fallocate.py

#!/usr/bin/python
# -*- coding: utf-8 -*-
import ctypes
import ctypes.util
import os

# reserve space for the contents of file before you write it
# (hope to disable preallocation)

OK = 0
NOTOK = 0x01

# the mode argument to fallocate is defined in /usr/include/linux/falloc.h

FALLOC_FL_KEEP_SIZE = 0x01  # default is extend size
FALLOC_FL_PUNCH_HOLE = 0x02  # de-allocates range


# this function is used if we can't load the real libc function

def noop_libc_function(*args):
    return OK


# I have no idea what this code really does, but strace says it works.
# does this code work under Cygwin?

def load_libc_function(func_name):
    func = noop_libc_function
    try:
        libc = ctypes.CDLL(ctypes.util.find_library('c'))
        func = getattr(libc, func_name)
    except AttributeError:
        # print("Unable to locate %s in libc.  Leaving as a no-op."% func_name)
        pass
    return func


# do this at module load time

_posix_fallocate = load_libc_function('fallocate64')


# mode is one of FALLOC constants above

def fallocate(fd, mode, offset, length):
    return _posix_fallocate(fd,
                            mode,
                            ctypes.c_uint64(offset),
                            ctypes.c_uint64(length))

# unit test

if __name__ == '__main__':
    fd = os.open('/tmp/foo', os.O_WRONLY | os.O_CREAT)
    assert fd > 0x02
    ret = fallocate(fd, FALLOC_FL_KEEP_SIZE, 0, 8)
    assert ret == OK
    ret = os.write(fd, 'hi there')
    assert ret == 8
    os.close(fd)