client socket unable to receive data using poll/select - c

Question

/* SEND FUNC. */
int mysend(unsigned char *buffer, int len) {
int sock,ret;
int status,flags;
struct sockaddr_in6 servaddr;
int opt = 1;
char *addr = "1101::1";
sock = socket(AF_INET6,SOCK_DGRAM,0);
if (sock < 0)
return -1;
if( setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (char *)&opt, sizeof(opt)) < 0 )
return -1;
flags = fcntl(sock, F_GETFL, 0);
fcntl(sock, F_SETFL, flags|O_NONBLOCK);
servaddr.sin6_family = AF_INET6;
servaddr.sin6_port = htons(61616);
status = inet_pton(AF_INET6, addr, &servaddr.sin6_addr);
if (status <= 0) {
perror("inet_pton");
return -1;
}
/* send message to server */
status = sendto(sock, buffer, len, 0, (struct sockaddr *)&servaddr, sizeof(servaddr));
if (status < 0) {
perror("sendto");
return -1;
}
close(sock);
printf("MESSAGE SENT SUCCESSFULLY\n");
return 0;
}
/* RECEIVE FUNC. */
int myrcv() {
int sock,ret;
int status,len,rx_bytes;
int timeout,nfds =1;
struct sockaddr_in6 servaddr;
struct timeval wait;
unsigned char rxbuff[1024];
char *rcv;
char *addr = "1101::1";
fd_set rd;
struct pollfd *fds;
sock = socket(AF_INET6,SOCK_DGRAM,0);
if (sock < 0)
return -1;
servaddr.sin6_family = AF_INET6;
servaddr.sin6_port = htons(61616);
status = inet_pton(AF_INET6, addr, &servaddr.sin6_addr);
if (status <= 0)
return -1;
bind(sock,(struct sockaddr *)&servaddr,sizeof(servaddr));
timeout = (1* 1000);
wait.tv_sec = 10;
wait.tv_usec = 0;
len = sizeof(servaddr);
fds->fd = sock;
fds->events = POLLIN;
for(;;) {
//FD_ZERO(&rd);
//FD_SET(sock,&rd);
printf("Waiting for data....\n");
ret = poll(fds,nfds,timeout);
//ret = select(1,&rd,NULL,NULL,&wait);
if(ret < 0)
break;
if(fds->revents == 0)
printf("revents 0 %d\n",ret);
if(ret == 0)
continue;
memset(rxbuff,0,1024);
//if(FD_ISSET(sock,&rd)) {
printf("receiving message\n");
rx_bytes = recvfrom(sock,rxbuff,1024,0,(struct sockaddr *)&servaddr,&len);
memcpy(rcv,rxbuff,rx_bytes);
//}
}
close(sock);
return 0;
}
int main()
{
/* call mysend() periodically using sigaction() */
/* create a thread that continuously monitors(calls myrcv()) for incoming data */
return 0;
}
I'm unable to receive the packets from the server, but I could see the packets in the tcpdump output. Above are the sample client code snippets, which tries to receive and send the data from/to the server. The scenario is: the client needs to send data periodically to server and should also be able to receive any data from the server.
I have tried using both poll and select methods but failed to receive. Please let me know if I'm missing anything. Thanks for your support.

Answer 1

The problem you have with receiving is that you need to bind the receiving socket to the local port.
You also have other things that can be improved, like creating a single socket for both sending and receiving and using SO_REUSEADDR on the sending socket (not needed on a write-only socket).
What you should do is:
Create socket
Set socket options
Bind to local address (Use IN6ADDR_ANY_INIT to bind to all interfaces)
Write to server
Poll for reply

Answer 2

Several things:
Your receive function (myrcv) isn't specifying a listen port via the bind() call. That's the most likely problem. Ditto for your send function, although a port is chosen randomly for you.
In you myrcv() function, I don't see where you have actually initialized fds or nfsd prior to calling poll().
Re-opening and closing the socket on each call to mysend() looks problematic. If you are expecting the server to send back to the same client on the same port it received the message on, chances are high you have already closed the socket. You should just open one socket for both sending and receiving. You can share the same socket between your send thread and your receive thread.