IPC 使用多个管道和分叉进程来运行 Python 程序

Question

我的作业遇到了问题。我正在尝试执行 3 个并发进程（在 C++ 中），其中 2 个是 Python 程序，其中一个是 C++ 程序。

我的 C++ 程序（sample.cpp）：

#include <unistd.h>
#include <stdio.h>
#include <string.h>  
#include <string>
#include <stdlib.h>
#include <iostream>
#include <vector> 
#include <signal.h>

using namespace std;

int main()
{
    while (true)
    {
        cout << "lol" << endl;
        sleep(2);
    }
    return 0;
}

我的 Python 程序 1 (sample.py)：

import sys

while True:
    line = sys.stdin.readline().strip()
    print "Python says: " + str(line)

我的 Python 程序 2 (sample2.py)：

import sys

while True:
    line = sys.stdin.readline().strip()
    print "Python 2 says: " + str(line)

这是我的驱动程序 C++ 程序，它分叉进程：

#include <unistd.h>
#include <stdio.h>
#include <string.h>  
#include <string>
#include <stdlib.h>
#include <iostream>
#include <vector> 
#include <signal.h>

using namespace std;

int main()
{
    vector<pid_t> kids;

    int fd[2];
    if (pipe(fd) < 0)
    {
        cout << "Error"; 
        return 1; 
    }

    int fd2[2];
    if (pipe(fd2) < 0)
    {
        cout << "Error";
        return 1;
    }

    pid_t pid;

    pid = fork();
    if (pid == 0)
    {
        dup2(fd[1], STDOUT_FILENO);
        close(fd[1]);
        close(fd[0]);

        while (true)
        {
            execvp("./sample", NULL);
        }
    }
    else
    {
        kids.push_back(pid);

        pid = fork();
        if (pid == 0)
        {
            dup2(fd[0], STDIN_FILENO);
            close(fd[0]);
            close(fd[1]);

            dup2(fd2[1], STDOUT_FILENO);
            close(fd2[1]);
            close(fd2[0]);

            char * python = "/usr/bin/python";
            char * pythonProgram = "./sample.py"; 
            char * pythonArgs[] = {python, pythonProgram, NULL, NULL};
            execvp(python, pythonArgs);
        }
        else
        {
            kids.push_back(pid);
            pid = fork();
            if (pid == 0)
            {
                dup2(fd2[0], STDIN_FILENO); 
                close(fd2[0]);
                close(fd2[1]);

                char * python = "/usr/bin/python";
                char * pythonProgram = "./sample2.py"; 
                char * pythonArgs[] = {python, pythonProgram, NULL, NULL};
                execvp(python, pythonArgs);
            }
            else
            {
                kids.push_back(pid);
            }
        }
    }

    close(fd[0]);
    close(fd[1]);
    close(fd2[0]);
    close(fd2[1]);

    for (pid_t k : kids) 
    {
        int status;
        //kill (k, SIGTERM);
        waitpid(k, &status, 0);
    }
}

当我运行这个程序时，我应该会看到“Python 2 说：Python 说：lol”。但是，我什么也没看到（完全空白）......它只是挂起。我究竟做错了什么？我尝试查找了很多东西，但没有运气。

Answer 1

./sample 开头的 while 循环是没有意义的，除非您期望 execvp 失败。对 exec* 的成功调用将永远不会返回。对 execvp 的实际调用也是错误的：

execvp("./sample", NULL);

第二个参数应该是char *const[]。

您应该为 execvp:s 添加错误处理（例如带有std::exit(1) 的行）。否则，如果 execvp 失败，您将在程序的主流程中运行子进程。

python 程序需要无缓冲运行，否则消息需要很长时间才能显示。您还应该检查 readline 是否成功。

sample.py

import sys

while True:
    line = sys.stdin.readline().strip()
    if not line: break
    print "Python says: " + str(line)

sample2.py

import sys

while True:
    line = sys.stdin.readline().strip()
    if not line: break
    print "Python 2 says: " + str(line)

驱动程序.cpp

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

using namespace std;

int main()
{
    vector<pid_t> kids;

    int fd[2];
    if (pipe(fd)==-1)
    {
        clog << "Error\n";
        return 1;
    }

    int fd2[2];
    if (pipe(fd2)==-1)
    {
        clog << "Error\n";
        return 1;
    }

    pid_t pid;

    pid = fork();
    if (pid == 0)
    {
        dup2(fd[1], STDOUT_FILENO);
        close(fd[1]);
        close(fd[0]);

        char* const args[] = { NULL };
        execvp("./sample", args);
        std::clog << "sample failed\n";
        std::exit(1);
    }
    else
    {
        kids.push_back(pid);

        pid = fork();
        if (pid == 0)
        {
            dup2(fd[0], STDIN_FILENO);
            close(fd[0]);
            close(fd[1]);

            dup2(fd2[1], STDOUT_FILENO);
            close(fd2[1]);
            close(fd2[0]);

            char const* python = "/usr/bin/python";
            char const* pythonProgram = "./sample.py";
            char const* pythonArgs[] = {python, "-u", pythonProgram, NULL};
            execvp(python, const_cast<char* const*>(pythonArgs));
            std::clog << "sample.py failed\n";
            std::exit(1);
        }
        else
        {
            kids.push_back(pid);
            pid = fork();
            if (pid == 0)
            {
                dup2(fd2[0], STDIN_FILENO);
                close(fd2[0]);
                close(fd2[1]);

                char const* python = "/usr/bin/python";
                char const* pythonProgram = "./sample2.py";
                char const* pythonArgs[] = {python, "-u", pythonProgram, NULL};
                execvp(python, const_cast<char* const*>(pythonArgs));
                std::clog << "sample2.py failed\n";
                std::exit(1);
            }
            else
            {
                kids.push_back(pid);
            }
        }
    }

    close(fd[0]);
    close(fd[1]);
    close(fd2[0]);
    close(fd2[1]);

    for (pid_t k : kids)
    {
        int status;
        //kill (k, SIGTERM);
        waitpid(k, &status, 0);
    }
}