Modifying binary file with Python

Question

i am trying to patch a hex file. i have two patch files (hex) named "patch 1" and "patch 2"

the file to be patched is a 16 MB file named "file.bin".

i have tried many different way for the past 6 or 7 hours to figure out how to do it. I can write a string to a file all day long, but i am trying to do the following:

open patch1.bin with read bytes open patch2.bin with read bytes open file.bin with write bytes

i want to seek to positions 0xc0010, and 0x7c0010, and apply patch1.bin then i want to seek to 0x040000 and apply patch2.bin

so all in all i will have 3 patches applied, then close the "file.bin"

if someone cold give me an example i would very much appreciate it :)

i tried this first:

patch1 = open("patch1", "r");
patch2 = open("patch2", "r");
main = open("file.bin", "w");
main.seek(0xC0010);
main.write(patch1);
main.seek(0x7C0010);
main.write(patch1);
main.seek(0x40000);
main.write(patch2);
main.close();

but was informed i was was trying to write a string to a file, when indeed its not what i wanted, lol then i tried this:

infile1 = open("patch1.bin", "rb") 
new_pos1 = int("0x00", 16)
infile1.seek(new_pos1, 0)
infile2 = open('file.bin', 'wb')
new_pos2 = int('0xc0010', 16)
infile2.seek(new_pos2, 0xc0010)
chunk1 = int("6FFFE0", 16) #this is how long patch1 file is
data1 = infile1.read(chunk1)
with open("file.bin", "a") as outfile:
 outfile.write(data1)

but it did not work either, as no matter what i tried, i could not get it to write the data at he correct offset.

I did manage a few times to write the patch1 to file.bin, but it did not patch at the right offset, as a matter of fact it deleted the file.bin and just copied patch1 in its place. which ofcourse is wrong.

i must remind you i am new to python and programming, but i am really trying to dig my feet into it and learn, so any good examples will be examined and hopefully will be a good learning lesson for me :)

thanks guys and gals for helping me figure out what i was doing wrong :)

• Antony Hatchkins: What did you try? Quote your code.
• unwind: That is not a "hex file"; it's a binary file. A hex file would be a text file containing hexadecimal numbers. These are often used to represent binary data, when you want to work with text.

Answer 1

You only need to use seek and write. Use seek to jump to the position and write to overrwite the existing data.

with file('patch1.bin', 'rb') as fh:
 patch1 = fh.read()
with file('patch2.bin', 'rb') as fh:
 patch2 = fh.read()
with file('file.bin', 'r+b') as fh:
 # apply patch1
 fh.seek(0xc0010)
 fh.write(patch1)
 fh.seek(0x7c0010)
 fh.write(patch1)
 # apply patch2
 fh.seek(0x040000)
 fh.write(patch2)

• james28909: ok i figured out what it was, with file('file.bin', 'w') needed to be 'r+'.... after i changed that the code works flawless. thank you very much :)
• Simon Callan: When opening the patch files, you should open them in binary mode - open("<fn>", "rb"), while the file to be patched should be opened in "r+b" mode.
• Antony Hatchkins: w mode truncates the file. Use r+b for patching existing file.
• james28909: well, i tested the files byte for byte, and when i run the script, it is different than when i manually patch the file, i tested with HxD and use file compare function. is there a way to make it more accurate?
• james28909: nvm, i figured it out. while i was screwing things up, i managed to delete all the data in one of my patch files, lol. i recopied it and all is well and the files are identical. thanks everyone :)

Answer 2

1.txt:

asdf

a.py:

with open('1.txt','r+b') as f:
 f.seek(2)
 f.write('D')

1.txt:

asDf

This should give you a clue.

• james28909: yes, and i now know what i was doing worng ;) thanks for schooling a noob lol

Answer 3

You need to use the r+b mode for editing the target file. wb is the mode for writing without updating and will truncate an existing file. Check out http://docs.python.org/2/library/functions.html#open or your OS' man page for fopen to get details on the different file modes.

• Antony Hatchkins: w+b truncates the file. Use r+b in this case.