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

         printf("second child pid=%d\n", 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();
         printf("second child, parent pid = %d, second child exit\n", getppid());
         exit();
     }

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

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\n", WEXITSTATUS(status));
     else if (WIFSIGNALED(status))
         printf("abnormal termination, signal number = %d%s\n", WTERMSIG(status),
 #ifdef WCOREDUMP
         WCOREDUMP(status) ? ("core file generated") : (""));
 #else
     "");
 #endif
     else if (WIFSTOPPED(status))
         printf("child stopped, signal number=%d\n", WSTOPSIG(status));
 }

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

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

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

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

     exit();
 }

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\n", WEXITSTATUS(status));
     else if (WIFSIGNALED(status))
         printf("abnormal termination, signal number = %d%s\n", WTERMSIG(status),
 #ifdef WCOREDUMP
         WCOREDUMP(status) ? ("core file generated") : (""));
 #else
     "");
 #endif
     else if (WIFSTOPPED(status))
         printf("child stopped, signal number=%d\n", WSTOPSIG(status));
 }

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

 void sig_usr(int signo)
 {
     if (signo == SIGUSR1)
         printf("received SIGUSR1\n");
     else if (signo == SIGUSR2)
         printf("received SIGUSR2\n");
     else
         printf("received signal %d\n", 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()) < )
     {
         printf("fork error\n");
         return -;
     }
     else if (pid == )
     {
         printf("child exit\n");
         return ;
     }
     else
     {
         printf("parent sleep(100)\n");
         sleep();
         printf("parent exit\n");
     }

     exit();
 }

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\n", WEXITSTATUS(status));
     else if (WIFSIGNALED(status))
         printf("abnormal termination, signal number = %d%s\n", WTERMSIG(status),
 #ifdef WCOREDUMP
         WCOREDUMP(status) ? ("core file generated") : (""));
 #else
     "");
 #endif
     else if (WIFSTOPPED(status))
         printf("child stopped, signal number=%d\n", WSTOPSIG(status));
 }

 void sig_usr(int signo)
 {
     if (signo == SIGUSR1)
         printf("received SIGUSR1\n");
     else if (signo == SIGUSR2)
         printf("received SIGUSR2\n");
     else
         printf("received signal %d\n", 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 = |SA_NOCLDWAIT;
     sigaction(SIGCHLD, &act, &oact);

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

     exit();
 }