Translating c code help required

I am trying to decipher/translate some code that I have no experience
with.

I am trying to find out how the checksum is computed for a Megellan
Explorist GPS Waypoint (POI) file. Below is the first 3 lines of a
typical
file. I understand everything except how the "a*23" at the end of the
line
is arrived at.

$PMGNFMT,%WPL,LAT,HEMI,LON,HEMI,ALT,UNIT,NAME,MSG, ICON,CHKSUM,
%META,ASCII
$PMGNWPL,3137.54374,S,01914.37622,E,0,M,WPT001,,a* 23
$PMGNWPL,3122.18567,S,01906.50683,E,0,M,WPT002,,a* 21

Below is an extract of the code for a program that generates lines
like the above and calculates the checksum
(It looks like the relevant section). The full code can be found here:
http://gpsbabel.cvs.sourceforge.net/...revision=1.155

The code is in C of which I understand nothing. My only experience at
this
stage is Excel VBA. Can someone please explain in plain text (or VBA)
how
the checksum is calculated.

Many thanks
Laurence


/*
* Given a protocol message, compute the checksum as needed by
* the Magellan protocol.
*/
unsigned int
mag_checksum(const char * const buf)
{
int csum = 0;
const char *p;

for(p = buf; *p; p++) {
csum ^= *p;
}

return csum;
}
static unsigned int
mag_pchecksum(const char * const buf, int len)
{
int csum = 0;
const char *p = buf;
for (; len ; len--) {
csum ^= *p++;
}
return csum;
}

static void
mag_writemsg(const char * const buf)
{
unsigned int osum = mag_checksum(buf);
int retry_cnt = 5;
int i;
char obuf[1000];

if (debug_serial) {
warning("WRITE: $%s*%02X\r\n",buf, osum);
}

retry:

i = sprintf(obuf, "$%s*%02X\r\n",buf, osum);
termwrite(obuf, i);
if (magrxstate == mrs_handon || magrxstate == mrs_awaiting_ack) {
magrxstate = mrs_awaiting_ack;
mag_readmsg(trkdata);
if (last_rx_csum != osum) {
if (debug_serial) {
warning("COMM ERROR: Expected %02x, got %02x",
osum, last_rx_csum);
}
if (retry_cnt--)
goto retry;
else {
mag_handoff();
fatal(MYNAME
": Too many communication errors.\n");
}
}
}
}

re: Translating c code help required

In article <1190786469.669276.131850@57g2000hsv.googlegroups. com>,
lombardm <lombardm@mweb.co.zawrote:Start the checksum at 0.
Loop over every character in the buffer until you get to the end
of the string (which is marked by a character which is binary 0)
xor each character into the checksum.

Because xor never creates new "on" bits where everything used
to be "off", this the result will never be more than one character
wide. That one character is later printed in hexadecimal.
--
Okay, buzzwords only. Two syllables, tops. -- Laurie Anderson

re: Translating c code help required

On Tue, 25 Sep 2007 23:01:09 -0700, lombardm wrote:
This is the bitwise exclusive-or of the values of characters in
buf.
(I don't get why on earth len is signed; what happens if is
negative? What's wrong with while (len--) where len is a size_t?Ditto as above, but it stops after len characters instead of at
the first null.It writes "$", the contents of buf, osum as two hex digits, a
carriage return and a line feed to obuf.I guess it writes obuf somewhere.Without knowing what these are I can't help...(*cough cough* Hasn't anybody told them 'bout do {} while()? )--
Army1987 (Replace "NOSPAM" with "email")
A hamburger is better than nothing.
Nothing is better than eternal happiness.
Therefore, a hamburger is better than eternal happiness.

re: Translating c code help required

Sorry folks - I know NOTHING about C and all your replies are Greek to
me.

I hoping for a bit of pseudo VBA code like the following (in my
example code the Ascii cvalues of the individual characters of
InputLine are added together and converted to hexadecimal)

Checksum = 0
For i = 1 to length(Inputline)
Checksum = checksum + Chr(Mid(InputLine(i,1))
Next i
HChecksum = Hex(Checksum)

Thanks
Laurence


Army1987 wrote:

re: Translating c code help required

lombardm <lombardm@mweb.co.zawrites:<snip><snip>Please don't top post. You should interleave you reply and trim any
material not required.
The reply you got said it in English. Surely you can turn that into
VBA? If not, then this is not the right place to ask. Take the
snipped of C and the line that says what it does ("exclusive or" is the
key bit) and post that to a VBA group. They can then do the translation.

.... although it is possible that:
replace with:
Checksum = checksum Xor Chr(Mid(InputLine(i,1))does it.

--
Ben.

re: Translating c code help required

Thanks for your reply. My apologies for not adhering to protocol.
Laurence

re: Translating c code help required

Walter Roberson wrote:.... snip ...Or other mechanisms.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>



--
Posted via a free Usenet account from http://www.teranews.com

re: Translating c code help required

lombardm wrote:Don't top-post. That has lost all the history. So I (and others)
have no idea what the thread is really about.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home.att.net>



--
Posted via a free Usenet account from http://www.teranews.com

re: Translating c code help required

lombardm wrote:These are NMEA 0183 messages. If you do a web search for NMEA checksum
VB you should find examples how to calculate them using VB.

Kevin.

re: Translating c code help required

Kevin Bagust wrote:Thank you very much - this is exactly what I am looking for - found
the solution right away and it works. Thought the information was out
there - just need to find it!
Thanks once again