inherited by process 1, init, and the process’s par-
ent is sent a SIGCHLD signal.
The value status is returned to the parent process as the process’s
exit status, and can be collected using one of the wait() family of
calls.
The function _Exit() is equivalent to _exit().
RETURN VALUE
These functions do not return.
CONFORMING TO
SVr4, POSIX.1-2001, 4.3BSD. The function _Exit() was introduced by
C99.
NOTES
For a discussion on the effects of an exit, the transmission of exit
status, zombie processes, signals sent, etc., see exit(3).
The function _exit() is like exit(), but does not call any functions
registered with atexit() or on_exit(). Whether it flushes standard I/O
buffers and removes temporary files created with tmpfile(3) is imple-
mentation dependent. On the other hand, _exit() does close open file
descriptors, and this may cause an unknown delay, waiting for pending
