Get position for file descriptor in Python

问题:

Say, I have a raw numeric file descriptor and I need to get the current position in file based on it.

import os, psutil

# some code that works with file
lp = lib.open('/path/to/file')

p = psutil.Process(os.getpid())
fd = p.get_open_files()[0].fd  # int

while True:
    buf = lp.read()
    if buf is None:
        break
    device.write(buf)
    print tell(fd)  # how to find where we are now in the file?

In the following code lib is a compiled library, that doesn't give access to the file object. In the loop I use the embedded method read which returns the processed data. The data and it's length doesn't relate to the file position, so I can't calculate the offset mathematically.

I tried to use fdopen, as fd = fdopen(p.get_open_files()[0].fd), but print fd.tell() returned only the first position in the file, which was not updating in the loop.

Is there a way to get the current real time position in a file based on file descriptor?

回答1:

So, the answer seems to be quite easy. I had to use os.lseek with SEEK_CUR flag:

print os.lseek(fd, 0, os.SEEK_CUR)

I don't know if it is the only approach, but at least it works fine.

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