Interpreting binary files as ASCII

Question

I have a binary file (which I've created in C) and I would like to have a look inside the file. Obviously, I won't be able to "see" anything useful as it's in binary. However I do know that contains certain number of rows with numbers in double precision. I am looking for a script to just read some values and print them so I can verify the if they are in the right range. In other words, it would be like doing head or tail in linux on an text file. Is there a way of doing it? Right now I've got something in Python, but it does not do what I want:

CHUNKSIZE = 8192
file = open('eigenvalues.bin', 'rb')
data = list(file.read())
print data

Martijn Pieters · Accepted Answer · 2013-05-10 11:19:15Z

3

Use the array module to read homogenous binary-representation numbers:

from array import array

data = array('d')
CHUNKSIZE = 8192
rowcount = CHUNKSIZE / data.itemsize  # number of doubles we find in CHUNKSIZE bytes

with open('eigenvalues.bin', 'rb') as eg:
    data.fromfile(eg, rowcount)

The array.array type otherwise behaves just like a list, only the type of values it can hold is constricted (in this case to float).

Depending on the input data, you may need to add a data.byteswap() call after reading to switch between little and big-endian. Use sys.byteorder to see what byteorder was used to read the data. If your data was written on a platform using little-endianess, swap if your platform uses the other form, and vice-versa:

import sys

if sys.byteorder == 'big':
    # data was written in little-endian form, so swap the bytes to match
    data.byteswap()

edited May 10, 2013 at 11:19

answered May 10, 2013 at 10:59

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Martijn Pieters Over a year ago

@Seidr: array is a nice complimentary module to struct, if you are dealing with a sequence of just one type of C-standard binary data.

Seidr · Accepted Answer · 2013-05-10 10:55:42Z

3

You can use struct.unpack to convert binary data into a specific data type.

For example, if you want to read the first double from the binary data. (not tested, but believe this is correct)

struct.unpack("d",inputData[0:7])

http://docs.python.org/2/library/struct.html

answered May 10, 2013 at 10:55

Seidr

4,9363 gold badges29 silver badges39 bronze badges

Comments

jlliagre · Accepted Answer · 2013-05-10 11:29:02Z

3

You can see each byte of your file represented in unsigned decimal with this shell command:

od -t u1 eigenvalues.bin | less

Should you want to see a particular area and decode floating point numbers, you can use dd to extract them and od -F option to decode them, eg:

dd status=noxfer if=eigenvalues.bin bs=1 skip=800 count=16 | od -F

will show two double precision numbers stored at offset 800 and 808 in the binary file.

Note that according to the Linux tag set to your question, I assume you are using Gnu versions of dd and od.

answered May 10, 2013 at 11:29

jlliagre

31.1k7 gold badges65 silver badges75 bronze badges

Collectives™ on Stack Overflow

Interpreting binary files as ASCII

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related