Nathan McBride wrote:
> Yup, I got some help in IRC. What I ended up doing was using regex to
> pull out each "/dev/loopX". Then
> took the X and fed it to max which in turn gave me the highest numbered
> loop device in use. After which I
> then just added 1 to X and added it to the end of "/dev/loop_". The
> only other thing I had to do was put a
> check in incase there were no used loop devices in which case then it
> defaults to "/dev/loop0".
One might also consider reimplementing 'losetup -f' in python, but after
my own attempt I realize it might not be that practical, and is
potentially dangerous I suppose -- if, like me, you don't fully
understand the underlying system calls. I've attached my attempt for the
sake of discussion only, and not as a solution -- perhaps someone with
interest will correct any errors and make it usable. I would definitely
appreciate it.
Drifting off topic now... I copied most of the find_unused_loop_device
implementation in util-linux/lomount.c[1]. The main points of interest,
and potential doom due to my ignorance, are related to the fcntl.ioctl call:
1) the LOOP_GET_STATUS ioctl op const, isn't exposed in any of the
typical python modules that I can find, and as such I'm worried that
it's value is somehow platform dependent.
2) for a similar reason, I am passing a string of the largest allowed
length as the 3rd arg to the fcntl.ioctl call on line 33, since the size
of the returned data seems to be governed by a struct defined in loop.h,
which needs dev_t from a kernel header. Whew. This seems to work fine on
my ubuntu system, if sloppy. But, since I don't know, I tend to assume
it could cause problems with stability or security.
Anyway, thanks for the interesting question Nathan. Now I have some
reading to do. :)
Marty
[1]
http://www.google.com/codesearch?hl=en&q=show:3q3vE6vLdaY:0lRVP2J7BtU:j-QqODsRp3s&sa=N&ct=rd&cs_p=ftp://ftp.kernel.org/pub/linux/utils/util-linux/testing/util-linux-2.13-pre7.tar.gz&cs_f=util-linux-2.13-pre7/mount/lomount.c&start=1
#!/usr/bin/env python2.5
import os
import stat
import errno
import fcntl
if os.uname()[4] == 'x86_64':
LOOP_GET_STATUS = 0x4C05
else:
LOOP_GET_STATUS = 0x4C03
def find_unused_loop_device():
"""
Return the next unused loop device node, returns None if the
next device cannot be determined (and swallows exceptions
encountered along the way: permission denied, no such file, etc).
"""
for loop_format in ['/dev/loop%d', '/dev/loop/%d']:
for i in range(256):
dev = loop_format % i
try:
st = os.stat(dev)
except OSError:
break # assume invalid loop_format
if stat.S_ISBLK(st.st_mode):
try:
fd = os.open(dev, os.O_RDONLY)
except OSError:
pass # assume permission denied
else:
try:
fcntl.ioctl(fd, LOOP_GET_STATUS, 1024*'\x00')
except IOError, e:
if e.errno == errno.ENXIO:
os.close(fd)
return dev
os.close(fd)
if __name__ == '__main__':
print find_unused_loop_device()
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