Trouble establishing pipe communication between python and cpp

You have some problems in your C++ program (like undefined behavior because you read your string literal out of bounds in write(w_sub, "message back", MSGSIZE);) but the first problem is that your file descriptors aren't inherited by the started program - so they are all bad file descriptors in the C++ program. Always check - don't take anything for granted.

You can set the inheritance mode explicitly in python:

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

... or open the pipes with os.pipe2(0) which makes them inheritable by default.

And make sure they aren't all closed when you start the sub-process:

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

For any who are wondering what the corrected version is:

I set the file descriptors inheritable, and when I passed them to the sub-process, I made sure that they were not closing (as per @Ted Lyngmo mentioned). In c++ I set the write length as the message's length to avoid reading out of bounds.

#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;
}

The results come as were initially desired:

python3 test.py

[Py]Parent process is writing : r_py5 w_py4r_sub3 w_sub6

[Py]Written text: message

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

[c++]I received file descriptors: r_sub3 w_sub6 r_py5 w_py4

[c++]Received message: message

[c++]Finish

[Py]Received Message:message back

Thank you again @Ted Lyngmo for your swift support towards the community!