cron

[Bradley W. Fisher]

In article <1990Feb10.193851.16968 at eddie.mit.edu>, aryeh at eddie.mit.edu (Aryeh M. Weiss) writes:
> The problem stems from the fact that cron is a 8086 program.  The way cron
> works is that it computes how long it has until the next job and executes
> sleep.  Unfortunately in the 8086 world the sleep argument is a 16-bit int,
> which is inadequate if you only have one cron job a day!  (24 hours = 86400 
> seconds)  The fix is to run a job every hour, like date, and redirect output
> to /dev/null (or wherever you like).

I agree with your belief that cron must be nudged every once in a while ...
my situation was a little different, so I thought I'd through it up here for
all to muse ...

I've got two identical 386 AT clones running 2.3.1, and cron works just fine on 
one and not on the other. The one that doesn't work though DOES have a problem
with the CMOS clock running way too fast! Gains about 10 minutes an hour. My 
fix (all does seem to work fine now) was to write a short script file to 
update the CMOS clock with the "real time" (kernel's) clock. This works
because both cpu's run 24 hrs a day, and once set, the real time clock doesn't
change and is pretty accurate. Here's the top part of my crontab ...

#------------------------------------------------------------------------------
#Min	Hour	Day	Month	Day	Command
#of	of	of	of	of	
#Hour	Day	Month	Year	Week
#0-59	0-23	1-31	1-12	0-6(0=Sunday)	
#------------------------------------------------------------------------------
# I'm taking out the next line temporarily, 'cause cron won't run
#1	2	*	*	*	cat -s /dev/clock >/dev/null 2>&1 || exit 0;/etc/setclock `date +\%m\%d\%H\%M\%y`
58	*	*	*	*	/usr/lib/news/fixclock #fix cmos clock

... and here's what I put in "fixclock" ...

# "fixclock" ... takes output from the "date" command and sets the CMOS clock.
/etc/setclock `date +\%m\%d\%H\%M\%y`
#<EOF>

... now obviously this idea was taken from the remarked line in crontab. Which
brings up another question ... does that line (the remarked one, which came
from the orginal crontab unremarked) make sense? I read it to mean that if 
the first command retruns a non-zero exit status(cat -s /dev/clock) then 
the second command(exit 0) will be executed. When would "cat -s /dev/clock"
exit with any status other than "0", even if the date is wrong? And even
if it did, "exit 0" would dump you right out, so when would the last
command(/etc/setclock ....) ever be executed? Seems wrong to me so that's
why I made "fixclock" and run it once every hour.


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