sendto : Resource temporarily unavailable (errno 11)

Question

I am having a problem with sendto.

I have a receiver who receives UPD packets with recvfrom and then replies to the sender using sendto.

Unfortunately, I am getting errno 11 (Resource temporarily unavailable). I am using two sockets.

The first packet is actually sent but not the ones afterwards:

sendto :: Success

error: 0.

sendto :: Resource temporarily unavailable

error: 11.

sendto :: Resource temporarily unavailable

...

This is an extract of my code:

    int sockfd, sockSend;

    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
            perror("socket");

    if ((sockSend = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
            perror("socket");

    if (fcntl(sockfd, F_SETOWN, getpid()) < 0) {
            perror("fcntl"); 
    }
    if (fcntl(sockfd, F_SETFL, O_RDONLY | O_NONBLOCK | FASYNC) < 0) {
            perror("fcntl"); 
    } 

    if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr))
                    < 0)
            perror("bind");

And in a SIGIO handler:

    len = sizeof(recv_addr);
    char buffer[payload];
    bzero(buffer, payload);
    n = recvfrom(sockfd, buffer, payload, MSG_DONTWAIT, (struct sockaddr *)&recv_addr, &len);

    while (n > 0) {

                            sprintf(response, "%d\n%d\n%d\n", items, target_buf, pb_sp);          
                            sendto(sockSend, response, strlen(response), 0, (struct sockaddr *) &recv_addr, sizeof(recv_addr));
                            // sleep(1);

                            perror("sendto :");
                            printf("error: %d.\n", errno);

     }

Could this issue come because the port is still hot, and I need to wait before reusing it? I've tried to change port but it hasn't helped.

Update: If the sleep(1) is commented out, then the packets actually get send!

Thanks a lot for your help.

Answer 1

The error you are getting:

EAGAIN or EWOULDBLOCK: The socket is marked nonblocking and the requested operation would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.

You set the socket to non-blocking (O_NONBLOCK). The socket is still busy sending the previous message. You cannot send another until the first has finished sending. That's why sleeping helped.

Don't set it to non-blocking, or try again after select says you can.