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

[krishna...@gmail.com]

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?


Thanks!

[mar...@python.net]

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

[Roy Smith]

In article <50bf9366-46e0-4a7f...@googlegroups.com>,

In general, seek() works on special files, when it makes sense. But,
the "in general" part is critical. Not all devices support the seek
operation. I have no idea what /dev/relpcfpga is (a google search for
relpcfpga came up with exactly one hit -- your post!) so I can't tell
you if it supports seek() or not.

[Andreas Perstinger]

On 14.05.2013 21:00, krishna...@gmail.com wrote:
> # first, open the file as a plain binary
> try:
> self.file = open(/dev/relpcfpga, "r+b", buffering=0)

Aren't you missing the quotes for "/dev/relpcfpga"?

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

Are you sure that your device is seekable?
Try

f = open("/dev/relpcfpga", "r+b", buffering=0)
print(f.seekable())

Bye, Andreas