how to read info from binary file

Question

I have a binary file and specifications:

after 'abst' (0x61627374):
var1 Unsigned 8-bit integer
var2 Unsigned 24-bit integer
var3 Sequence of Unicode 8-bit characters (UTF-8), terminated with 0x00 

How to read var1,var2,var3 from file ?

Answer 1

Quick and dirty and not tested:

# assumption: the file is small enough to fit into the RAM
# and also that 'abst' does not occur in the dataset
for hunk in input.split('abst')[1:]: # skip first hunk, since it is the stuff befor the first 'abst' occurence
 var1 = ord(hunk[0])
 var2 = ord(hunk[1]) + ord(hunk[2])*256 + ord(hunk[3])*256*256
 var3 = hunk[4:].split('\x00')[0]

• John Machin: also if there is guff before 'abst' you will unpack that.
• Rudi: @John Thank you. (And I even got caught by the first hunk error yesterday grml)

Answer 2

The bitstring module might be helpful here as you have unusual bit lengths, and it can be a bit more readable than unpacking values 'by hand':

import bitstring
bitstring.bytealigned = True
s = bitstring.ConstBitStream(your_file)
if s.find('0x61627374'): # seeks to your start code
 start_code, var1, var2 = s.readlist('bytes:4, uint:8, uint:24')
 p1 = s.pos
 p2 = s.find('0x00', start=p1) # find next '\x00'
 var3 = s[p1:p2+8].bytes # and interpret the slice as bytes