Try to send data from C++ to Python and reversed using socket C++, C++ sendto() and python recvfrom() working but not the reversed

TLDR; I try to send data from C++ program to Python program and reversed, i managed to send data from C++ and received on python, but not send from python to C++

Okay before going into code, i will explain what iam trying to do (Hope it will make understanding my code easier): I wanted to create a server which will listen and read an image from C++ Client and return another image

(Server Side-Python) First i created a socket and bind it with localhost at port 5001

(Client Side-C++) I created a socket, read an image (Here i use OpenCV) send image size first then send the image itself, and wait for server to respond

(Server Side-Python) Read the size and read the image using the size it received

-- At this point everything work as expected --

(Server Side-Python) Server read and image and send it back

(Client Side-C++) Client is freeze since it dont receive anything?

It took me 4 hours with no result :( this is my code:

Server Python

import socket
import cv2
import numpy
import time


UDP_IP = '127.0.0.1'
UDP_PORT = 5001

#Create socet here
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind((UDP_IP, UDP_PORT))

#Get image size here
length, addr = s.recvfrom(16)

#read image data here
stringData,addr = s.recvfrom(int(length))

#Convert to opencv image format
data = numpy.fromstring(stringData, dtype='uint8')
decimg=cv2.imdecode(data,0)

############################################################################

#Read an image
sendBackImg = cv2.imread("cat.jpeg",0)

#Encode file
encode_param=[int(cv2.IMWRITE_JPEG_QUALITY),90]
result, imgencode = cv2.imencode('.jpg', sendBackImg, encode_param)
data = numpy.array(imgencode)
stringData = data.tostring()

#Send image size
s.sendto(str(len(stringData)).zfill(16), (UDP_IP, UDP_PORT))
# s.sendto(stringData, (UDP_IP, UDP_PORT))

#Show image to check if it work correctly
cv2.imshow('SERVER-SEND',sendBackImg)
cv2.imshow('SERVER-SEND',decimg)

cv2.waitKey(3000)

s.close()

cv2.destroyAllWindows()

Client C++

#undef UNICODE

#define WIN32_LEAN_AND_MEAN
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>

#include <iostream>
#include <string.h>
#include <sys/socket.h> 
#include <arpa/inet.h> 
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

#define PORT 5001

using namespace std;
using namespace cv;

int main(void)
{
    int sock = 0, valread;
    struct sockaddr_in serv_addr;

    // read an image and show it so later and can compare with result on server side
    Mat frame = imread("dog.jpg",0);
    imshow("CLIENT",frame);

    // Encode image
    vector<uchar> buf;
    imencode(".jpg",frame,buf);

    const char *data = reinterpret_cast<char*>(buf.data());

    int strLength = buf.size();

    std::string strSizeTmp = std::to_string(strLength);
    strSizeTmp = string(16-strSizeTmp.length(),'0')+strSizeTmp ;
    char strSize[strSizeTmp.length()+1];
    strcpy(strSize, strSizeTmp.c_str());

    // Create socket
    memset(&serv_addr, 0, sizeof(serv_addr)); 
    serv_addr.sin_family = AF_INET; 
    serv_addr.sin_port = htons(PORT); 
    serv_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    if ((sock = socket(AF_INET, SOCK_DGRAM, 0)) < 0) 
    { 
        printf("\n Socket creation error \n"); 
        return -1; 
    } 

    //Send image size and image data
    sendto(sock, (const char *)strSize, 16, 
        MSG_CONFIRM, (const struct sockaddr *) &serv_addr,  
            sizeof(serv_addr));
    sendto(sock, (const char *)data, strLength, 
        MSG_CONFIRM, (const struct sockaddr *) &serv_addr,  
            sizeof(serv_addr));

    //////////////////////////////////////////////////////////

    //Wait for respond
    char bufferTmp[16] ; 
    cout << "Wait" << endl;
    recvfrom(sock, (char *)bufferTmp, 16,  
                MSG_WAITALL, (struct sockaddr *) NULL, 
                NULL);
    // The program just nerver reach here
    cout << bufferTmp << endl;


    close(sock);

    waitKey(3000);
}

There is one thing i noticed that c++ recvfrom doesnt required ip and port, how does it know which port to listen to?