一、多进程创建和回收

• 孤儿进程：父进程先退出了，子进程没有退出，成为孤儿进程，父进程变成1号进程。
• 僵尸进程：父进程没有退出，子进程退出了，但是父进程没有回收子进程资源，导致子进程变成僵尸进程。

1. fork()

创建子进程函数，一个进程可以创建多个子进程。

pid_t fork(void)
返回值
    小于0 创建失败
    等于0 是子进程
    大于0 是父进程

示例：

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

int main()
{
   
    pid_t ret = fork();
    if (ret < 0)
    {
   
        perror("fork error");
        return -1;
    }
    else if (ret > 0)
    {
   
        printf("here is father,pid is [%d], child pid is [%d]\n", getpid(), ret);
        sleep(1);
    }
    else if (ret == 0)
    {
   
        printf("here is child pid is [%d],father is [%d]\n", getpid(), getppid());
    }

    return 0;
}

2. wait()

父进程回收子进程，其中分为wait()和waitpid()函数

pid_t wait(int *status);
返回值
    大于0 返回值为回收的进程id
    返回-1 回收失败

参数 status 进程回收状态
    可以使用下面的宏来说明当前的回收状态
    WIFEXITED(wstatus) 如果正常返回，返回true
    WEXITSTATUS(wstatus) 用于输出正常返回的状态 用 %d 格式化输出

    WIFSIGNALED(wstatus) 如果被信号杀死，返回true
    WTERMSIG(wstatus) 用于输出被哪个信号杀死 用 %d 格式化输出

    WIFSTOPPED(wstatus) 如果子进程停止了，返回true
    WSTOPSIG(wstatus) 用于输出进程停止是由于哪个信号
    WIFCONTINUED(wstatus) 如果进程被信号SIGCONT重启，返回true

    WCOREDUMP(wstatus) 如果子进程发生核心转储，返回true

示例：

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

// pid_t wait(int *status);

int main()
{
   
    pid_t ret = fork();
    if (ret < 0)
    {
   
        perror("fork error");
        return -1;
    }
    else if (ret > 0)
    {
   
        int waitstatus;
        printf("here is father,pid is [%d], child pid is [%d]\n", getpid(), ret);
        pid_t waitid = wait(&waitstatus);
        if (waitid == -1)
        {
   
            perror("wait error");
            return -1;
        }
        else if (waitid > 0)
        {
   
            printf("child pid [%d] is over\n", waitid);
        }

        if (WIFEXITED(waitstatus))
        {
   
            printf("exited, status=%d\n", WEXITSTATUS(waitstatus));
        }
        else if (WIFSIGNALED(waitstatus))
        {
   
            printf("killed by signal %d\n", WTERMSIG(waitstatus));
        }
        else if (WIFSTOPPED(waitstatus))
        {
   
            printf("stopped by signal %d\n", WSTOPSIG(waitstatus));
        }
        else if (WIFCONTINUED(waitstatus))
        {
   
            printf("continued\n");
        }
    }
    else if (ret == 0)
    {
   
        printf("here is child pid is [%d],father is [%d]\n", getpid(), getppid());
    }

    return 0;
}

//输出
here is father,pid is [24290], child pid is [24291]
here is child pid is [24291],father is [24290]
child pid [24291] is over
exited, status=0

3. waitpid()

waitpid中的第三个参数可以让回收进程变成非阻塞的。

pid_t waitpid(pid_t pid, int *status, int options);
返回值
    大于0 成功，返回的是回收的进程id
    等于0 返回0是因为第三个参数设置为了 WNOHANG ，等待是非阻塞的，并且这个时候没有子进程需要被回收
    小于0 回收失败

 参数 pid 进程id
    小于-1 :等待该数字绝对值的所在组所有子进程，例如-21328，则等待21328组内所有子进程
    -1 :等待所有子进程
    0 :等待组ID等于调用进程的组ID的子进程
    大于0 :等待某个子进程
参数 status 进程回收状态
        同wait函数的
参数 options 回收选项
        0 :不添加选项
        WNOHANG :如果没有子进程需要被回收，就立即返回
        WUNTRACED :如果一个子进程被停止了，就返回
        WCONTINUED :如果一个子进程被SIGCONT从停止转变为运行，就返回

示例：

创建多个子进程并回收它们

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

// pid_t waitpid(pid_t pid, int *status, int options);

int main()
{
   
    int i = 0;
    for (i = 0; i < 3; i++)
    {
   
        pid_t ret = fork();
        if (ret < 0)
        {
   
            perror("fork error");
            return -1;
        }
        else if (ret > 0)
        {
   

            printf("here is father,pid is [%d], child pid is [%d]\n", getpid(), ret);
            // pid_t waitid = wait(&waitstatus);
        }
        else if (ret == 0)
        {
   
            sleep(3);
            printf("here is child [%d] pid is [%d],father is [%d]\n", i, getpid(), getppid());
            break;
        }
    }

    if (i == 3)
    {
   
        int waitstatus;
        int num = 0;
        do
        {
   
            pid_t waitid = waitpid(-1, &waitstatus, WNOHANG);
            if (waitid == -1)
            {
   
                perror("wait error");
                return -1;
            }
            else if (waitid > 0)
            {
   
                printf("child pid [%d] is over\n", waitid);
                num++;
            }
            else if (waitid == 0)
            {
   
                // 没有进程被回收，可以做其他事情
                sleep(1);
                continue;
            }

            if (WIFEXITED(waitstatus))
            {
   
                printf("[%d] exited, status=%d\n", waitid, WEXITSTATUS(waitstatus));
            }
            else if (WIFSIGNALED(waitstatus))
            {
   
                printf("[%d] killed by signal %d\n", waitid, WTERMSIG(waitstatus));
            }
            else if (WIFSTOPPED(waitstatus))
            {
   
                printf("[%d] stopped by signal %d\n", waitid, WSTOPSIG(waitstatus));
            }
            else if (WIFCONTINUED(waitstatus))
            {
   
                printf("[%d] continued\n", waitid);
            }

            if (num == i)
            {
   
                break;
            }
        } while (1);
    }

    return 0;
}
//输出
here is father,pid is [24553], child pid is [24554]
here is father,pid is [24553], child pid is [24555]
here is father,pid is [24553], child pid is [24556]
here is child [0] pid is [24554],father is [24553]
here is child [2] pid is [24556],father is [24553]
here is child [1] pid is [24555],father is [24553]
child pid [24554] is over
[24554] exited, status=0
child pid [24555] is over
[24555] exited, status=0
child pid [24556] is over
[24556] exited, status=0