SHRI SHRI - 1 month ago 12
C Question

Not able to receive data from UDP server in win32 UDP socket application

I have an hardware ARM control as UDP server and I am communicating to it via the code below written in C#. PC is UDP Client. Server simply echoes the data.

This works fine without any issue and this is stable.

using System.Net.Sockets;
using System.Net;
using System.Text;
using System;

namespace UDPSocket
{
class UDPSender
{
static void Main(string[] args)
{
UInt32 i=0;
Int32 PORT = 45555;
for (; i < 15; )
{
Console.WriteLine(i++);
Socket s = new Socket(AddressFamily.InterNetwork, SocketType.Dgram,
ProtocolType.Udp);

IPAddress broadcast = IPAddress.Parse("192.168.0.10");

byte[] sendbuf = Encoding.ASCII.GetBytes("Shriganesh Damle, Infineon Techonologies India Pvt Ltd, Bangalore");
IPEndPoint ep = new IPEndPoint(broadcast, PORT);

//Creates a UdpClient for reading incoming data.
UdpClient receivingUdpClient = new UdpClient(PORT);

s.SendTo(sendbuf, ep);

Console.WriteLine("Message sent to the broadcast address");

//Creates an IPEndPoint to record the IP Address and port number of the sender.
// The IPEndPoint will allow you to read datagrams sent from any source.
IPEndPoint RemoteIpEndPoint = new IPEndPoint(IPAddress.Any, 0);

// Blocks until a message returns on this socket from a remote host.
Byte[] receiveBytes = receivingUdpClient.Receive(ref RemoteIpEndPoint);

string returnData = Encoding.ASCII.GetString(receiveBytes);

Console.WriteLine("This is the message you received " +
returnData.ToString());
Console.WriteLine("This message was sent from " +
RemoteIpEndPoint.Address.ToString() +
" on their port number " +
RemoteIpEndPoint.Port.ToString());
//Console.Read();
receivingUdpClient.Close();
}
Console.Read();
}
}
}


Now, I want the same PC code in Win32 C application. I tried the code below.

/*
Simple udp client
*/
#include<stdio.h>
#include<winsock2.h>

#pragma comment(lib,"ws2_32.lib") //Winsock Library

#define SERVER "192.168.0.10" //ip address of udp server
#define BUFLEN 512 //Max length of buffer
#define PORT 45555 //The port on which to listen for incoming data

int main(void)
{
struct sockaddr_in si_other;
int s, slen=sizeof(si_other);
char buf[BUFLEN];
char message[BUFLEN];
WSADATA wsa;

//Initialise winsock
printf("\nInitialising Winsock...");
if (WSAStartup(MAKEWORD(2,2),&wsa) != 0)
{
printf("Failed. Error Code : %d",WSAGetLastError());
exit(EXIT_FAILURE);
}
printf("Initialised.\n");

//create socket
if ( (s=socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == SOCKET_ERROR)
{
printf("socket() failed with error code : %d" , WSAGetLastError());
exit(EXIT_FAILURE);
}

//setup address structure
memset((char *) &si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(PORT);
si_other.sin_addr.S_un.S_addr = inet_addr(SERVER);

//start communication
while(1)
{
printf("Enter message : ");
gets(message);

//send the message
if (sendto(s, message, strlen(message) , 0 , (struct sockaddr *) &si_other, slen) == SOCKET_ERROR)
{
printf("sendto() failed with error code : %d" , WSAGetLastError());
exit(EXIT_FAILURE);
}

//receive a reply and print it
//clear the buffer by filling null, it might have previously received data
memset(buf,'\0', BUFLEN);
//try to receive some data, this is a blocking call
if (recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, &slen) == SOCKET_ERROR)
{
printf("recvfrom() failed with error code : %d" , WSAGetLastError());
exit(EXIT_FAILURE);
}

puts(buf);
}

closesocket(s);
WSACleanup();

return 0;
}


This code is able to send to the UDP server and it echoes back. But recvfrom call will be waiting for data till infinite time. recvfrom is blocking call. I am giving correct IP address and PORT. Still I am not able to receive data from server in Win32 app. Can you help?

Answer

If you want to receive data on a socket, you need to bind it to one or more local addresses and a local port. Your C# version achieves this via new UdpClient(PORT) (note the PORT argument), but your C version does not do anything comparable.

After creating the socket, do

sockaddr_in localAddr;

localAddr.sin_family = AF_INET;
localAddr.sin_port = htons(PORT);
localAddr.sin_addr.S_un.S_addr = INADDR_ANY;

bind(s, &localAddr, sizeof(sockaddr_in));

and verify that it returns 0 to indicate success.

Alternatively, the C version would be more parallel to the C# version if you created a second socket for receiving the server response. I don't think it's necessary to do that in C (in Winsock2, to be more precise), but it is possible that using the same socket to both send and receive would require you to bind to a specific local address instead of to INADDR_ANY.

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