在 python 和 cpp 之间建立管道通信时遇到问题

Question

附加的第一段代码是我使用的python 代码（在“test.py”中）。第二个是 c++ 代码（在我编译为“test.out”的“test.cpp”中）。我正在使用 ubuntu (18.04) wsl 来运行这些程序。

我首先建立了允许两个进程之间相互通信的管道。使用 fork 我创建了一个子进程来调用“test.out”，它是 c++ 代码的可执行文件。我将文件描述符作为参数传递给被调用程序。

#Python

import os
import subprocess
import time
#establishing communication pipes
r_sub, w_py = os.pipe()
r_py, w_sub = os.pipe()
#creating a subprocess to run c++ code
pid = os.fork()

if pid > 0:
#Parent
    os.close(r_sub)
    os.close(w_sub)

    print("[Py]Parent process is writing : r_py" + str(r_py) + " w_py" + str(w_py) +  "r_sub" + str(r_sub) + " w_sub" + str(w_sub))
    text = b"message"
    #Writing message to c++ .exe
    os.write(w_py,text)
    print("[Py]Written text:", text.decode())

    os.close(w_py)
    #Reading c++ message
    rec = os.fdopen(r_py)
    print("[Py]Received Message " + rec.read())
else:
    #Child
    print("[SubP]Calling c++ .exe : r_sub" + str(r_sub) + " w_sub" + str(w_sub) + " r_py" + str(r_py) + " w_py" + str(w_py))
    #Calling .exe of c++ code
    subprocess.call(["./test.out",str(r_sub),str(w_sub),str(r_py),str(w_py)])

//c++

#include <iostream>
#include <stdio.h>
#include <unistd.h>
#include <string>
#define MSGSIZE 16

int main(int argc, char *argv[]) {
    if(argc < 5){
        printf("[c++]File descriptors are not present");
        return -1;
    }
    printf("[c++]I received file descriptors: r_sub%s w_sub%s r_py%s w_py%s\n",argv[1],argv[2],argv[3],argv[4]);
    //Arguments are received as character arrays -> turning them to integers to use them as file descriptors
    int r_sub = std::stoi(argv[1]), w_sub = std::stoi(argv[2]), r_py = std::stoi(argv[3]), w_py = std::stoi(argv[4]);
    char buffer[MSGSIZE] = "";

    close(r_py);
    close(w_py);
    //Here I find out that read fails
    if(read(r_sub,buffer,MSGSIZE) == -1)
        printf("\n:(\n");

    printf("[c++]Received message: %s\n",buffer);

    //Attempting to send message back to python
    close(r_sub);
    write(w_sub,"message back",MSGSIZE);
    std::cout << "[c++]Finish\n\n";
    return 0;
}

既没有编译错误，也没有“错误文件描述符”错误，但通信不起作用。实际上，两端都没有收到任何信息。 结果：

---

python3 test.py

[Py]父进程正在写入：r_py5 w_py4r_sub3 w_sub6

[Py]书面文本：消息

[SubP]调用 c++ .exe : r_sub3 w_sub6 r_py5 w_py4

[c++]我收到文件描述符：r_sub3 w_sub6 r_py5 w_py4

:(

[c++]收到消息：

[c++]完成

[Py]收到的消息：

---

Answer 1

对于任何想知道更正版本是什么的人：

我将文件描述符设置为可继承，当我将它们传递给子进程时，我确保它们没有关闭（正如@Ted Lyngmo 提到的那样）。在 c++ 中，我将写入长度设置为消息的长度，以避免读取越界。

#Python

import os
import subprocess
import time
#establishing communication pipes
r_sub, w_py = os.pipe()
r_py, w_sub = os.pipe()
#creating a subprocess to run c++ code
pid = os.fork()
os.set_inheritable(r_sub, True)
os.set_inheritable(w_sub, True)
os.set_inheritable(r_py, True)
os.set_inheritable(w_py, True)

if pid > 0:
#Parent
    os.close(r_sub)
    os.close(w_sub)

    print("[Py]Parent process is writing : r_py" + str(r_py) + " w_py" + str(w_py) +  "r_sub" + str(r_sub) + " w_sub" + str(w_sub))
    text = b"message"
    #Writing message to c++ .exe
    os.write(w_py,text)
    print("[Py]Written text:", text.decode())

    os.close(w_py)
    #Reading c++ message
    rec = os.fdopen(r_py)
    print("[Py]Received Message:" + rec.read())
else:
    #Child
    print("[SubP]Calling c++ .exe : r_sub" + str(r_sub) + " w_sub" + str(w_sub) + " r_py" + str(r_py) + " w_py" + str(w_py))
    #Calling .exe of c++ code
    subprocess.call(["./test.out",str(r_sub),str(w_sub),str(r_py),str(w_py)], close_fds=False)

//c++

#include <iostream>
#include <stdio.h>
#include <unistd.h>
#include <string>
#define MSGSIZE 16

int main(int argc, char *argv[]) {
    if(argc < 5){
        printf("[c++]File descriptors are not present");
        return -1;
    }
    printf("[c++]I received file descriptors: r_sub%s w_sub%s r_py%s w_py%s\n",argv[1],argv[2],argv[3],argv[4]);
    //Arguments are received as character arrays -> turning them to integers to use them as file descriptors
    int r_sub = std::stoi(argv[1]), w_sub = std::stoi(argv[2]), r_py = std::stoi(argv[3]), w_py = std::stoi(argv[4]);
    char buffer[MSGSIZE] = "";
    char buf[] = "message back";

    close(r_py);
    close(w_py);

    //Attempting to receive message
    if(read(r_sub,buffer,MSGSIZE) == -1)
        printf("\n:(\n");

    printf("[c++]Received message: %s\n",buffer);

    //Attempting to send message back to python
    close(r_sub);
    if(write(w_sub,buf,sizeof(buf)) == -1)
        printf("\n:(\n");
    std::cout << "[c++]Finish\n\n";
    return 0;
}

结果如最初所愿：

---

python3 test.py

[Py]父进程正在写入：r_py5 w_py4r_sub3 w_sub6

[Py]文字：消息

[SubP]调用 c++ .exe : r_sub3 w_sub6 r_py5 w_py4

[c++]我收到文件描述符：r_sub3 w_sub6 r_py5 w_py4

[c++]收到的消息：message

[c++]完成

[Py]收到消息：返回消息

---

再次感谢@Ted Lyngmo 对社区的迅速支持！

Answer 2

您的 C++ 程序中存在一些问题（例如 未定义的行为，因为您在 write(w_sub, "message back", MSGSIZE); 中读取了超出范围的字符串文字）但第一个问题是您的文件描述符不是由启动的程序 - 所以它们都是 C++ 程序中的错误文件描述符。始终检查 - 不要认为任何事情都是理所当然的。

你可以在python中显式设置继承模式：

os.set_inheritable(r_sub, True) # do this for all fds that should be inherited

... 或使用os.pipe2(0) 打开管道，这使得它们默认可继承。

并确保在启动子流程时它们没有全部关闭：

subprocess.call(["./test.out",str(r_sub),str(w_sub),str(r_py),str(w_py)], close_fds=False)
#                                                                         ^^^^^^^^^^^^^^^