Hello,
the following code is the main part of an example prepared for the workshop, emulating
"tail -f":
- snip --
# install "tail -f"
Event->io(
fd=>\*IN,
cb=>sub {
# check for real news
return if eof(IN);
# read new line and report it
print "New line detected: ", scalar(<IN>), "\n";
}
);
- snip ---
Please note the lines
- snip ---
# check for real news
return if eof(IN);
- snip ---
which I had to add because an event occurs even if nothing new arrives in the file.
Well, this was ok for me in a demonstration focussed on an io watcher, but now someone
asked me if there is a way to avoid callback invokation for EOF. Running the script
again, I recognized that it enforces a very high load.
Ok, I suppose this is because of the underlaying select() call. I read the manpage of
IO::Select which mentions EOF as an ERROR (to be detected by has_error()), not a
"ready to be read" event (checked by can_read()). Nevertheless, even with IO::Select
and usage of can_read() EOF is detected as an event as well:
- snip --
use IO::Select;
$s=IO::Select->new;
open(IN, "tailtest.tmp");
$s->add(\*IN);
while ($s->can_read(0))
{
next if $s->has_error(0);
print "New line detected: ", scalar(<IN>), "\n";
}
- snip --
This surely is a basic problem. The question is if there is a solution? What I'm
looking for is to get a read event (poll=>'r') ONLY if there is something to read, not
in case of EOF (which could be matched by poll=>'e'). Ideally, this solution would
result in a lower load than the example above.
Jochen
