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poll of filesystem

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  poll of filesystem Belisko Marek <marek.belisko@gmail.com> - 2011-07-02 11:40 +0200
    Re: poll of filesystem Nobody <nobody@nowhere.com> - 2011-07-02 18:49 +0100
    Re: poll of filesystem Tim Roberts <timr@probo.com> - 2011-07-02 15:37 -0700

#8660 — poll of filesystem

Hi,

just want to use poll method to get data from /proc file system. Use
simple code for it. Problem is it seems poll return POLLIN flag but
when I try to read data it's always empty. Could be a problem changes
are so fast that print can't print it?

Code:

#!/usr/bin/python

import select
from select import POLLIN, POLLERR

p = select.poll()

fd = open("/proc/loadavg", "r")

p.register(fd.fileno(), POLLIN)

while True:
events = p.poll(1000)
	if (events == -1):
		print "timeout"
	fd0, event = events[0]
	if (event == POLLIN):
		print fd.read()

Thanks,

marek
-- 
as simple and primitive as possible
-------------------------------------------------
Marek Belisko - OPEN-NANDRA
Freelance Developer

Ruska Nova Ves 219 | Presov, 08005 Slovak Republic
Tel: +421 915 052 184
skype: marekwhite
icq: 290551086
web: http://open-nandra.com

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#8673

On Sat, 02 Jul 2011 11:40:46 +0200, Belisko Marek wrote:

> just want to use poll method to get data from /proc file system. Use
> simple code for it. Problem is it seems poll return POLLIN flag but
> when I try to read data it's always empty. Could be a problem changes
> are so fast that print can't print it?

poll() doesn't work for files; they are always readable. It's meant for
pipes, sockets and character devices (ttys, etc). If you're looking for a
way to find out when a file has changed, there isn't one (Linux has
inotify, but that isn't portable and it doesn't work with /proc).

The reason why fd.read() doesn't return any data after the first call is
that you're already at EOF; you need to reset the file position with
fd.seek(0) to read more data (however: for /proc, I would recommend
closing the file and reopening it).

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#8690

Belisko Marek <marek.belisko@gmail.com> wrote:
>
>just want to use poll method to get data from /proc file system. Use
>simple code for it. Problem is it seems poll return POLLIN flag but
>when I try to read data it's always empty. Could be a problem changes
>are so fast that print can't print it?

Poll doesn't make sense here.  The data in /proc/loadavg data is not text
data that gets updated periodically.  There is no file on disk somewhere
called /proc/loadavg.  Instead, reading /proc/loadavg causes a call into a
kernel driver, and reads data directly from variables stored in the kernel.
The data is ALWAYS available.

If you want the data once a second, just do it the easy way:

    import time
    while True:
        print open('/proc/loadavg').read()
        time.sleep(1)
-- 
Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.

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