Fork, parent, child process and infinite loop - c++

Question

I am trying to create a C++ program that computes some values in a simulation and then passes the output to a Python script for drawing. Both processes operate in an infinite loop, the C++ program computes steps in the simulation and passes it to the Python program via a pipe, the Python program repeatedly reads from the pipe and displays the data.
So far I've tried achieving this by forking the current process and executing the Python script under the child process however when I start the infinite loop in the parent process it blocks all IO and nothing is ever outputted to the screen until I terminate the parent process and turn the child process into a zombie. The C++ program that starts the script is as follows:
int main(int argc, char* argv[]) {
int pipe_to_py[2];
if (::pipe(pipe_to_py)) {
std::cout << "Failed to open pipe for uni-directional communication";
}
pid_t pid = fork();
// if pid is the child process then start up the python script and pass the file descriptors over to it
if (pid == 0) {
::close(pipe_to_py[1]); // close the "write end"
std::ostringstream oss;
oss << "export PY_READ_FD=" << std::to_string(pipe_to_py[0]) << " && "
<< "export PYTHONUNBUFFERED=true && "
<< "python3 script.py";
::system(oss.str().c_str()); // call py script
} else if (pid < 0) { exit(1);
} else {
::close(pipe_to_py[0]);
program_loop(&pipe_to_py[1]); // function with while(true) loop
}
return 0;
}
While the Python script is:
# the physical animation function draws data from the pipe and plots it
def animate(i):
data = read_data_from_pipe()
draw(data)
anim = FuncAnimation(fig, animate, interval=100)
fig.show()
plt.show()
I'm just wondering what exactly I'm doing wrong and why I/O is being blocked for the child process.