asynchronous udp server client application - c

Question

I am trying to make an asynchronous UDP chat application, currently having only one client and server.
When I run my server, a lot of redundant data is displayed. Afterward, when some text is typed,
Error sending the file!
is displayed.
Could someone please look at the code and let me know where I am going wrong?
Server:
u_long iMode=1;
ioctlsocket(sd,FIONBIO,&iMode);
int n=sd+1;
fd_set readfds,writefds;
while(1)
{
FD_ZERO(&readfds);
FD_ZERO(&writefds);
FD_SET(sd,&readfds);
FD_SET(sd,&writefds);
int rv = select(n, &readfds, &writefds, NULL, NULL);
if(rv==-1)
{
printf("Error in Select!!!\n");
exit(0);
}
if(rv==0)
{
printf("Timeout occurred\n");
}
if (FD_ISSET(sd, &readfds))
{
FD_CLR(sd,&readfds);
int client_length = (int)sizeof(struct sockaddr_in);
memset(&buffer,0,sizeof(buffer));
int bytes_received = recvfrom(sd, buffer,SIZE, 0, (struct sockaddr *)&client, &client_length);
if (bytes_received < 0)
{
fprintf(stderr, "Could not receive datagram.\n");
closesocket(sd);
WSACleanup();
exit(0);
}
}
printf("\nClient says: %s",buffer);
printf("\nWrite :");
fgets(buffer,SIZE,stdin);
if(FD_ISSET(sd,&writefds))
{
FD_CLR(sd,&writefds);
int client_length = (int)sizeof(struct sockaddr_in);
if(sendto(sd, buffer,strlen(buffer), 0, (struct sockaddr *) &client,client_length)<0)
{
printf("Error sending the file! \n");
exit(1);
}
}
}
closesocket(sd);
WSACleanup();
return 0;
}

Answer 1

I see a problem. This line:
int rv = select(n, &readfds, &writefds, NULL, NULL);
Will nearly always return immediately with the readfds empty. But writefds will almost ALWAYS be set with the "sd" socket indicating that it is ready for writing/sending.
Hence, your code correctly skips the attempt to call recvfrom(), but nothing stops it from falling through to the sending code path. All of the variables such client, client_length, and buffer as are likely uninitialized at that point. Or worse, buffer and client are exactly what they were from the last successful call in the loop. That likely explains the redundant data.
My advice would be to not have a "writefds" set in the select call at all. Then only "send" when you actually read. The sendto call won't block for any significant amount of time anyway.