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 :)

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)

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.