echo server not responding udp - c++

Question

Am trying to implement a echo client. Client using sendto() is transmitting the message and server is receiving n displaying it. but then server is not sending the message (echo back). here is the code of both server and client. Can anyone help me with this?
Server:
char msg[100]="";
int conn_sock,n;
struct sockaddr_in server_addr,client_addr;
conn_sock=socket(AF_INET,SOCK_DGRAM,0);
server_addr.sin_family=AF_INET;
server_addr.sin_port=htons(1234);
server_addr.sin_addr.s_addr=inet_addr("127.0.0.1");
err=bind(conn_sock, (struct sockaddr *)&server_addr,sizeof(server_addr));
n=recvfrom(conn_sock,msg,sizeof(msg),0,(struct sockaddr *)&client_addr,(socklen_t *)&client_addr);
n=sendto(conn_sock,msg,sizeof(msg),0,(struct sockaddr *)&client_addr,sizeof(client_addr));
in recvfrom the value of n is number of bytes. but in sendto() value of n is -1
Client:
char msg[100];
int conn_sock,n,err;
struct sockaddr_in server_addr;
conn_sock=socket(AF_INET,SOCK_DGRAM,0);
server_addr.sin_family=AF_INET;
server_addr.sin_port=htons(1234);
server_addr.sin_addr.s_addr=inet_addr("127.0.0.1");
cin>>msg;
n=sendto(conn_sock,msg,strlen(msg),0,(struct sockaddr *)&server_addr,sizeof(server_addr));
n=recvfrom(conn_sock, msg, 15, 0, (struct sockaddr*) &server_addr,(socklen_t *)&server_addr);
recvfrom() is not receiving any data from the server.

Answer 1

These two lines are both wrong:
// In the server
n=recvfrom(conn_sock,msg,sizeof(msg),0,(struct sockaddr *)&client_addr,(socklen_t *)&client_addr);
// In the client
n=recvfrom(conn_sock, msg, 15, 0, (struct sockaddr*) &server_addr,(socklen_t *)&server_addr);
The last two parameters of recvfrom() are supposed to point to two separate variables. One of them receives the other peer's network address, and the other one receives the length of that network address. That you're passing the same pointer value into both means that the address length is overwriting the first several bytes of the address data, corrupting it and resulting in badness.
Here's how the server should work, receiving the length into a separate variable:
struct sockaddr_storage client_addr;
socklen_t client_addr_len = sizeof(client_addr); // This is an in+out parameter
n=recvfrom(..., (struct sockaddr *)&client_addr, &client_addr_len);
n=sendto(..., (struct sockaddr *)&client_addr, client_addr_len);
Note that I used a sockaddr_storage instead of sockaddr_in. A sockaddr_storage is guaranteed to be large enough to hold any valid socket address type for supported address families, so that this code will be forward-compatible with IPv6.
Likewise, here's how the client should work:
n=sendto(..., &server_addr, sizeof(server_addr));
do
{
struct sockaddr_storage peer_addr;
socklen_t peer_addr_len = sizeof(peer_addr);
n=recvfrom(..., (struct sockaddr *)&peer_addr, &peer_addr_len);
} while (!areSockAddrsEqual((struct sockaddr *)&server_addr, (struct sockaddr *)&peer_addr));
...
bool areSockAddrsEqual(struct sockaddr *addr1, struct sockaddr *addr2)
{
if (addr1->sa_family != addr2->sa_family)
return false;
switch (addr1->sa_family)
{
case AF_INET:
struct sockaddr_in *addr1_in = (struct sockaddr_in *)addr1;
struct sockaddr_in *addr2_in = (struct sockaddr_in *)addr2;
return (addr1_in->sin_port == addr2_in->sin_port &&
addr1_in->sin_addr.s_addr == addr2_in->sin_addr.s_addr);
...
// Other address families such as AF_INET6 left as an exercise
}
}
Here again we make sure to pass a separate socklen_t pointer to receive the address length, and then I also added a loop to make sure that the actual packet we received was the one we were expecting from the intended server. If we instead received a different packet (say, due to another peer that just happened to send us a packet at the wrong time), we ignore it.
The check for whether or not two socket addresses are equal is a little gnarly, since it depends on the address family, and supporting both IPv4 and IPv6 is tricky.