Path: utzoo!attcan!uunet!lll-winken!lll-lcc!pyramid!leadsv!ereed
From: ereed@leadsv.UUCP (Ed Reed)
Newsgroups: comp.sys.mac.programmer
Subject: Checksum does not add up
Message-ID: <3567@leadsv.UUCP>
Date: 25 Jul 88 21:55:24 GMT
Distribution: na
Organization: LMSC-LEADS, Sunnyvale, Ca.
Lines: 60


I am having a problem with a program I am writing on a Macintosh II.  The 
program is being written in Lightspeed C ver. 3.0, and what follows is the
applicable code:

main()
{
    .....other declarations
    char       DataIn[256];
    long       HeaderIn;
    short      CheckSumIn;
    short      CheckSum;
    char       *DataInPtr;
    long       *HeaderInPtr;
    short      *CheckSumPtr;

    .....other initializations
    DataInPtr = &DataIn[0];
    HeaderInPtr = &HeaderIn;
    CheckSumPtr = &CheckSumIn;

    .....other statements to open the serial driver(port a)
                          to reset the serial port to 19.2 Kb
                          to get hard disk information
                          to create a data file
                          to open a data file

    while (!(Button())
    {
        CheckSum = 0;
        FSRead( -6, &HeaderCount, &HeaderInPtr );
        FSRead( -6, &DataCount, &DataInPtr );
        FSRead( -6, &SumCount, &CheckSumPtr );

        for (i = 0; i < 256; i++)
        {
        CheckSum += (short) DataIn[i];
        }

        if (CheckSum != CheckSumIn)
        {
            .....write buffer to file
                 exit program
        }
    }
}

The problem is that when I do the for loop the CheckSum total does not add up
right. I added a print statement inside the loop in order to make sure what
was being added was correct. I was annoyed to find out it wasn't. Instead of
0 - 255, which was being sent, I was getting 9-39 hex, 50-ff hex, 0-a hex, and
then odd junk. But the interesting thing was when I did a hex dump of the file
it was correct. Now does anyone out there know why I can't do a checksum. Does
the Mac o.s. cause this problem or am I getting the structure of the read
command wrong? I read "Inside Mac, vol 2" which describes the I/O calls and I
think I'm right, so what is wrong?

Please respond directly, do not post your reply.

Joe Hughes
pyramid!leadsv!jhughes