UDP c++ receive - c++

Question

I want to send/receive data from a device via UDP. The device acts as a server and if I send it a value it sends me some values back. First I write a little python code which works without any problems:
import socket
import time
UDP_IP = "192.168.42.33"
UDP_PORT = 5004
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(('',UDP_PORT))
while True:
MESSAGE = input('Write new message: ')
sock.sendto(bytes.fromhex(MESSAGE), (UDP_IP, UDP_PORT))
time.sleep(0.1)
data, addr = sock.recvfrom(1076)
a = len(data)
print ("Byte: ", data.hex()[46], data.hex()[47])
I can write a value to a register with this script and get an array with all updated register values from the device back. I write an equal program in c++:
#pragma once
#pragma comment(lib, "Ws2_32.lib")
#include <sdkddkver.h>
#include <WinSock2.h>
#include <Windows.h>
#include <iostream>
#include <string>
#define SCK_VERSION2 0x0202
#define BUFLEN 2048
using namespace std;
int main()
{
int inputI;
long SUCCESSFUL;
WSAData WinSockData;
WORD DLLVersion;
SOCKET sock;
char msg_input[] = { 0x60, 0x01, 0x00, 0x00}; //Write REG mode, 1 byte
char* SU_IP = "192.168.42.33"; //IP Address of Scanner Unit
u_short SU_PORT = 5004; //Port of Scanner Unit
SOCKADDR_IN ADDRESS;
char buf[BUFLEN];
int slen = sizeof(ADDRESS);
DLLVersion = MAKEWORD(2, 1);
SUCCESSFUL = WSAStartup(DLLVersion, &WinSockData);
sock = socket(AF_INET, SOCK_DGRAM, 0);
ADDRESS.sin_addr.s_addr = inet_addr(SU_IP);
ADDRESS.sin_family = AF_INET;
ADDRESS.sin_port = htons(SU_PORT);
connect(sock, (SOCKADDR*)&ADDRESS, sizeof(ADDRESS));
while (1)
{
cout << "Adresse des Registers (hex): ";
cin >> hex >> inputI;
msg_input[2] = inputI;
cout << "Wert des Registers (hex): ";
cin >> hex >> inputI;
msg_input[3] = inputI;
send(sock, msg_input, sizeof(msg_input), NULL);
//recv(sock, buf, BUFLEN, 0);
}
}
I am able to sent values and the device sends its register values back so the communication works (I checked this with wireshark). But I can't receive the data in my program. I oncomment the recv function because the program gets stuck at this point if I want to receive. The recvfrom() function doesn't work at this point too. I tried the bind() function instead of connect because in the python script it works with sock.bind. But than I cant send or receive. I was reading several posts about UDP receive function but can't find my mistake. Can someone help me?

Answer 1

The Python code binds to all existing local interfaces at port 5004, whereas the C code will implicitly bind to a local free port during the call to connect(), thus if the remote peer is hard-coded to respond to port 5004, the socket will not receive it.
You should add a call to bind() right after creating your socket:
bind(sock, (sockaddr *)&ADDRESS, sizeof(ADDRESS));
And see what happens :)