linux系統程式設計之程式（四）：程式退出exit，_exit區別即atexit函式

mickole發表於2013-07-12
Linux程式設計函式

一，程式終止有5種方式：

正常退出：

  • 從main函式返回
  • 呼叫exit
  • 呼叫_exit

異常退出：

  • 呼叫abort
  • 由訊號終止

二，exit和_exit區別：

QQ截圖20130712154916

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()函式定義在stdlib.h中，而_exit()定義在unistd.h中，

注：exit()就是退出，傳入的引數是程式退出時的狀態碼，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兩個巨集，用exit(EXIT_SUCCESS);

_exit()函式的作用最為簡單：直接使程式停止執行，清除其使用的記憶體空間，並銷燬其在核心中的各種資料結構；exit() 函式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()函式與_exit()函式最大的區別就在於exit()函式在呼叫exit系統呼叫之前要檢查檔案的開啟情況，把檔案緩衝區中的內容寫回檔案，就是"清理I/O緩衝"。

exit()在結束呼叫它的程式之前，要進行如下步驟：
1.呼叫atexit()註冊的函式（出口函式）；按ATEXIT註冊時相反的順序呼叫所有由它註冊的函式,這使得我們可以指定在程式終止時執行自己的清理動作.例如,儲存程式狀態資訊於某個檔案,解開對共享資料庫上的鎖等.

2.cleanup()；關閉所有開啟的流，這將導致寫所有被緩衝的輸出，刪除用TMPFILE函式建立的所有臨時檔案.

3.最後呼叫_exit()函式終止程式。

_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執行完清理工作後就呼叫_exit來終止程式。

三，atexit()

atexit可以註冊終止處理程式，ANSI C規定最多可以註冊32個終止處理程式。

終止處理程式的呼叫與註冊次序相反

       #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.

示例程式：

#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);
}

執行結果：

QQ截圖20130712162502

 

當呼叫fork時，子程式繼承父程式註冊的atexit：

示例程式：

#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;
}

執行結果：

QQ截圖20130712163016

 

當atexit註冊的函式中有一個沒有正常返回或被kill，則後續的註冊函式都不會被執行

示例程式：

#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);
}

執行結果：

QQ截圖20130712164236

可見最後那個fun1沒有執行

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