一，进程终止有5种方式：
正常退出：
异常退出：
调用abort 由信号终止 二，exit和_exit区别：
关于_exit()：
#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(2) family of calls.
The function _Exit() is equivalent to _exit().
关于exit()：
#include <stdlib.h>
void exit(int status);
DESCRIPTION The exit() function causes normal process termination and the value of status & 0377 is returned to the parent (see wait(2)).
All functions registered with atexit(3) and on_exit(3) are called, in the reverse order of their registration. (It is possible for one of these functions to use atexit(3) or on_exit(3) to register an addi- tional function to be executed during exit processing; the new regis- tration is added to the front of the list of functions that remain to be called.) If one of these functions does not return (e.g., it calls _exit(2), or kills itself with a signal), then none of the remaining functions is called, and further exit processing (in particular, flush- ing of stdio(3) streams) is abandoned. If a function has been regis- tered multiple times using atexit(3) or on_exit(3), then it is called as many times as it was registered.
All open stdio(3) streams are flushed and closed. Files created by tmpfile(3) are removed.
The C standard specifies two constants, EXIT_SUCCESS and EXIT_FAILURE, that may be passed to exit() to indicate successful or unsuccessful termination, respectively.
和exit比较一下，exit()函数定义在stdlib.h中，而_exit()定义在unistd.h中，
注：exit()就是退出，传入的参数是程序退出时的状态码，0表示正常退出，美国服务器，其他表示非正常退出，一般都用-1或者1，标准C里有EXIT_SUCCESS和EXIT_FAILURE两个宏，网站空间，用exit(EXIT_SUCCESS);
_exit()函数的作用最为简单：直接使进程停止运行，清除其使用的内存空间，并销毁其在内核中的各种数据结构；exit() 函数则在这些基础上作了一些包装，在执行退出之前加了若干道工序。exit()函数与_exit()函数最大的区别就在于exit()函数在调用exit系统调用之前要检查文件的打开情况，把文件缓冲区中的内容写回文件，就是"清理I/O缓冲"。
exit()在结束调用它的进程之前，要进行如下步骤： 1.调用atexit()注册的函数（出口函数）；按ATEXIT注册时相反的顺序调用所有由它注册的函数,这使得我们可以指定在程序终止时执行自己的清理动作.例如,保存程序状态信息于某个文件,解开对共享数据库上的锁等.
2.cleanup()；关闭所有打开的流，这将导致写所有被缓冲的输出，删除用TMPFILE函数建立的所有临时文件.
3.最后调用_exit()函数终止进程。
_exit做3件事（man）： 1，香港空间，Any open file descriptors belonging to the process are closed 2，any children of the process are inherited by process 1, init 3，the process’s parent is sent a SIGCHLD signal
exit执行完清理工作后就调用_exit来终止进程。
三，atexit()
atexit可以注册终止处理程序，ANSI C规定最多可以注册32个终止处理程序。
终止处理程序的调用与注册次序相反
#include <stdlib.h>
int atexit(void (*function)(void));
向上攀爬的。
