Basically, I have a daemon process which I want to periodically check
a fifo node made with mknod (STAT_FILE, S_IFIFO | 0444, 0) and spit up
a status message.
Something like this:
writefd = open (STAT_FILE, O_NDELAY | O_WRONLY);
if (writefd > 0) {
int len;
len = give_ctrl_status (writefd);
close (writefd);
} else if (errno != ENXIO) {
error ("Error (%s) reading " STAT_FILE, strerror (errno));
all_done = 1;
}
Yay. All's well until there's multiple processes consuming the message
out of STAT_FILE.
The bash script below simulates the problem. If there's multiple readers,
only one of them gets the message out of the fifo. This is not a Linux
specific deal, Solaris does it too.
Here's the output of the script on Linux:
[mjfrazer@frogger fifo]$ ./fifo.test
after 1000 iterations:
winner[1] = 999
winner[2] = 1
winner[3] = 0
winner[4] = 0
[mjfrazer@frogger fifo]$ uname -a
Linux frogger 2.4.17 #1 Fri Jan 18 16:06:33 EST 2002 i686 unknown
and on Solaris:
[mjfrazer@toronto-fs fifo]$ ./fifo.test
after 100 iterations:
winner[1] = 19
winner[2] = 18
winner[3] = 16
winner[4] = 47
[mjfrazer@toronto-fs fifo]$ uname -a
SunOS toronto-fs 5.8 Generic_108528-10 sun4u sparc SUNW,Ultra-4
As far as I can tell, I'm borked. What think the filesystem types
of adding a flag to mknod or something to tell the pipe not to boot off
unserviced readers?
-mark
#!/bin/bash
if [ `uname` = Linux ] ; then
MAX=1000
WAIT='usleep 100000'
elif [ `uname` = SunOS ] ; then
MAX=100
WAIT='sleep 1'
else
echo "Only runs on Solaris or Linux"
exit 1
fi
readers="1 2 3 4"
for i in $readers ; do
winner[$i]=0
done
iter=0
while [ $iter -lt $MAX ] ; do
echo -ne " $(( $iter + 1 )) \r"
/bin/rm -f fifo
mkfifo fifo
trap "echo hello > fifo" INT
for i in $readers ; do
cat fifo > out.$i &
done
$WAIT
# ps -ef | grep 'cat fifo' | grep -v grep
echo hello > fifo
trap - INT
wait
ps -ef | grep 'cat fifo' | grep -v grep
for i in $readers ; do
if [ -s out.$i ] ; then
# echo winner is $i
winner[$i]=$(( ${winner[$i]} + 1 )) ;
fi
done
iter=$(( $iter + 1 ))
done
echo "after $iter iterations:"
for i in $readers ; do
echo winner\[$i] = ${winner[$i]}
done
-
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This archive was generated by hypermail 2b29 : Wed Jan 23 2002 - 21:01:06 EST