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From: 0004219666@MCIMAIL.COM (Bob Stine)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Problems with TCP, read(), and write()
Message-ID: <03910404143730.0004219666NB1EM@mcimail.com>
Date: 4 Apr 91 14:37:00 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 42

>I am having problems with TCP, write(), and read(). I want to send and
>receive 32 byte structures....
>My problem is that if I do no put a sizeable delay in the
>send or receive loop, I end up receiving only part of a 32 byte chunk
>after a number of chunks have been received.

Shawn,

As you have discovered, TCP does not preserve record boundaries.  One
work-around makes use the value returned by read(), which is the number of the
bytes read.  Keep reading until you get your entire 32 bytes.  I.e.,

  int bf_ln,  /* number of bytes requested */
      bc,     /* byte count returned by read() */
      s;      /* socket */

  struct shawns_struct { /* whatever, 32 bytes worth */} in_rec;

  char *bptr;

  /* get the connection. Then, to load a structure... */

  bf_ln = sizeof(struct shawns_struct);
  bptr  = (char *) in_rec;
  while (bf_ln)
  {
      bc = read(s,bptr,bf_ln);
      if (bc < 1)
         exit(-1);
      bf_ln -= bc;
      bptr  += bc;
   }

Note that I exit if read() returns a value of zero.  In BSD, select() will
indicate that a closed socket is ready to read, but read() will return zero
bytes.  Hence, if the above loop didn't exit when read() returned a zero, then
a closed socket would keep it busy for quite some time... :-)

Regards,

Bob Stine
bstine@MCIMail.com