Question

我正在编写一个简单的管道程序，它将要求两个命令然后运行程序，好像它们以require 'pg' conn = PG.connect("IP-Address", 5432, '', '', "database-name", "username", "password") res = conn.exec('select product_id, description, price from product') res.each do |row| puts row end的形式输入bash。然后它应循环并再次询问，直到其中一个命令为cmd1 | cmd2。

我已经写了这么多：

quit

当我运行它并输入#include<iostream> #include<string> #include<sys/types.h> #include<sys/wait.h> #include<string.h> #include<unistd.h> #include<stdlib.h> #include<stdio.h> #include<signal.h> using namespace std; int main(int argc, char *argv[]) { int pid1, pid2, errchk; int pip[2]; char cmd1[128]; char cmd2[128]; int i = 0; int status; errchk = pipe(pip);//make pipe if(errchk == -1)//check for error in pipe { perror("pipe"); exit(1); } while(i<3) { i++; //Enter commands here cout<<"Enter cmd1: "; cin>>cmd1; cout<<"Enter cmd2: "; cin>>cmd2; //if a command is quit... quit if(strcmp(cmd1,"quit")==0 || strcmp(cmd2,"quit") == 0) { cout<<"Quitting...\n"; exit(1); } pid1 = fork(); cout<<"first fork makes pids: "<<pid1<<endl; if(pid1 < 0) { perror("fork"); exit(1); } else if(pid1 == 0) //MAIN CHILD { cout<<"In main child with pid: "<<pid1<<endl; pid2 = fork(); cout<<"second fork makes pids: "<<pid2<<endl; if(pid2 == 0)//SUB CHILD 2 to run cmd2 { cout<<"In child of second fork"<<endl; close(0); close(pip[1]); dup(pip[0]); execvp(cmd2,argv);//execute command2 and die } else if(pid2 > 0)//SUB CHILD 1 to run cmd1 { cout<<"In parent of second fork"<<endl; close(1); close(pip[0]); dup(pip[1]); execvp(cmd1,argv);//execute command1 and die } } else if(pid1 > 0) //MAIN PARENT { wait(NULL);//wait for cmd1 wait(NULL);//wait for cmd2 cout<<"DONE executing"<<endl; //keep going } } return 0; }和ls时我得到的输出是：

wc

我遇到的主要问题是我应该Enter cmd1: ls Enter cmd2: wc first fork makes pids: 5785 first fork makes pids: 0 In main child with pid: 0 second fork makes pids: 5786 In parent of second fork second fork makes pids: 0 In child of second fork DONE executing 5 5 54 Done executing而不是相反。我认为ls|wc不起作用，但我不确定。

请指教。谢谢。

Answer 1

你有：

parent
  1. child1
    2. child2
    2. exec()
  1. exec()

当你在child1中执行时，child2被重新定义为init，你不能再等待child2终止了。

所以我们需要做一些重构才能得到类似的东西：

parent
  1. child1
  1. exec()
  2. child2
  2. exec()
wait() x 2

基于您自己的代码，通过重构来完成类似外观的代码（一些注释在线）：

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

using namespace std;

int main(int argc, char *argv[])
{
    int pid1, pid2, errchk;
    int pip[2];
    char cmd1[128];
    char cmd2[128];
    int status;

    while(true)
    {
        errchk = pipe(pip);//make pipe
        if(errchk == -1)//check for error in pipe
        {
            perror("pipe");
            exit(1);
        }

        //Enter commands here
        cout<<"Enter cmd1: ";
        cin>>cmd1;
        cout<<"Enter cmd2: ";
        cin>>cmd2;
        //if a command is quit... quit
        if(strcmp(cmd1,"quit")==0 || strcmp(cmd2,"quit") == 0)
        {
            cout<<"Quitting...\n";
            exit(1);
        }

        pid1 = fork();
        cout<<"first fork makes pids: "<<pid1<<endl;
        if(pid1 < 0)
        {
            perror("fork");
            exit(1);
        }
        if (pid1 == 0) // in child
        {
            cout<<"In child of first fork"<<endl;
            close(pip[0]); // close read-end of pipe
            close(0); // close stdin
            close(1); // close stdout
            dup2(pip[1], 1); // write-end of pipe is stdout of cmd1
            argv[0] = cmd1; // make it look like the command in the ps output
            execvp(cmd1,argv);//execute command1 and die
            fprintf(stderr, "execvp(1): `%s': %s\n", cmd1, strerror(errno));
            return 0;
        }
        pid2 = fork();
        cout<<"second fork makes pids: "<<pid2<<endl;
        if (pid2 < 0)
        {
            perror("fork2");
            exit(1);
        }
        if (pid2 == 0)
        {
            cout<<"In child of second fork"<<endl;
            close(pip[1]); // close write-end of pipe
            close(0); // close stdin
            dup2(pip[0], 0); // read-end of pipe is stdin of cmd2
            argv[0] = cmd2; // update ps output
            execvp(cmd2,argv);//execute command2 and die
            fprintf(stderr, "execvp(1): `%s': %s\n", cmd2, strerror(errno));
            return 0;
        }
        if(pid1 > 0) //MAIN PARENT
        {
            // close remaining pipe handles in parent
            close(pip[0]);
            close(pip[1]);
            wait(0);//wait for one command to run
            wait(0);//wait for a second command to run
            cout<<"DONE executing"<<endl; //keep going
        }
    }
    return 0;
}

wait（NULL）似乎导致子进程停止执行

1 个答案: