waitpid

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <stdlib.h>

int main(int argc, char* argv[])
{
    pid_t pid1,pid2,wait_pid;
    int status;

    if(0 == (pid1 = fork())) /* 子进程1 */
    {
        printf("child process1 :%d has created. \n", getpid());
        pause();
        _exit(0);
    }
    else if(0 == (pid2 = fork()))  /* 子进程2 */
    {
        printf("child process2 :%d has created. \n",getpid());
        pause();
        _exit(0);
    }
    else /* 父进程 */
    {
        while((wait_pid = waitpid(pid1,&status,WUNTRACED|WCONTINUED)) && (-1 != wait_pid))
        {
            if(WIFEXITED(status))
            {
                printf("child process %d exites, status:%d.\n", wait_pid, WEXITSTATUS(status));
            }
            if(WIFSIGNALED(status))
            {
                printf("child process %d is killed by signal:%d.\n", wait_pid, WTERMSIG(status));
            }
            if(WIFSTOPPED(status))
            {
                printf("child process %d is stopped by signal:%d.\n", wait_pid, WSTOPSIG(status));
            }
            if(WIFCONTINUED(status))
            {
                printf("child process %d is resume running...\n", wait_pid);
            }
        }
    }
    _exit(0);
    return 0;
}

运行结果：
lisong@lisong:~/code/experiment/proc/wait_pid$ ./waitpid &
[1] 2996
child process1 :2997 has created.
child process2 :2998 has created.
lisong@lisong:~/code/experiment/proc/wait_pid$ ps
  PID TTY          TIME CMD
 2181 pts/1    00:00:01 bash
 2996 pts/1    00:00:00 waitpid
 2997 pts/1    00:00:00 waitpid
 2998 pts/1    00:00:00 waitpid
 3003 pts/1    00:00:00 ps
lisong@lisong:~/code/experiment/proc/wait_pid$ kill -19 2997
child process 2997 is stopped by signal:19.
lisong@lisong:~/code/experiment/proc/wait_pid$ kill -19 2998
lisong@lisong:~/code/experiment/proc/wait_pid$ ps
  PID TTY          TIME CMD
 2181 pts/1    00:00:01 bash
 2996 pts/1    00:00:00 waitpid
 2997 pts/1    00:00:00 waitpid
 2998 pts/1    00:00:00 waitpid
 3014 pts/1    00:00:00 ps
lisong@lisong:~/code/experiment/proc/wait_pid$ kill -18 2998
lisong@lisong:~/code/experiment/proc/wait_pid$ ps
  PID TTY          TIME CMD
 2181 pts/1    00:00:01 bash
 2996 pts/1    00:00:00 waitpid
 2997 pts/1    00:00:00 waitpid
 2998 pts/1    00:00:00 waitpid
 3015 pts/1    00:00:00 ps
lisong@lisong:~/code/experiment/proc/wait_pid$ kill -18 2997
child process 2997 is resume running...
lisong@lisong:~/code/experiment/proc/wait_pid$ ps
  PID TTY          TIME CMD
 2181 pts/1    00:00:01 bash
 2996 pts/1    00:00:00 waitpid
 2997 pts/1    00:00:00 waitpid
 2998 pts/1    00:00:00 waitpid
 3020 pts/1    00:00:00 ps
lisong@lisong:~/code/experiment/proc/wait_pid$
lisong@lisong:~/code/experiment/proc/wait_pid$
lisong@lisong:~/code/experiment/proc/wait_pid$ kill -9 2998
lisong@lisong:~/code/experiment/proc/wait_pid$ ps
  PID TTY          TIME CMD
 2181 pts/1    00:00:01 bash
 2996 pts/1    00:00:00 waitpid
 2997 pts/1    00:00:00 waitpid
 2998 pts/1    00:00:00 waitpid
 3022 pts/1    00:00:00 ps
lisong@lisong:~/code/experiment/proc/wait_pid$ kill -9 2997
child process 2997 is killed by signal:9.
lisong@lisong:~/code/experiment/proc/wait_pid$ ps
  PID TTY          TIME CMD
 2181 pts/1    00:00:01 bash
 3027 pts/1    00:00:00 ps
[1]+  完成                  ./waitpid

分析：
可以看到，对子进程2（pid：2998） 发送信号，并没有得到父进程处理。杀死子进程2之后，由于父进程不处理，子进程2变成了僵尸进程(defunct)。当父进程结束之后，子进程2被init进程消灭。