waitpid()函数

waitpid函数

 作用同于wait，但可指定pid进程清理，可以不阻塞。

pid_t waitpid(pid_t pid,int *status,int options);成功：返回清理掉的子进程ID；失败：-1（无子进程）

特殊参数和返回情况：

参数pid：

       >0 回收指定ID的子进程

       -1 回收任意子进程（相当于wait）

       0 回收和当前调用waitpid一个组的所有子进程

       < -1 回收指定进程组内的任意子进程

返回0：参数3为WNOHANG，且子进程正在运行。

注意：一次wait或waitpid调用只能清理一个子进程，清理多个子进程需要用到循环

/***
loop_wait.c
***/
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
    int n = 5, i;                //默认创建5个子进程
    pid_t p, q;

    if(argc == 2){    
        n = atoi(argv[1]);
    }

    for(i = 0; i < n; i++)     {//出口1,父进程专用出口
        p = fork();
        if(p == 0) {
            break;            //出口2,子进程出口,i不自增
        } else if (i == 3){
            q = p;
        }
    }

    if(n == i){
        sleep(n);
        printf("I am parent, pid = %d
", getpid(), getgid());
        //pid_t pid = waitpid(q, NULL, WNOHANG);
//        pid_t pid = wait(NULL);
        //printf("child pid = %d
", pid);
        while(1);

    } else {
        sleep(i);
        printf("I'm %dth child, pid = %d, gpid=%d
", 
                i+1, getpid(), getgid());
        while(1);
    }

    return 0;
}

/***
waitpid.c
***/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>

int main(void)
{
    pid_t pid, pid2, wpid;
    int flg = 0;

    pid = fork();
    pid2 = fork();

    if(pid == -1){
        perror("fork error");
        exit(1);
    } else if(pid == 0){        //son
        printf("I'm process child, pid = %d
", getpid());
        sleep(5);                
        exit(4);
    } else {                    //parent
        do {
            wpid = waitpid(pid, NULL, WNOHANG);
            //wpid = wait(NULL);
            printf("---wpid = %d--------%d
", wpid, flg++);
            if(wpid == 0){
                printf("NO child exited
");
                sleep(1);        
            }
        } while (wpid == 0);        //子进程不可回收

        if(wpid == pid){        //回收了指定子进程
            printf("I'm parent, I catched child process,"
                    "pid = %d
", wpid);
        } else {
            printf("other...
");
        }
    }

    return 0;
}

/***
waitpid2.c
***/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/wait.h>

int main(void)
{
    pid_t pid, pid2, wpid;
    int flg = 0;

    pid = fork();
    pid2 = fork();

    if(pid == -1){
        perror("fork error");
        exit(1);
    } else if(pid == 0){        //son
        printf("I'm process child, pid = %d
", getpid());
        sleep(5);                
        exit(4);
    } else {                    //parent
        do {
            wpid = waitpid(pid, NULL, WNOHANG);
            //wpid = wait(NULL);
            printf("---wpid = %d--------%d
", wpid, flg++);
            if(wpid == 0){
                printf("NO child exited
");
                sleep(1);        
            }
        } while (wpid == 0);        //子进程不可回收

        if(wpid == pid){        //回收了指定子进程
            printf("I'm parent, I catched child process,"
                    "pid = %d
", wpid);
        } else {
            printf("other...
");
        }
    }

    return 0;
}

/***
waitpid3.c
***/
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
    int n = 5, i;                
pid_t p, q;

    if(argc == 2){    
        n = atoi(argv[1]);
    }
    q = getpid();

    for(i = 0; i < n; i++)     {
        p = fork();
        if(p == 0) {
            break;            
        } 
    }

    if(n == i){  // parent
        sleep(n);
        printf("I am parent, pid = %d
", getpid());
        for (i = 0; i < n; i++) {
            p = waitpid(0, NULL, WNOHANG);
            printf("wait  pid = %d
", p);
        }
    } else {
        sleep(i);
        printf("I'm %dth child, pid = %d
", 
                i+1, getpid());
    }
    return 0;
}

waitpid：

       参1：    pid  > 0       指定进程id回收

                     pid = -1        回收任意子进程

                     pid = 0          回收本组任意子进程

                     pid < -1        回收该进程组的任意子进程

       参2：    status：

                     返回：成功：pid  失败 -1

                     status：传出参数

                     1： 阻塞等待子进程

                     2： 回收子进程资源

                     3:    获取子进程结束状态：1）WIFEXITED（）真

                                                                      WEXITSTATUS（）获取子进程退出状态

                                                               2）WIFSIGNALED（） 真

                                                                      WTERMSIG（）获取导致子进程终止的信号的                                                                                           编码

参3：    0 ：（wait）阻塞回收

              WBNIOHANG：非阻塞回收（轮询）

返回值：       成功：pid  失败 -1  返回 0 值： 参3传WNOHANG，并且子进程尚未结束。