linux系统编程之进程（四）：进程退出exit，

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 <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，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 <stdlib.h>

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 <unistd.h>

#include <stdlib.h>

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 <unistd.h>
#include <stdlib.h>
#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 <stdio.h>

#include <unistd.h>

#include <stdlib.h>

#include <signal.h>

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