[RESOLVED] Read returns 0 or -1 (while data are present)

[duboism]

Hello everybody,

I'm not a socket expert but I have some experience with serial ports I/O under linux...

I am working on client/server project for a home-brew protocol. There was a previous implementation of the client and the server but I have to make a cleaner one. We use the BSD sockets.

On the client side I have to send a message (which is OK) and then wait for the header of the response. The header is a 16 bytes structures.

I use select to wait for data. select returns 1 indicating that there is data to read... However, read() always returns -1 or 0.

I have absolutely no idea of what is going on... Any help would be appreciated.

Thanks in advance,
Mathieu

[hoxsiew]

Please post some code. It's impossible to diagnose based on what you've said.

[duboism]

Hello,

The socket is open as usual:

  *sockfd = socket(AF_INET, SOCK_STREAM, 0);
  if (*sockfd < 0) {
    sockerror("in connect_to_robot (socket creation)");
    return DOCONNECT_FAIL;
  };

  /* for stream (TCP) sockets, specify "nodelay" */
  int opt = TRUE;
  if (setsockopt(*sockfd, IPPROTO_TCP, TCP_NODELAY, (const void *)&opt, sizeof(opt)) < 0) {
    sockerror("in connect_to_robot (setsockopt(TCP_NODELAY))");
    closesocket(*sockfd);
    *sockfd = SOCKET_ERROR;
    return DOCONNECT_FAIL;
  }

Connection is OK. I can write on this socket.

To read the header I use the code ("header" is the variable where I want to put the header; its a struct with the right fields and HEADER_SIZE is the size of such structure i.e. 16 bytes):

        uint8_t *rcvptr, *buffer = (uint8_t *)header;
	size_t to_read=HEADER_SIZE, total_read=0;
	ssize_t n_read;

        /* Wait for response */
	FD_ZERO(&readfds);
	FD_SET(fd, &readfds);
	readyfds = readfds;

	rcvptr = buffer;
	while (to_read) {
		ready = select(FD_SETSIZE, &readyfds, NULL, NULL, NULL);
		fprintf(stderr, "ready=%d\n", ready);
		if (ready < 0) { /* select returned an error */
			if (errno!=EINTR) { /* this is not an interruption */
				perror("read_header");
				return READ_MESSAGE_FAIL;
			} else {
				continue;
			}
		}
		if (ready == 0) { /* select timed out */
			return READ_MESSAGE_FAIL;
		}

		/* We can read in the buffer */
		if (FD_ISSET(fd, &readfds)) {
	  		fprintf(stderr, "read=to_read=%zu\n", to_read);
			n_read = read(fd, rcvptr, to_read);
        	        fprintf(stderr, "read=n_read=%zd\n", n_read);
			if (n_read < 0) {
				perror("read_header");
				return READ_MESSAGE_FAIL;
        		}
			rcvptr += n_read;
			total_read += (size_t)n_read;
			to_read -= n_read;
		}
	}

Executing the code I get:

ready=1
read=to_read=16
read=n_read=0
ready=1
read=to_read=16
read=n_read=0
ready=1
read=to_read=16
read=n_read=0
....

AFAI the line "ready=1" indicates that the socket is ready for reading. But read() always returns 0...

Thanks for your reply.

[duboism]

Hello,

I have realized that there is a nasty bug in my code (I don't send enough data to the server). I can't test now (it's too late) but I will keep you informed later...

[duboism]

Hum, apparently this was not the cause...

[duboism]

Hum, in fact I was sending too much data to the server...

Now select never returns...

[duboism]

Hello,

After some intense hacking my colleagues and I realized that we misunderstood the protocol... In fact the server was replying on another port! This seems a rather strange design...

Anyway it works now.

Thanks for replying (and kudos to my colleagues).