Unix Device File Emulation

Discussion in 'Python' started by blaine, Apr 23, 2008.

  1. blaine

    blaine Guest

    Hey everyone,
    So I've got a quick query for advice.

    We have an embedded device in which we are displaying to an LCD
    device that sits at /dev/screen. This device is not readily available
    all the time, so I am needing to write an emulator. This will
    basically just monitor a file, /dev/screen for example, and write the
    commands to a TK or WxWindows canvas.

    So sending 'line 0 0 10 10' will draw a line on my canvas from (0,0)
    to (10,10).

    My question: Whats the best way to set up a monitor (in python) of
    this file? Would I simply open up the file for read, check for
    changes, get any updated data, and clear the file? Or is there some
    standard way of doing something like this that guarantees no overlap
    or data loss?

    example usage: echo 'line 0 0 10 10' > /dev/screen

    On the actual embedded device this is handled by a kernel module. We
    can spit commands into it as fast as we can and the kernel module can
    keep up. This is typical unix device file behavior.

    Any suggestions or advice would be splendid. Thanks!
    Blaine
     
    blaine, Apr 23, 2008
    #1
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  2. blaine wrote:
    > Hey everyone,
    > So I've got a quick query for advice.
    >
    > We have an embedded device in which we are displaying to an LCD
    > device that sits at /dev/screen. This device is not readily available
    > all the time, so I am needing to write an emulator. This will
    > basically just monitor a file, /dev/screen for example, and write the
    > commands to a TK or WxWindows canvas.
    >
    > So sending 'line 0 0 10 10' will draw a line on my canvas from (0,0)
    > to (10,10).
    >
    > My question: Whats the best way to set up a monitor (in python) of
    > this file? Would I simply open up the file for read, check for
    > changes, get any updated data, and clear the file? Or is there some
    > standard way of doing something like this that guarantees no overlap
    > or data loss?
    >
    > example usage: echo 'line 0 0 10 10' > /dev/screen
    >
    > On the actual embedded device this is handled by a kernel module. We
    > can spit commands into it as fast as we can and the kernel module can
    > keep up. This is typical unix device file behavior.
    >
    > Any suggestions or advice would be splendid. Thanks!
    > Blaine


    It looks as if you need something like the Unix 'tail -f' command.
    Perhaps here is some help

    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/414771
    http://mail.python.org/pipermail/python-list/2002-August/157510.html
    http://pyinotify.sourceforge.net/


    --
    Helmut Jarausch

    Lehrstuhl fuer Numerische Mathematik
    RWTH - Aachen University
    D 52056 Aachen, Germany
     
    Helmut Jarausch, Apr 23, 2008
    #2
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  3. blaine wrote:

    > example usage: echo 'line 0 0 10 10' > /dev/screen
    >
    > On the actual embedded device this is handled by a kernel module. We
    > can spit commands into it as fast as we can and the kernel module can
    > keep up. This is typical unix device file behavior.
    >
    > Any suggestions or advice would be splendid. Thanks!


    Assuming you are on unix, have you considered FIFO's, os.mkfifo()?
     
    Ville M. Vainio, Apr 23, 2008
    #3
  4. blaine

    Dan Upton Guest

    (let's try this again, and actually send it to the list this time)

    On Wed, Apr 23, 2008 at 11:02 AM, blaine <> wrote:
    > Hey everyone,
    > So I've got a quick query for advice.
    >
    > We have an embedded device in which we are displaying to an LCD
    > device that sits at /dev/screen. This device is not readily available
    > all the time, so I am needing to write an emulator. This will
    > basically just monitor a file, /dev/screen for example, and write the
    > commands to a TK or WxWindows canvas.
    >
    > So sending 'line 0 0 10 10' will draw a line on my canvas from (0,0)
    > to (10,10).
    >
    > My question: Whats the best way to set up a monitor (in python) of
    > this file? Would I simply open up the file for read, check for
    > changes, get any updated data, and clear the file? Or is there some
    > standard way of doing something like this that guarantees no overlap
    > or data loss?
    >
    > example usage: echo 'line 0 0 10 10' > /dev/screen
    >
    > On the actual embedded device this is handled by a kernel module. We
    > can spit commands into it as fast as we can and the kernel module can
    > keep up. This is typical unix device file behavior.
    >
    > Any suggestions or advice would be splendid. Thanks!
    > Blaine
    > --
    > http://mail.python.org/mailman/listinfo/python-list
    >


    've only interacted with device files from python that I was only
    reading from. And I guess technically they were under /proc and /sys,
    rather than /dev, although they may be handled the same way. Anyway,
    in that case my method was basically to open the file, read it, close
    it, do whatever processing I needed to do on the data, sleep for some
    interval...lather, rinse, repeat. Sleeping for some interval may or
    may not be appropriate in your case. I know in the case of /proc
    files and /sys files, the files are generated on demand by the kernel
    and relevant kernel structures are basically printed to a string and
    copied into a page in memory that the user program can access. AFAIK,
    you can seek back and forth through them, but I don't know whether the
    data in the page is updated on a seek, so you may have to close and
    reopen the file ever iteration to see what's changed.

    HTH,
    -dan
     
    Dan Upton, Apr 23, 2008
    #4
  5. blaine

    blaine Guest

    On Apr 23, 11:17 am, "Ville M. Vainio" <> wrote:
    > blaine wrote:
    > > example usage: echo 'line 0 0 10 10' > /dev/screen

    >
    > > On the actual embedded device this is handled by a kernel module. We
    > > can spit commands into it as fast as we can and the kernel module can
    > > keep up. This is typical unix device file behavior.

    >
    > > Any suggestions or advice would be splendid. Thanks!

    >
    > Assuming you are on unix, have you considered FIFO's, os.mkfifo()?


    Thank you - this is exactly what I need, I believe. I'm having a
    problem though. The os.mkfifo() works fine, but when I read from the
    file my blocking calls dont work as intended... See below:

    # Fake Nokia Screen Emulator
    import sys, os

    class nokia_fkscrn:
    def __init__(self, file):
    if not os.path.exists(file):
    os.mkfifo(file)
    self.fifodev = open(file, 'r')
    def read(self):
    while 1:
    r = self.fifodev.readline()
    print r

    nokia = nokia_fkscrn('dev.file')
    nokia.read()

    This works at first, but when I write to the 'dev.file' for the first
    time, the text is displayed as intended, but then the program just
    keeps spitting out blank lines. I can continue to write to the file
    (using echo 'test\n' > dev.file) and this shows up in my output, but
    amist a giant mass of scrolling blank lines. This also causes my CPU
    usage to shoot up to 100%.

    Any ideas? This is OS X 10.4
    -Blaine
     
    blaine, Apr 23, 2008
    #5
  6. blaine

    blaine Guest

    On Apr 23, 12:27 pm, "Martin Blume" <> wrote:
    > "blaine" schrieb
    >
    >
    >
    >
    >
    > > # Fake Nokia Screen Emulator
    > > import sys, os

    >
    > > class nokia_fkscrn:
    > > def __init__(self, file):
    > > if not os.path.exists(file):
    > > os.mkfifo(file)
    > > self.fifodev = open(file, 'r')
    > > def read(self):
    > > while 1:
    > > r = self.fifodev.readline()
    > > print r

    >
    > > nokia = nokia_fkscrn('dev.file')
    > > nokia.read()

    >
    > > This works at first, but when I write to the 'dev.file'
    > > for the first time, the text is displayed as intended,
    > > but then the program just keeps spitting out blank lines.
    > > I can continue to write to the file
    > > (using echo 'test\n' > dev.file)
    > > and this shows up in my output, but amist a giant mass
    > > of scrolling blank lines. This also causes my CPU
    > > usage to shoot up to 100%.

    >
    > > Any ideas? This is OS X 10.4

    >
    > while 1:
    > r = self.fifodev.readline()
    > if r: print r
    >
    > According to my docs, readline() returns an empty string
    > at the end of the file.
    > Also, you might want to sleep() between reads a little bit.
    >
    > IMHO. HTH.
    > Martin


    Oh ok, that makes sense. Hmm. So do I not want to use readline()? Or
    is there a way to do something like 'block until the file is not
    empty'?
     
    blaine, Apr 23, 2008
    #6
  7. blaine

    blaine Guest

    On Apr 23, 2:01 pm, "Martin Blume" <> wrote:
    > "blaine" schrieb
    >
    >
    >
    > > > while 1:
    > > > r = self.fifodev.readline()
    > > > if r: print r

    >
    > > > According to my docs, readline() returns an empty
    > > > string at the end of the file.
    > > > Also, you might want to sleep() between reads a
    > > > little bit.

    >
    > > Oh ok, that makes sense. Hmm. So do I not want to use
    > > readline()? Or is there a way to do something like
    > > 'block until the file is not empty'?

    >
    > No,
    > while 1:
    > r = self.fifodev.readline()
    > if r: print r
    > else: time.sleep(0.1)
    > is ok (note the "if r:" clause).
    >
    > Martin


    Beautiful! Thanks Martin!
     
    blaine, Apr 23, 2008
    #7
  8. blaine

    blaine Guest

    On Apr 24, 3:38 am, "A.T.Hofkamp" <> wrote:
    > On 2008-04-23, blaine <> wrote:
    >
    > > On Apr 23, 2:01 pm, "Martin Blume" <> wrote:
    > >> "blaine" schrieb
    > >> No,
    > >> while 1:
    > >> r = self.fifodev.readline()
    > >> if r: print r
    > >> else: time.sleep(0.1)
    > >> is ok (note the "if r:" clause).

    >
    > >> Martin

    >
    > > Beautiful! Thanks Martin!

    >
    > yes, but you have to follow the usual file handling rules, and close/re-open
    > the fifo after detecting EOF. You will be blocked on attempting to re-open
    > until there is another writing process.
    >
    > while 1:
    > fp = open('my_fifo', 'r')
    > while 1:
    > line = fp.readline()
    > if line == '':
    > break
    > print line.rstrip() # To prevent printing of \n in line
    > fp.close()
    >
    > Albert


    Oh, good call. Thanks a lot, that really helps. I felt that the
    previous solution was workable, but not perhaps 100% correct.

    Thanks Albert!
     
    blaine, Apr 25, 2008
    #8
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