For illustration purposes, this script creates a file mapfile containing the content of the files, given as arguments, prepended by a binary header with a sha1 checksum, that allows for duplication detection on subsequent runs.
What's needed here is a hashable ctypes.c_char replacement, that can hold sha1 checksums with minimum fuzz, but not choking on '\0' bytes.
# -*- coding: utf8 -*
import io
import mmap
import ctypes
import hashlib
import logging
from collections import OrderedDict
log = logging.getLogger(__file__)
def align(size, alignment):
"""return size aligned to alignment"""
excess = size % alignment
if excess:
size = size - excess + alignment
return size
class Header(ctypes.Structure):
Identifier = b'HEAD'
_fields_ = [
('id', ctypes.c_char * 4),
('hlen', ctypes.c_uint16),
('plen', ctypes.c_uint32),
('name', ctypes.c_char * 128),
('sha1', ctypes.c_char * 20),
]
HeaderSize = ctypes.sizeof(Header)
class CtsMap:
def __init__(self, ctcls, mm, offset = 0):
self.ctcls = ctcls
self.mm = mm
self.offset = offset
def __enter__(self):
mm = self.mm
offset = self.offset
ctsize = ctypes.sizeof(self.ctcls)
if offset + ctsize > mm.size():
newsize = align(offset + ctsize, mmap.PAGESIZE)
mm.resize(newsize)
self.ctinst = self.ctcls.from_buffer(mm, offset)
return self.ctinst
def __exit__(self, exc_type, exc_value, exc_traceback):
del self.ctinst
self.ctinst = None
class MapFile:
def __init__(self, filename):
try:
# try to create initial file
mapsize = mmap.PAGESIZE
self._fd = open(filename, 'x+b')
self._fd.write(b'\0' * mapsize)
except FileExistsError:
# file exists and is writable
self._fd = open(filename, 'r+b')
self._fd.seek(0, io.SEEK_END)
mapsize = self._fd.tell()
# mmap this file completely
self._fd.seek(0)
self._mm = mmap.mmap(self._fd.fileno(), mapsize)
self._offset = 0
self._toc = OrderedDict()
self.gen_toc()
def gen_toc(self):
while self._offset < self._mm.size():
with CtsMap(Header, self._mm, self._offset) as hd:
if hd.id == Header.Identifier and hd.hlen == HeaderSize:
self._toc[hd.sha1] = self._offset
log.debug('toc: [%s]%s: %s', len(hd.sha1), hd.sha1, self._offset)
self._offset += HeaderSize + hd.plen
else:
break
del hd
def add_data(self, datafile, data):
datasize = len(data)
sha1 = hashlib.sha1()
sha1.update(data)
digest = sha1.digest()
if digest in self._toc:
log.debug('add_data: %s added already', digest)
return None
log.debug('add_data: %s, %s bytes, %s', datafile, datasize, digest)
with CtsMap(Header, self._mm, self._offset) as hd:
hd.id = Header.Identifier
hd.hlen = HeaderSize
hd.plen = datasize
hd.name = datafile
hd.sha1 = digest
del hd
self._offset += HeaderSize
log.debug('add_data: %s', datasize)
blktype = ctypes.c_char * datasize
with CtsMap(blktype, self._mm, self._offset) as blk:
blk.raw = data
del blk
self._offset += datasize
return HeaderSize + datasize
def close(self):
self._mm.close()
self._fd.close()
if __name__ == '__main__':
import os
import sys
logconfig = dict(
level = logging.DEBUG,
format = '%(levelname)5s: %(message)s',
)
logging.basicConfig(**logconfig)
mf = MapFile('mapfile')
for datafile in sys.argv[1:]:
if os.path.isfile(datafile):
try:
data = open(datafile, 'rb').read()
except OSError:
continue
else:
mf.add_data(datafile.encode('utf-8'), data)
mf.close()
Run: python3 hashable_ctypes_bytes.py somefiles*
Invoking it a second time, it reads through the file collecting all items in an ordered dict, using the sha1 digest as key. Unfortunately, the c_char array semantics are a little wired, because it also behaves like '\0' terminated c string, resulting in truncated checksums here.
See line 3 and 4:
DEBUG: toc: [20]b'\xcd0\xd7\xd3\xbf\x9f\xe1\xfe\xffr\xa6g#\xee\xf8\x84\xb5S,u': 0
DEBUG: toc: [20]b'\xe9\xfe\x1a;i\xcdG0\x84\x1b\r\x7f\xf9\x14\x868\xbdVl\x8d': 1273
DEBUG: toc: [19]b'\xa2\xdb\xff$&\xfe\x0f\xb4\xcaB<F\x92\xc0\xf1`(\x96N': 3642
DEBUG: toc: [15]b'O\x1b~c\x82\xeb)\x8f\xb5\x9c\x15\xd5e:\xa9': 4650
DEBUG: toc: [20]b'\x80\xe9\xbcF\x97\xdc\x93DG\x90\x19\x8c\xca\xfep\x05\xbdM\xfby': 13841
DEBUG: add_data: b'\xcd0\xd7\xd3\xbf\x9f\xe1\xfe\xffr\xa6g#\xee\xf8\x84\xb5S,u' added already
DEBUG: add_data: b'\xe9\xfe\x1a;i\xcdG0\x84\x1b\r\x7f\xf9\x14\x868\xbdVl\x8d' added already
DEBUG: add_data: b'../python/tmp/colorselect.py', 848 bytes, b'\xa2\xdb\xff$&\xfe\x0f\xb4\xcaB<F\x92\xc0\xf1`(\x96N\x00'
DEBUG: add_data: 848
DEBUG: add_data: b'../python/tmp/DemoCode.py', 9031 bytes, b'O\x1b~c\x82\xeb)\x8f\xb5\x9c\x15\xd5e:\xa9\x00p\x0f\xc04'
DEBUG: add_data: 9031
DEBUG: add_data: b'\x80\xe9\xbcF\x97\xdc\x93DG\x90\x19\x8c\xca\xfep\x05\xbdM\xfby' added already
Usual suggestion is replace the c_char * 20 with c_byte * 20, and loosing the transparent bytes handling on that way. Apart from the data conversion hassles, c_byte arrays aren't hashable due to being bytearrays. I haven't found a practical solution without going through heavy conversion troubles all the way back and forth, or resorting to hexdigests, that doubles the sha1 digest size requirements.
I think, that the c_char design decision to mix it with C zero termination semantics was a fault in the first place. To cope with this, I could imagine to add a c_char_nz type to ctypes, that resolves this issue.
For those of you, that read the code carefully, you might wonder about the del statements of the ctypes structures. A discussion of it can be found here:.
While the below code is doing the back-and-forth conversion you mentioned, it does nicely hide the issue. I found a hash that contained a null and the field can now be used as a dictionary key. Hope it helps.
from ctypes import *
import hashlib
class Test(Structure):
_fields_ = [('_sha1',c_ubyte * 20)]
@property
def sha1(self):
return bytes(self._sha1)
@sha1.setter
def sha1(self, value):
self._sha1 = (c_ubyte * 20)(*value)
test = Test()
test.sha1 = hashlib.sha1(b'aaaaaaaaaaa').digest()
D = {test.sha1:0}
print(D)
Output:
{b'u\\\x00\x1fJ\xe3\xc8\x84>ZP\xddj\xa2\xfa#\x89=\xd3\xad': 0}
来源：https://stackoverflow.com/questions/41078754/python-ctypes-hashable-c-char-array-replacement-without-tripping-over-0-byte