Unbuffered communication between C++ and Python - c++

Question

I want to pass data from C++ to Python in Linux and I do this through a pipe.
My C++ program looks like this:
#include<iostream>
#include<stdio.h>
#include<sstream>
#include<math.h>
int main() {
FILE *cmd;
std::ostringstream ss;
double val;
cmd = popen("python plot.py","w");
for (int i=0; i<100; i++) {
val = cos(3.14159*i/40);
ss.str("");
ss << val << std::endl;
fputs(ss.str().c_str(),cmd);
fflush(cmd);
}
fputs("\n",cmd);
fflush(cmd);
pclose(cmd);
return 0;
}
The corresponding python function plot.py looks like:
fig, = plt.plot([],[])
index = 0
plt.ion()
while True:
data = sys.stdin.readline().strip()
if not data:
plt.ioff()
break
index = index + 1
val = [float(n) for n in self.data.split()]
fig.set_xdata(numpy.append(fig.get_xdata(),index))
fig.set_ydata(numpy.append(fig.get_ydata(),val))
axis = list(plt.axis())
min_x = 0
max_x = max(self.fig.get_xdata())
min_y = min(self.fig.get_ydata())
max_y = max(self.fig.get_ydata())
axis[0] = min_x
axis[1] = max_x
axis[2] = min_y
axis[3] = max_y
plt.axis(axis)
plt.draw()
plt.show()
Now my problem is that there seems to be a buffering of data from my c++ function before data is being passed to the python function, which I am not able to get rid of. I have searched for a solution but have not been able to find any. The best suggestion I could find was to use setvbuf in the c++ function, so I also tried to include the following line to the c++ function
setvbuf(cmd,NULL,_IONBUF,0)
But that did not have any effects.
Does anyone know how I can pass data from my c++ function to python in an unbuffered manner?