Illegal seek error with seek() and os.lseek() in python

Question

I am trying to use os.open() and os.lseek() methods to operate on a device file in Linux. My code goes something like this -

# first, open the file as a plain binary
try:
self.file = open(/dev/relpcfpga, "r+b", buffering=0)

except IOError:
raise IOError ('Failed to open.')

# Figure out file size
self.file.seek(0, 2)
self.file_size = self.file.tell()

The method seek() complains "OSError: [Errno 29] Illegal seek"
The device relpcfpga is a char device.

The same code works with a normal text file.
I have tried to use os.open() and os.lseek() methods, but see the same error.
Is there a different method to operate on device files?

Answer 1

Some file streams are not seekable. Specifically, some (all?) char
devices aren't seekable (because e.g. they can't be rewound or they have
no end). You'd get the same error in C (well it would return -1).

See also: http://www.linuxintro.org/wiki/Device