一，进程终止有5种方式：

正常退出：

从main函数返回

调用exit

调用_exit

异常退出：

调用abort

由信号终止

二，exit和_exit区别：

关于_exit()：

#include

void _exit(int status);

#include

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

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，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，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，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

int atexit(void (*function)(void));

DESCRIPTION

The atexit() function registers the given function to be called at nor-

mal process termination, either via exit(3) or via return from the pro-

gram’s main(). Functions so registered are called in the reverse order

of their registration; no arguments are passed.

The  same  function may be registered multiple times: it is called once

for each registration.

POSIX.1-2001 requires that an implementation allow at least  ATEXIT_MAX

(32) such functions to be registered.  The actual limit supported by an

implementation can be obtained using sysconf(3).

When a child process is created via fork(2), it inherits copies of  its

parent’s  registrations.   Upon a successful call to one of the exec(3)

functions, all registrations are removed.

RETURN VALUE

The atexit() function returns the value 0 if successful;  otherwise  it

returns a non-zero value.

示例程序：

#include

#include

#include

void fun1()

{

printf("fun1 is called\n");

}

void fun2()

{

printf("fun2 is called\n");

}

int main(void)

{

printf("main function\n");

atexit(fun1);

atexit(fun2);

atexit(fun1);

exit(EXIT_SUCCESS);

}

运行结果：

当调用fork时，子进程继承父进程注册的atexit：

示例程序：

#include

#include

#include

#define ERR_EXIT(m) \

do\

{\

perror(m);\

exit(EXIT_FAILURE);\

}\

while (0)\

void fun1()

{

printf("fun1 is called\n");

}

void fun2()

{

printf("fun2 is called\n");

}

int main(void)

{

pid_t pid;

pid = fork();

atexit(fun1);

atexit(fun2);

atexit(fun1);

if(pid == -1)

ERR_EXIT("fork error");

if(pid == 0){

printf("this is child process\n");

}

if(pid > 0){

printf("this is parent process\n");

}

return 0;

}

运行结果：

当atexit注册的函数中有一个没有正常返回或被kill，则后续的注册函数都不会被执行

示例程序：

#include

#include

#include

#include

void fun1()

{

printf("fun1 is called\n");

}

void fun2()

{

printf("fun2 is called\n");

kill(getpid(),SIGINT);

}

int main(void)

{

printf("main function\n");

if(signal(SIGINT,SIG_DFL) == SIG_ERR){

perror("signal error");

exit(EXIT_FAILURE);

}

atexit(fun1);

atexit(fun2);

atexit(fun1);

exit(EXIT_SUCCESS);

}

运行结果：

可见最后那个fun1没有执行