zombie process

僵尸进程：子进程退出后，父进程还没有回收子进程的资源，那么这个子进程就处于僵尸状态。
Q1:“资源”是些什么?
Q2:父进程如何回收子进程的资源?

内核为每个终止子进程保存了一定量的信息,所以当终止进程的父进程调用wait或waitpid时,可以得到这些信息。
这些信息至少包括进程ID,该进程的终止状态,以及该进程使用的CPU时间总量.
内核可以释放终止进程所使用的所有存储区,关闭其所有打开的文件.

如果编写一个长期运行的程序，他调用fork产生了很多子进程,那么除非父进程等待来取得子进程的终止状态,否则这些子进程终止后就会变成僵尸进程。

避免僵尸进程的方法
方法1：调用两次fork可以避免僵尸进程的产生。

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


int main(void)
{
    pid_t pid;

    if ((pid = fork()) < 0)
    {
        printf("fork error
");    
        exit(-1);
    }
    else if (0==pid) /*first child*/
    {
        printf("1first child pid=%d
", getpid());
        if ((pid = fork())<0)
        {
            printf("fork error
");    
            exit(-1);
        }
        else if (pid > 0) /*parent from second fork == first child*/
        {
            printf("2first child pid=%d, first child exit
", getpid());
            exit(0);//first child exit
        }

        printf("second child pid=%d
", getpid());
        /*I am the second child, my parent become init as soon as my real parent calls exit() in the statement above
         * Here's where we'd continue executing, knowing that when we're done, init will reap out status*/
        sleep(30);
        printf("second child, parent pid = %d, second child exit
", getppid());
        exit(0);
    }

    printf("pararent pid=%d
", getpid());
    if (waitpid(pid, NULL, 0) != pid) /*wait for first child*/
    {
        printf("waitpid error
");    
        exit(-1);
    }
    printf("parent exit");


    /*I am the parent(the original process); I continue executing, knowing that I'm not the parent of the second child*/

    exit(0);
}

方法2：当SIGCHLD的处理方式是系统默认时，父进程调用了wait()以避免僵尸进程的产生，此方法中，父进程阻塞。

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <errno.h>
#include <signal.h>

void print_exit(int status)
{
    if (WIFEXITED(status))
        printf("normal termination, exit status = %d
", WEXITSTATUS(status));
    else if (WIFSIGNALED(status))
        printf("abnormal termination, signal number = %d%s
", WTERMSIG(status),
#ifdef WCOREDUMP
        WCOREDUMP(status) ? ("core file generated") : (""));
#else
    "");
#endif
    else if (WIFSTOPPED(status))
        printf("child stopped, signal number=%d
", WSTOPSIG(status));
}

void sig_child(int signo)
{
    int status;
    int ret;
    ret = wait(&status);
    printf("pid:%d, res:%d, status=%d, %s
", getpid(), ret, status, strerror(errno));
    print_exit(status);
}

void sig_usr(int signo)
{
    if (signo == SIGUSR1)
        printf("received SIGUSR1
");
    else if (signo == SIGUSR2)
        printf("received SIGUSR2
");
    else
        printf("received signal %d
", signo);
}

int main(int argc, char** argv)
{
    pid_t pid;
    int status;
    int ret;

    if ((pid=fork()) < 0)
    {
        printf("fork error
");
        return -1;
    }
    else if (pid == 0)
    {
        printf("child exit
");
        return 0;
    }
    else
    {
        //printf("parent sleep(100)
");
        //sleep(100);
        ret = wait(&status);
        print_exit(status);
        printf("parent exit
");
    }


    exit(0);
}

方法3：当SIGCHLD的处理方式是捕获时，在其信号处理程序中调用wait()函数以避免僵尸进程的产生，此方法中，父进程不阻塞。

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <errno.h>
#include <signal.h>

void print_exit(int status)
{
    if (WIFEXITED(status))
        printf("normal termination, exit status = %d
", WEXITSTATUS(status));
    else if (WIFSIGNALED(status))
        printf("abnormal termination, signal number = %d%s
", WTERMSIG(status),
#ifdef WCOREDUMP
        WCOREDUMP(status) ? ("core file generated") : (""));
#else
    "");
#endif
    else if (WIFSTOPPED(status))
        printf("child stopped, signal number=%d
", WSTOPSIG(status));
}

void sig_child(int signo)
{
    int status;
    int ret;
    ret = wait(&status);
    printf("pid:%d, res:%d, status=%d, %s
", getpid(), ret, status, strerror(errno));
    print_exit(status);
}

void sig_usr(int signo)
{
    if (signo == SIGUSR1)
        printf("received SIGUSR1
");
    else if (signo == SIGUSR2)
        printf("received SIGUSR2
");
    else
        printf("received signal %d
", signo);
}

int main(int argc, char** argv)
{
    pid_t pid;
    struct sigaction act, oact;
    int status;
    int ret;

    act.sa_handler = sig_child;
    sigemptyset(&act.sa_mask);
    //act.sa_flags = 0|SA_NOCLDWAIT;
    sigaction(SIGCHLD, &act, &oact);

    if ((pid=fork()) < 0)
    {
        printf("fork error
");
        return -1;
    }
    else if (pid == 0)
    {
        printf("child exit
");
        return 0;
    }
    else
    {
        printf("parent sleep(100)
");
        sleep(100);
        printf("parent exit
");
    }


    exit(0);
}

从打印中可以看出，信号处理程序由父进程调用
方法4：设置SIGCHLD为SA_NOCLDWAIT,当子进程终止时，不创建僵尸进程。父进程中不需调用wait。

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <errno.h>
#include <signal.h>

void print_exit(int status)
{
    if (WIFEXITED(status))
        printf("normal termination, exit status = %d
", WEXITSTATUS(status));
    else if (WIFSIGNALED(status))
        printf("abnormal termination, signal number = %d%s
", WTERMSIG(status),
#ifdef WCOREDUMP
        WCOREDUMP(status) ? ("core file generated") : (""));
#else
    "");
#endif
    else if (WIFSTOPPED(status))
        printf("child stopped, signal number=%d
", WSTOPSIG(status));
}

void sig_usr(int signo)
{
    if (signo == SIGUSR1)
        printf("received SIGUSR1
");
    else if (signo == SIGUSR2)
        printf("received SIGUSR2
");
    else
        printf("received signal %d
", signo);
}

int main(int argc, char** argv)
{
    pid_t pid;
    struct sigaction act, oact;
    int status;
    int ret;

    act.sa_handler = sig_usr;
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0|SA_NOCLDWAIT;
    sigaction(SIGCHLD, &act, &oact);

    if ((pid=fork()) < 0)
    {
        printf("fork error
");
        return -1;
    }
    else if (pid == 0)
    {
        printf("child exit
");
        return 0;
    }
    else
    {
        printf("parent sleep(100)
");
        sleep(100);
        //ret = wait(&status);
        //print_exit(status);
        printf("parent exit
");
    }


    exit(0);
}