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To: cs551@merlot.usc.edu
Subject: Re: Misunderstanding TCP stream?
Date: Sun, 16 Nov 2008 10:56:49 -0800
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > How would we read in a our sha1 hash hex encoded string and convert it into
  > a 20 byte buffer? 

Well, you can write a function that convert 2 characters to
one byte:

    unsigned char HexstringOfTwoToOneByte(char *in)
        /* assuming that everything in in[] are lowercase */
    {
        unsigned char hi_nibble=0;
        unsigned char lo_nibble=0;

        switch (in[0]) {
        case '0': hi_nibble = 0; break;
        case '1': hi_nibble = 1; break;
        ...
        case 'e': hi_nibble = 14; break;
        case 'f': hi_nibble = 15; break;
        }
        switch (in[1]) {
        case '0': lo_nibble = 0; break;
        case '1': lo_nibble = 1; break;
        ...
        case 'e': lo_nibble = 14; break;
        case 'f': lo_nibble = 15; break;
        }
        return (hi_nibble*16)+lo_nibble;
    }

Then call this function 20 times with the input
buffer shifted by 2 characters each time and the
output buffer shifted by 1 byte each time.
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>




  -----Original Message-----
  From: Bill Cheng [mailto:william@bourbon.usc.edu] 
  Sent: Saturday, November 15, 2008 7:35 AM
  To: cs551@merlot.usc.edu
  Subject: Re: Misunderstanding TCP stream?
  
  Someone wrote:
  
    > Due to the buffer size constraints, I first send the message common header
    > and metadata. I then iteratively read in 'chunks' of the file data, and send
    > each 'chunk' individually. The common header of course tells the client how
    > much data to expect, including the file data.
    >
    > When I attempt this though, for some reason, the client reads in the correct
    > number of bytes, but the file data bytes are incorrect. And then a few
    > seconds later the client seg faults. Am I misunderstanding / misusing the
    > TCP stream? I thought this should work?
  
  Sounds like you are doing it correctly.  This is also what we did for warmup
  #1 (except that instead of reading and writing one block at a time, we were
  reading and writing one byte at a time).  If your warmup #1 is working
  properly, you can change it to do it the same way you are doing and you
  should see that it works.
  
  So, my guess is that the bug is somewhere else.
  --
  Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>