How to find binary data within ConstBitStream starting from a given offset?

Question

I'm trying to find specific bytes within a file loaded as ConstBitStream:

s = ConstBitStream(filename=myFile)
found = s.find('0x4140', bytealigned=False)

This is working fine for the first occurrence. After finding the first sequence, I want to find the next one by using the find method again but starting from an offset now:

s.bytepos = position_after_the_first_occurrence + my_offset
found = s.find('0x4140', start=s.bytepos, bytealigned=False)

This doesn't seem to work. I'm always getting the position from the first occurrence of my binary sequence.

What's wrong?

UPDATE:

(values of first found and s.bytepos):

found = {tuple} (54784, )
s.bytepos = {int} 6848

(values of second found and s.bytepos):

s.bytepos = {int} 32969
found = {tuple} (54784, )

It seems that setting start=s.bytepos does not have any effect.

Can you print out the values of `found` (the first value) and `s.bytepos`? — Alex Hall, Apr 23 '16 at 22:55
Don't use `s.bytepos` as a value for `start` argument, use `s.pos` instead. — user2683246, Apr 24 '16 at 14:32
Be sure that `my_offset` counts bytes, not bits. Be aware that `found` value is a tuple with bit offset, not byte offset. — user2683246, Apr 24 '16 at 17:42
s.pos solved the problem but did not work in my case anyway. i wrote my own search algorithm. you should write this as an answer so i can accept. — mr.proton, Apr 24 '16 at 20:10

score 1 · Accepted Answer · answered May 05 '16 at 20:40

The start parameter is the bit position to start the search, not the byte position. To get the next occurrence you need to use start=s.bitpos + 1.

Another thing to note is that if you use bytealigned=False (which is the default) then you can't in general even use s.bytepos afterwards as the current position might not be byte aligned (it will raise a ByteAlignError). Chances are you want bytealigned=True, which is also a fair bit faster.

Also note you can just use

g = s.findall('0x4140')

which returns a generator that gives all the positions without having to do multiple find calls (just use g.next() repeatedly or list(g) to get them all at once).

How to find binary data within ConstBitStream starting from a given offset?

1 Answers1