Query size of block device file in Python

问题

I have a Python script that reads a file (typically from optical media) marking the unreadable sectors, to allow a re-attempt to read said unreadable sectors on a different optical reader.

I discovered that my script does not work with block devices (e.g. /dev/sr0), in order to create a copy of the contained ISO9660/UDF filesystem, because os.stat().st_size is zero. The algorithm currently needs to know the filesize in advance; I can change that, but the issue (of knowing the block device size) remains, and it's not answered here, so I open this question.

I am aware of the following two related SO questions:

Determine the size of a block device (/proc/partitions, ioctl through ctypes)
how to check file size in python? (about non-special files)

Therefore, I'm asking: in Python, how can I get the file size of a block device file?

回答1:

The “most clean” (i.e. not dependent on external volumes and most reusable) Python solution I've reached, is to open the device file and seek at the end, returning the file offset:

def get_file_size(filename):
    "Get the file size by seeking at end"
    fd= os.open(filename, os.O_RDONLY)
    try:
        return os.lseek(fd, 0, os.SEEK_END)
    finally:
        os.close(fd)

回答2:

Linux-specific ioctl-based solution:

import fcntl
import struct

device_path = '/dev/sr0'

req = 0x80081272 # BLKGETSIZE64, result is bytes as unsigned 64-bit integer (uint64)
buf = ' ' * 8
fmt = 'L'

with open(device_path) as dev:
    buf = fcntl.ioctl(dev.fileno(), req, buf)
bytes = struct.unpack('L', buf)[0]

print device_path, 'is about', bytes / (1024 ** 2), 'megabytes'

Other unixes will have different values for req, buf, fmt of course.

回答3:

Another possible solution is

def blockdev_size(path):
    """Return device size in bytes.
    """
    with open(path, 'rb') as f:
        return f.seek(0, 2) or f.tell()

or f.tell() part is there for Python 2 portability's sake — file.seek() returns None in Python 2.

Magic constant 2 may be substituted with io.SEEK_END.

回答4:

Trying to adapt from the other answer:

import fcntl
c = 0x00001260 ## check man ioctl_list, BLKGETSIZE
f = open('/dev/sr0', 'ro')
s = fcntl.ioctl(f, c)
print s

I don't have a suitable computer at hand to test this. I'd be curious to know if it works :)

来源：https://stackoverflow.com/questions/2773604/query-size-of-block-device-file-in-python

标签

python

Linux

filesize

device