UDP sockets use of select()

Question

Hi I'm creating a proxyserver that wait for packets from client with an UDP connection and checks if all of them are effectively arrived or same of them have been rejected .

In that case I should send an "ack" to the clients for each lost packet (with send_ack() ) but just after have sent the first ack the "if part" of the select loops sending unlimited acks never going back to the "else part" were the select listen data from client (receive _pkt() function )

fd_set rset, allset;
int maxfd, nready;
struct timeval timeout;

timeout.tv_sec = 4;
timeout.tv_usec = 150000; 
maxfd = socketfd;  
FD_ZERO(&allset);
FD_SET(socketfd, &allset);
rset = allset;

for( i=0; ;i++){
    do {
        nready=select( (maxfd +1), &rset, NULL, NULL,  &timeout); 
    } while ((nready<0) & (errno==EINTR)); 

    if( nready<0) {
        perror("Error main: select failed: ");
        exit(32);
    }  
    if( nready==0){ 
        send_ack(socketfd,head);
    }
    else{           
        receive_pkt(socketfd, head);
    }
}

Hope it's enough clear, thanks for the advices!

Answer 1

On some systems (Linux in particular), a select call modifies the timeout to show how much time is left. So in your case, if it waits 3 seconds for a packet, timeout will be reduced to 1.15 seconds, and after a total of 4.15 seconds, timeout will be 0, so later calls to select will immediately return with nready == 0.

If you want to wait again after sending an ack, you need to reset the timeout to non-zero.

Answer 2

One has to reset the fd_set rset before every call to select. The select call expects a bit set of field descriptors to monitor and overwrites with a bit set of field descriptors with notifications to read.

for( i=0; ;i++){
    do {
        rset = allset;
        nready=select( (maxfd +1), &rset, NULL, NULL,  &timeout); 
    } while ((nready<0) & (errno==EINTR));