zombie

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於程式中查看到signal(SIGCHLD,SIG_IGN)，了解一下用意

更新記錄

item note
20160908 第一版

目錄

彊屍程序(zombie)

僵屍 (zombie) 程序是什麼

造成僵屍程序的成因是因為該程序應該已經執行完畢，或者是因故應該要終止了， 但是該程序的父程序卻無法完整的將該程序結束掉，而造成那個程序一直存在記憶體當中

ps

  • ps

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    xx :[~]# ps
      PID USER       VSZ STAT COMMAND
        1 root      2164 S    init
        2 root         0 SW   [kthreadd]
        3 root         0 SW   [ksoftirqd/0]
    xx 
    1315 root      1604 S    ./a.out
    1316 root         0 Z    [a.out]
    1317 root         0 Z    [a.out]
    1318 root         0 Z    [a.out]
    1319 root         0 Z    [a.out]
    1320 root         0 Z    [a.out]

  • VSZ : 該 process 使用掉的虛擬記憶體量 (Kbytes)

  • STAT : 該程序目前的狀態，狀態顯示與 ps -l 的 S 旗標相同 (R/S/T/Z)
    • R (Running)
    • S (Sleep)：該程式目前正在睡眠狀態(idle)，但可以被喚醒(signal)
    • T ：停止狀態(stop)，可能是在工作控制(背景暫停)或除錯 (traced) 狀態
    • Z (Zombie)：僵屍狀態，程序已經終止但卻無法被移除至記憶體外

example (未加入SIGCHLD訊息處理)

  • 未加入SIGCHLD訊息處理

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

    int main(){
            int i;
            pid_t pid;

            for( i=0; i<5; i++) {
                    pid = fork();
                    if ( pid == 0 )
                            break;
            }
            if( pid > 0 ){
                    printf("press enter to exit ..");
                    getchar();
            }
            return 0;
    }

  • 結果如下
    當子程序結束，父程序仍在
    而父程序fork()之前未設置SIGCHILD信號處理或waitpid等待子程序結束
    此時會產生 僵屍程序(zombie), 若父程序一直存在不會結束，就會看到一堆zombie

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    1315 root      1604 S    ./a.out
    1316 root         0 Z    [a.out]
    1317 root         0 Z    [a.out]
    1318 root         0 Z    [a.out]
    1319 root         0 Z    [a.out]
    1320 root         0 Z    [a.out]

example (加入SIGCHLD訊息處理)

  • 加入SIGCHLD訊息處理

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    #include <stdio.h>
    #include <unistd.h>
    #include <sys/signal.h>

    int main(){
           int i;
           pid_t pid;
            signal(SIGCHLD, SIG_IGN);
            for( i=0; i<5; i++) {
                   pid = fork();
                   if ( pid == 0 )
                           break;
           }
           if( pid > 0 ){
                   printf("press enter to exit ..");
                   getchar();
           }
           return 0;
    }

  • 測試

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    1454 root      1604 S    ./a.out
    1460 root      2168 R    ps

fork

  • The purpose of fork() is to create a new process, which becomes the child process of the caller
  • If fork() returns a negative value, the creation of a child process was unsuccessful.
  • fork() returns a zero to the newly created child process.
  • fork() returns a positive value, the process ID of the child process, to the parent.
  • The fork() System Call

fork example

  • ex1

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    #include <stdio.h>
    #include <string.h>
    #include <sys/types.h>

    void main(void)
    {
            pid_t pid;
            int     i;
            char    buf[100];

            fork();
            pid = getpid();
            for(i =1; i <= 200; i++){
                    sprintf(buf, "This line is from pid %d, value = %d \n", pid, i);
                    write(1,buf,strlen(buf));
            }
    }

  • 測試

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    xx :[~]# ./a.out 
    This line is from pid 1498, value = 1 
    This line is from pid 1498, value = 2 
    This line is from pid 1498, value = 3 
    This line is from pid 1498, value = 4 
    This line is from pid 1498, value = 5 
    This line is from pid 1498, value = 6 
    This line is from pid 1498, value = 7 
    This line is from pid 1498, value = 8 
    This line is from pid 1498, value = 9 
    This line is from pid 1499, value = 1 
    This line is from pid 1498, value = 10 
    This line is from pid 1499, value = 2

fork之後會產生1個子程序，跟父程序同時進行，因此會看到有兩個pid，獨立算數
fork出來的子程序是由原本的父程序複製全部的變數(kernel需要的系統參數)，因此是跟父程序獨立分開的process

fork ex2

  • fork ex2

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    #include <stdio.h>
    #include <string.h>
    #include <unistd.h>
    #include <sys/types.h>

    void ChildProcess(){
            pid_t pid;
            pid = getpid();
            printf("child pid:%d\n",pid);
            getchar();
    }
    void ParentProcess(){
            pid_t pid;
            pid = getpid();
            printf("parent pid:%d\n",pid);
            getchar();
    }
    void main(void)
    {
            pid_t pid;
            int     i;
            char    buf[100];

            pid = fork();
            if ( pid == 0 )
                    ChildProcess();
            else
                    ParentProcess();

    }

  • 測試

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    1559 root      1604 S    ./a.out
    1560 root      1604 S    ./a.out

  • fork回傳0則為child process

參考