Getting problems with sockets and select - sockets

Question

I am implementing a socket programming project in C. I am using
select()
for waiting for data from client. I have two UDP sockets and select call is always ignoring one of my sockets. Can anybody briefly describe where should I start looking for it? this is what my server is doing
waitThreshold.tv_sec = 5000;
waitThreshold.tv_usec = 50;
if(sd > sd1)
max_sd = (sd + 1);
else if(sd1 > sd)
max_sd = (sd1 + 1);
FD_ZERO(&read_sds);
FD_SET(sd, &read_sds);
FD_SET(sd1, &read_sds);
ret = select(max_sd, &read_sds, NULL, NULL, &waitThreshold);
if(ret <0)
{
printf("\nSelect thrown an exception\n");
return 0;
}
else if(FD_ISSET(sd, &read_sds))
{
// code for socket one
}
else if(FD_ISSET(sd1, &read_sds))
{
// code for socket two
}

Answer 1

You wrote else if , so just one of them will run.

Answer 2

Generally speaking, when pooling multiple sockets using select() you want to use a for loop instead of branching the code with IFs. Also take note of the fact that select CHANGES the fd_set arguments (the read, write and error file descriptor sets - 2nd, 3rd and 4th arguments) and that you need to re-set them before each select(). A pretty general code layout for selecting sockets that have data to read with multiple concurrent connections would be something like this:
FD_ZERO(&master_sds);
FD_ZERO(&read_sds);
for (i=0; i<number_of_sockets); i++){
FD_SET(sd[i], &master_sds);
if sd[i] > max_sd {
max_sd=sd[i];
}
}
for(;;){
read_sds=master_sds;
ret = select(max_sd, &read_sds, NULL, NULL, &waitThreshold);
if(ret<0){
printf("\nSelect thrown an exception\n");
return 0;
}
for(i=0; i<max_sd; i++){
if FD_ISSET(sd[i], &read_fds){
// code for socket i
}
}
}
You might not want to have an endless loop to pool the sockets for data, you can insert some condition like receiving specific data on one of the sockets or specific user input as an exit condition. Hope this helps.