linux系統編程之進程（四）：進程退出exit，_exit區別即atexit函數
一，進程終止有5種方式：
- 正常退出：
- 從main函數返回
- 調用exit
- 調用_exit
- 異常退出：
- 調用abort
- 由信號終止
二，exit和_exit區別：
- 關於_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().
- 關於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比較一下，exit()函數定義在stdlib.h中，而_exit()定義在unistd.h中，
注：exit()就是退出，傳入的參數是程序退出時的狀態碼，0表示正常退出，其他表示非正常退出，一般都用-1或者1，標準C裡有EXIT_SUCCESS和EXIT_FAILURE兩個宏，用exit(EXIT_SUCCESS);
_exit()函數的作用最為簡單：直接使進程停止運行，清除其使用的內存空間，並銷燬其在內核中的各種數據結構；exit() 函數則在這些基礎上作了一些包裝，在執行退出之前加了若干道工序。
exit()函數與_exit()函數最大的區別就在於exit()函數在調用exit系統調用之前要檢查文件的打開情況，把文件緩衝區中的內容寫迴文件，就是"清理I/O緩衝"。exit()在結束調用它的進程之前，要進行如下步驟：
- 調用atexit()註冊的函數（出口函數）；按ATEXIT註冊時相反的順序調用所有由它註冊的函數,這使得我們可以指定在程序終止時執行自己的清理動作.例如,保存程序狀態信息於某個文件,解開對共享數據庫上的鎖等.
- cleanup()；關閉所有打開的流，這將導致寫所有被緩衝的輸出，刪除用TMPFILE函數建立的所有臨時文件.
- 最後調用_exit()函數終止進程。
_exit做3件事（man）：
- Any open file descriptors belonging to the process are closed
- any children of the process are inherited by process 1, init
- the process's parent is sent a SIGCHLD signal
exit執行完清理工作後就調用_exit來終止進程。
三，atexit()
atexit可以註冊終止處理程序，ANSI C規定最多可以註冊32個終止處理程序。
終止處理程序的調用與註冊次序相反
#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 <stdio.h>
#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);
}
運行結果：
當調用fork時，子進程繼承父進程註冊的atexit：
示例程序：
#include <stdio.h>
#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;
}
運行結果：
當atexit註冊的函數中有一個沒有正常返回或被kill，則後續的註冊函數都不會被執行
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);
}
運行結果：
可見最後那個fun1沒有執行