Question

在下面的程序中，我试图实现这些条件：
我正在尝试只实现第一个子进程来打印“hi”？
和打印“areyou”的根过程？

并且最终子进程必须退出系统而不做任何事情？

  #include <iostream>
  #include <sys/types.h>
  #include <unistd.h>
  using namespace std;

  pid_t pid1,pid2,pid3,pid4;
  int function(){                        
  pid1=fork(); 
  if(pid1>0)
  {
  cout << "hi" << getpid()<<" " << getppid()<< endl; /*first child process should print "hi"*/
  }
  pid2=fork();
  cout << "hell" << getpid()<<" " << getppid()<< endl;
  pid3=fork();
  cout << "how "  <<getpid() <<" "<<getppid() <<endl;
  pid4=fork();

  if(pid4>0){
  return 0;/* final child process should exit from the system with out doing anything*/                                      
            }
        else{
 cout << "areyou "<<getpid()<<" "<<getppid()<<endl; 
            }
        }

 int main() {

 /* and the root process should print  "are you"*/
    function();
   }

-with if（pid1＆gt; 0）我想我试图实现第一个孩子输出“hi”，我觉得我迷失了解我怎么才能得到只有根父进程来打印“areyou”，以及如何控制最后一个孩子退出而不做任何事情

Answer 1

if（pid1＆gt; 0）我猜我试图实现第一个孩子输出“hi”

不，它是父获得正pid（成功时），因为它获取刚刚分叉的子进程的id，或者如果fork调用失败则为-1。孩子收到的返回值为0。

你想做的事情是这样的：

if(pid1 < 0)
{
    cout << "fork failed to create a child process."
}
else if (pid1 > 0) // Parent
{
     cout << "areyou";
}
else // child
{
     cout << "hi";
}

Answer 2

您可以执行类似

的操作

void function()
{                        
    pid_t pid1, pid2, pid3, pid4;
    pid1 = fork(); 
    if (pid1 == 0)
    {
        // first child process should print "hi"
        cout << "hi " << getpid() << " " << getppid()<< endl;

    }
    pid2 = fork();
    cout << "hell " << getpid() <<" " << getppid() << endl;
    pid3 = fork();
    cout << "how "  << getpid() <<" "<<getppid() << endl;
    pid4 = fork(); // Mostly useless as only parent print something for this one

    if (pid1 == 0 && pid2 == 0 && pid3 == 0 && pid4 == 0){
        return; // final child process should exit from the system with out doing anything
    } else if (pid1 > 0 && pid2 > 0 && pid3 > 0 && pid4 > 0){
        cout << "areyou "<< getpid() << "  "<< getppid() << endl;
    }
}

Demo

需要使用fork（）函数实现条件输出的帮助

2 个答案: