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Old 10-19-2015, 12:52 PM   #1
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linux read(): after reading 20 bytes, reading again get "Resource temporarily unavail

I use funfd to read packets from a tun socket, which is registered as an even callback function with libevent.

at first I read the first 20 bytes to check the IP header, and then read the following bytes:

                nr_bytes = read(tunfd, buf, 20);
		assert(nr_bytes == 20);

		/* get the IP packet length and read the whole packet */
		iphdr = (struct ip *)buf;
		ip_len = ntohs(iphdr->ip_len);
		iphdr_len = iphdr->ip_hl*4;

		nr_bytes = read(tunfd, buf+20, ip_len-20);
		if(-1 == nr_bytes) perror("readtun() failed:\n");
but for the second read, `read(tunfd, buf+20, ip_len-20)`, I get "Resource temporarily unavailable", what are potential problems? and how to deal with it?
Old 10-20-2015, 05:52 AM   #2
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Are you sure such partial read is possible?
Old 10-20-2015, 08:02 AM   #3
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More information is needed:
  1. What exact attributes are you using with your open()?
  2. Are you changing any attributes after you open the descriptor? If so what?
  3. What exact ERRNO number are you receiving? Check the manual page for read(2) because that error string only makes sense if you aren't root but needed to be root to use this resource. Since you've read from it once already, and managed to open it, I'm assuming permissions are not the issue here.
  4. It is perfectly fine to get EAGAIN if there's no data and the socket is non-blocking, this just means you have no data. I regularly code reads in a loop where I screen for that errno and upon seeing it just continue my loop silently because I know it just means there's no data at the present time.


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