Thread: Modifying portion of binary files?

  1. #1
    Registered User
    Join Date
    Aug 2012

    Question Modifying portion of binary files?


    I have a large binary file and I only need my code to modify a small portion of it in the middle. What is the best way to read in, modify, and then save to a new file?

    My knowledge of C is basic (just one module at university) so please try and explain things which may not be obvious.


  2. #2
    Registered User
    Join Date
    Sep 2006
    I'm unsure what you're looking for, but I'll give you a big Welcome! to the board, anyway!

    Recommended way:
    Open the file in binary mode (read and write mode),
    read the data into a large buffer, writing out the parts that you don't want changed, to a temp file (which you have opened in binary mode as well).
    locate the part that you want changed, when it's in the buffer,
    make the change in the buffer, and finish writing out the correct data to the temp file.
    confirm the new file has the right size.
    delete the original file
    rename the temp file with the original file's name.

    Although you CAN write data to a binary file directly and make the change, it's fraught with the possibility of an error occurring, and wrecking a part, or all of, the data file. If the data file is all backed up already, it may not be a problem to take this risk - but that seems to never be the case, does it?

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