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Read from /dev/zero

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Join Date: Nov 2007

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mn_kthompson mn_kthompson is offline

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Read from /dev/zero

Nov 12th, 2007

I'm curious why this bit of code doesn't work. When run it returns a blank line. Note: this is making use of a linux device file so it should not be expected to run properly on Windows.

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 python Syntax (Toggle Plain Text) 
#!/usr/bin/env python
import os
 
infile = os.open("/dev/zero", os.O_RDONLY)
charout = os.read(infile,1)
os.close(infile)
print charout
#!/usr/bin/env python
import os

infile = os.open("/dev/zero", os.O_RDONLY)
charout = os.read(infile,1)
os.close(infile)
print charout

I'm pretty new to Python so I might be way off base. I appreciate any suggestions.

Last edited by mn_kthompson; Nov 12th, 2007 at 5:21 pm.

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Join Date: Nov 2007

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Re: Read from /dev/zero

Nov 13th, 2007

OK, I have been able to verify that something is being read from my device file, but nothing is printable. I have modified the code to read as follows

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
#!/usr/bin/env python
import os
 
charout = "XXX"
print len(charout)
infile = os.open("/dev/zero", os.O_RDONLY)
charout = os.read(infile,51)
os.close(infile)
print charout
print len(charout)
#!/usr/bin/env python
import os

charout = "XXX"
print len(charout)
infile = os.open("/dev/zero", os.O_RDONLY)
charout = os.read(infile,51)
os.close(infile)
print charout
print len(charout)

And here is the output:

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(Toggle Plain Text)

kevin@MSU1375020:~$ python read.py
3

51

So you can see that the charout file is getting 51 bytes of data, but it still wont print anything but a blank line. Does this give anyone any clues as to why I'm not getting a bunch of zeros on my screen?

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Join Date: Dec 2006

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woooee

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Re: Read from /dev/zero

Nov 13th, 2007

They are not printable characters (probably) and so who knows where they come from. To test this, try printing the decimal value of each character. It might just be a bunch of decimal zeroes (which is zero, the printable zero is decimal 48), or EOFs, or garbage created by some sort of "noise".

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
for each_char in charout:
     print ord(each_char),
print
for each_char in charout:
     print ord(each_char),
print

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Join Date: Nov 2007

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Re: Read from /dev/zero

Nov 14th, 2007

Hey that worked for me, I got the zeros I was expecting. I guess when you read characters from a device file you might not be getting printable characters. I thought that python would have casted the characters into strings
Thanks!

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Join Date: Dec 2006

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woooee

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Re: Read from /dev/zero

Nov 14th, 2007

Zero evidently means that nothing was read and it will probably continue trying to read until you tell it to stop, which is why there are multiple zeros.