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Path: utzoo!watmath!clyde!bonnie!akgua!sdcsvax!noscvax!marlin!sloane
From: sloane@marlin.UUCP
Newsgroups: net.unix-wizards
Subject: Help: 4.2bsd IPC routines/TCP sequence error?
Message-ID: <601@marlin.UUCP>
Date: Thu, 14-Mar-85 13:39:58 EST
Article-I.D.: marlin.601
Posted: Thu Mar 14 13:39:58 1985
Date-Received: Sat, 16-Mar-85 01:47:04 EST
Distribution: net
Organization: Naval Ocean Systems Center, San Diego
Lines: 98

OVERVIEW:
I am working on a series of programs that communicate via TCP over
a 10mb ethernet. The hosts are all Vaxen (750s and 780s) all running
4.2bsd. The application is written in C and uses the calls outlined
in the 4.2bsd Interprocess Communications Primer. Our problem is that
when we send records from one host to another, the order in which they
are received is not the same as the order they were sent. The problem
seems to be in the timing between repeated calls to send().

SPECIFICS:
Here is a sample of the code used to send and recieve info over the TCP
socket: the lines marked /* CROWBAR */ in SEND are necessary to make it work. 
We think this is because of a timing problem in the send and/or recv loops.
The socket is opened AF_INET/SOCK_STREAM. There is NO select() call made
as there is only one socket open. The TCP address is sent from the host
requesting the transfer via UDP (it works great).

The sample code opens a file on one host and uses the send() call to 
transfer it over a TCP socket to another host. The file is transferred 
line-by-line. The file consists of 64 records with sequence numbers from
0-63.

We know the data is not received in order because there is a test in the 
RECV code to make sure that the received sequence number is the same as 
the loop counter. Without the CROWBAR code these fragments transfer 
portions of the file (like maybe the 1st 14 records) and then trap at the 
"out of sequence" error in RECV. The amount of data transferred varies 
from try to try... With the CROWBAR code the entire transfer works
fine (all 64 records are transferred IN ORDER).

----------------------   SEND    -------------------------------------
/* open memo group file */
fd = fopen("memogrp.fil","r");

/* read memogroup records and send via TCP */
for(cnt=0; cnt<NGRPS; cnt++)
   {
   /* read a single group record */
   if(fgets(bufr, 66, fd) == NULL)
      {
      shutdown(tcp_socket);
      user_err(USR, 0, pgm, "bad data or premature EOF in readgrps()");
      }

   /* send it via TCP */
   if(send(tcp_socket, bufr, strlen(bufr)+1, 0) < 0)
      {
      shutdown(tcp_socket);
      user_err(SYS, errno, pgm, "send failure");
      }

   /* if these lines are commented out it FAILS!!! */
   fprintf(null, "%s", bufr); /* CROWBAR */
   fflush(null); /* CROWBAR */
   }

fclose(null); /* CROWBAR */

/* close memo group file */
fclose(fd);;
}

--------------------------------  RECV  -------------------------------
/* open the memogroup file */
mfd = fopen("memogrp.fil", "w");

/* receive the memogroup file from the memo daemon via TCP */
for(cnt=0; cnt<NGRPS; cnt++)
   {
   /* recieve a group record and load the structure */
   if(recv(tcp_socket, bufr, 68 , 0) < 0)
      {
      shutdown(tcp_socket);
      user_err(SYS, errno, pgm, "recv: call failed");
      }

   /* make sure receipt is in sequence */
   extract(recvd_seq,bufr,1);
   if(stoi(recvd_seq) != cnt)
      {
      shutdown(tcp_socket);
      user_err(USR, 0, pgm, "recv: memogroup received out of sequence");
      }
   /* write the record to the memogroup file */
   fprintf(mfd, "%s\n", bufr);
   }

/* close the memogroup file */
fclose(mfd);
}
-----------------------------------------------------------------------

    * **************************************************************** *
    * Gary K. Sloane/Computer Sciences Corporation/San Diego, CA 92110 *
    * DDD: (619) 225-8401                                              *
    * MILNET: sloane@nosc.ARPA                                         *
    * UUCP: [ihnp4,akgua,decvax,dcdwest,ucbvax]!sdcsvax!noscvax!sloane *
    * **************************************************************** *