Linux系统下进程回收--waitpid函数

waitpid函数的作用和wait函数的作用相同，只是有些参数不同。
在Linux终端下输入命令：man 2 waitpid查看waitpid函数的具体描述：

SYNOPSIS
       #include <sys/types.h>
       #include <sys/wait.h>

        pid_t waitpid(pid_t pid, int *wstatus, int options);
            作用：回收指定进程号的子进程，可以设置非阻塞(默认阻塞)。
            参数：
                pid_t pid：
                    < -1：回收某个进程组的组id的绝对值，回收指定进程组的子进程
                    -1：回收所有子进程，相当于wait()        //最常用
                    0：回收当前进程组的所有子进程
                    > 0:某个子进程的id

                The value of pid can be:

                    < -1   meaning wait for any child process whose process group ID is equal to the absolute value of pid.

                    -1     meaning wait for any child process.

                    0      meaning wait for any child process whose process group ID is equal to that of the calling process.

                    > 0    meaning wait for the child whose process ID is equal to the value of pid.
                
                
                int *wstatus：进程退出时的状态信息，传入的是一个int类型的地址，传出参数。
                    返回值：
                        - 成功：返回被回收的子进程的id
                        - 失败：返回-1（调用函数失败，或者所有子进程都被回收）

                int options：设置阻塞或非阻塞
                    0：阻塞
                    WNOHANG：非阻塞
            返回值：
                > 0：返回子进程的id
                = 0：options=WNOHANG，表示还有子进程没有退出（在非阻塞的情况下）
                -1 ：错误，或者没有子进程

        
        WIFEXITED(wstatus)
              returns true if the child terminated normally, that is, by calling exit(3) or _exit(2), or by  returning  from
              main().

       WEXITSTATUS(wstatus)
              returns  the  exit  status of the child.  This consists of the least significant 8 bits of the status argument
              that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main().
              This macro should be employed only if WIFEXITED returned true.

       WIFSIGNALED(wstatus)
              returns true if the child process was terminated by a signal.

下面是waitpid函数的简单案例：

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

using std::cout;
using std::endl;

namespace waitpid_test{
    void test(){

        //有一个父进程创建5个子进程
        pid_t pid;

        //创建5个子进程
        for(int i=0;i<5;++i){
            pid = fork();
            if(pid == 0)
                break;
        }

        if(pid>0){
            //父进程
            while(1){
                cout<<"parent,pid："<<getpid()<<endl;
                sleep(1);
                //int ret = wait(NULL);
                int st;
                
                //阻塞
                //int ret = waitpid(-1,&st,0);

                //非阻塞
                int ret = waitpid(-1,&st,WNOHANG);


                if(ret == -1)
                    break;
                
                else  if(ret == 0)
                    //说明还有子进程存在
                    continue;
                else if(ret > 0){
                
                    if(WIFEXITED(st))
                        //是不是正常退出
                        cout<<"退出的状态码："<<WEXITSTATUS(st)<<endl;
                    if(WIFSIGNALED(st))
                        //是不是异常终止
                        cout<<"被哪个信号干掉了："<<WTERMSIG(st)<<endl;
                    cout<<"child has died,pid："<<ret<<endl;
                }

                 
                
                
            }
        }
        else if(pid == 0){
            //子进程
            while(1){
            cout<<"child, pid："<<getpid()<<endl;
            sleep(1);    
            }
            exit(0);
        }
    }

}

int main(){

    //wait_test::test();
    waitpid_test::test();
    return 0;
}