Query size of block device file in Python

Question

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?

Answer 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)

Answer 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 = b' ' * 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.