"Connected" UDP sockets, communication in both directions? - c

Question

How do I achieve 2 way communication on a connected UDP sockets ?
I can send message from client to server, but can not get a message from server. Here is my code. I think that problem must be in the server side, but I don't know how to solve that problem. I have intentionally removed error checking just to post on SO & keep my post short. I am not receiving any errors on any side.
I can get this program running with not connected UDP sockets, but can't with connected socket.
Server.c
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <errno.h>
int main()
{
int sockfd;
struct sockaddr_in me;
char buffer[1024];
sockfd = socket(AF_INET, SOCK_DGRAM, 0);
memset(&me, '\0', sizeof(me));
me.sin_family = AF_INET;
me.sin_port = htons(8080);
me.sin_addr.s_addr = inet_addr("127.0.0.1");
bind(sockfd, (struct sockaddr *)&me, sizeof(me));
recv(sockfd, buffer, 1024, 0);
printf("[+]Data Received: %s\n", buffer);
strcpy(buffer, "Hello Client\n");
send(sockfd, buffer, 1024, 0);
printf("[+]Data Send: %s\n", buffer);
return 0;
}
Client.c
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <errno.h>
int main()
{
int sockfd;
struct sockaddr_in other;
char buffer[1024];
sockfd = socket(AF_INET, SOCK_DGRAM, 0);
memset(&other, '\0', sizeof(other));
other.sin_family = AF_INET;
other.sin_port = htons(8080);
other.sin_addr.s_addr = inet_addr("127.0.0.1");
connect(sockfd, (struct sockaddr *)&other, sizeof(other));
strcpy(buffer, "Hello Server\n");
send(sockfd, buffer, 1024, 0);
printf("[+]Data Send: %s\n", buffer);
recv(sockfd, buffer, 1024, 0);
printf("[+]Data Received: %s\n", buffer);
return 0;
}
Output of Server
[+]Data Received: Hello Server
[+]Data Send: Hello Client
Output of Client
[+]Data Send: Hello Server
// Here it does not receive the message sent by server.

Answer 1

On linux, straceing the executable, the server send does say this:
sendto(3, "Hello Client\n\0\0\0\310$\220\4J\177\0\0\0\0\0\0\0\0\0\0"...,
1024, 0, NULL, 0) = -1 EDESTADDRREQ (Destination address required)
i.e. the server socket indeed does not know the address it needs to send to. Any UDP socket must make the other end of the socket known by either connecting, or providing the destination socket address in sendto.
connect on an UDP socket means just setting a default address for send.
To connect the socket on the "server" side, with an unknown party you should use recvfrom to find out the socket address of the sending party - then you can connect using this address or continue using sendto. With sendto the same socket can communicate with many different parties concurrently.
The TCP server/client sockets are a different case, because listen/accept on the server side returns a new connected socket that is distinct from the original server socket.