recv with non-blocking socket

Question

I am trying to implement non-blocking for socket recv and the problem is that I got an error -1 when there in no data but I expect to get EAGAIN error.

Answer 1

You need to check errno, not the length value to get the reason why the socket failed to return data. Also, EWOULDBLOCK is the other error code you should check for in addition to EAGAIN. Change your while loop as follows:

while(1)
{
    int err;
    printf("1\n");
    length = recv(s, buffer, ETH_FRAME_LEN_MY, 0);
    err = errno; // save off errno, because because the printf statement might reset it
    printf("2\n");
    if (length < 0)
    {
       if ((err == EAGAIN) || (err == EWOULDBLOCK))
       {
          printf("non-blocking operation returned EAGAIN or EWOULDBLOCK\n");
       }
       else
       {
          printf("recv returned unrecoverable error(errno=%d)\n", err);
          break;
       }
    }           
    //printf ("buffer %s\n", buffer);
    print(buffer, length);
}

Of course this infinite loop will get into a CPU wasting busy cycle while it's waiting for data. There are a variety of ways to be notified of data arriving on a socket without having to call recv() in a spin loop. This includes select and poll calls.