Polling, Fifos, and Linux
andreas at kostyrka.org
Fri Jul 8 15:29:31 CEST 2005
On Thu, Jul 07, 2005 at 10:21:19PM -0700, Jacob Page wrote:
> Jeremy Moles wrote:
> > This is my first time working with some of the more lower-level python
> > "stuff." I was wondering if someone could tell me what I'm doing wrong
> > with my simple test here?
> > Basically, what I need is an easy way for application in userspace to
> > simply echo values "down" to this fifo similar to the way proc files are
> > used. Is my understanding of fifo's and their capabilities just totally
> > off base?
> You shouldn't need to use select.poll(), unless I'm missing something.
> I was able to get the following to work:
Ok, you miss something ;) The program you proposed does busy waiting
and without a time.sleep call will consume 100% CPU time :(
Actually, with a named fifo the situation gets even nastier:
import os, select, time
fifo = os.open("fifo", os.O_RDONLY)
print "SELECT", select.select([fifo],,)
string = os.read(fifo, 1)
nf = os.open("fifo", os.O_RDONLY)
fifo = nf
# Perhaps add a delay under an else
The problem is, that select (and poll) show a End-Of-File condition by returning
ready to read. But on a FIFO, when the first client terminates, the reading
end goes into a EOF state till somebody else reopens the fifo for writing.
[This bit of wisdom comes Advanced Programming in the UNIX Environment by
W.R. Stevens p. 400: 'If we encounter the end of file on a descriptor, that
descriptor is considered readbale by select.']
closing the old descriptor must be done after opening a new one, or else you
get a tiny moment where a O_WRONLY client is not able to open the file.
This way there is always a reading client of the fifo.
> import os
> fifo = os.open("fifo", os.O_RDONLY | os.O_NONBLOCK)
> while True:
> string = os.read(fifo, 1)
> if len(string):
> print string
> # Perhaps add a delay under an else
> The Python script, when run, does nothing until you put data into the
> fifo from another process. Then it immediately spits the data out,
> character by character.
> I'm assuming that you've already created the fifo and that it's in the
> current working directory.
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