wait(NULL) 似乎导致 child 停止执行

Question

我正在编写一个简单的管道程序，它将请求两个命令，然后运行程序就好像它们是作为 cmd1 | cmd2 输入到 bash 中一样。然后它应该循环并再次询问，直到其中一个命令是 quit.

我已经写这么多了：

#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;
}

当我运行它并输入 ls 和 wc 我得到的输出是：

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

我的主要问题是 Done executing 应该在 ls|wc 之后，而不是相反。我认为 wait(NULL) 不起作用，但我不确定。

请指教。还有谢谢。

Answer 1

你有：

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

当您在 child1 中执行时，child2 将重新设置为 init，您不能再等待 child2 终止。

所以我们需要做一些重构来得到这样的东西：

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代码的操作（一些内嵌注释）：

#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 child 停止执行

wait(NULL) seems to cause child to stop executing

c++

fork

pipe

wait