reading Ethernet data packet from raw sockets using recvfrom() in MSG_DONTWAIT mode

Question

I am using raw sockets to send and receive Ethernet data packets in C using recvFrom(). I want to read in non blocking mode so I am using MSG_DONTWAIT. But the recvFrom() is always returning -1 even if packet is received or not. I am new to C programming. I am able to receive my payload but I get message "Receive resource temporary unavailable" always.

Code Snippet:

    if ((sock = socket(AF_PACKET, SOCK_RAW, htons(0x8851))) < 0) { 
       perror("ERROR: Socket");
       exit(1);
    }


    while(1) {
       int flag=0;
       n=recvfrom(sock, buffer, 2048, MSG_DONTWAIT, NULL, NULL);

       if (n == -1) {
           perror("ERROR: Recvfrom");
           close(sock);
           flag=1;

       } 

       if (flag==0)  {
        // Read Packet

       }
    }

I added error message to question. I am closing it too everytime. — Tejzeratul
What do you think should happen if a packet is received while you don't have a socket? (Answer: it will be processed normally like any packet, but your program won't receive a copy because it doesn't have a raw socket) — user253751
It's always returning -1 and EAGAIN because it's never receiving a packet... — user253751

juhist juhist · Accepted Answer · 2015-03-19T15:06:11

If you use the MSG_DONTWAIT argument for recvfrom(), the system call will always immediately return whether or not there is any data to be read. If there's no data, the return value is -1 and then errno will be set to EAGAIN. In your application, I'm not completely sure that MSG_DONTWAIT is the right choice. If the only thing you're doing is reading packets from that one socket, you shouldn't use MSG_DONTWAIT. So, your program in practice will print lots of error messages in a loop. If you remove that error message for the case when errno == EAGAIN, your program would be slightly better but not much better: it would spin in a loop, consuming all CPU resources.

If, however, you are reading from multiple file descriptors at the same time, then using non-blocking I/O is the right choice. But instead of your loop, you should have a loop that polls for the readiness of multiple file descriptors using select(), poll() or epoll_wait(). Since you're running on Linux, I highly recommend epoll_wait() as it's the most scalable approach of these. See the epoll, epoll_wait and epoll_create manual pages on Linux for more information.

I heavily recommend not using MSG_DONTWAIT for now and check if the function call ever returns. If it never returns, it means it isn't receiving any packets.

reading Ethernet data packet from raw sockets using recvfrom() in MSG_DONTWAIT mode

1 Answers