how to write binary file without deleting its previous contents
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Thread: how to write binary file without deleting its previous contents

  1. #1
    Join Date
    Nov 2016
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    how to write binary file without deleting its previous contents

    I am writing a binary file like this:
    Code:
    const int sized = 300000000ULL;
    int ad[sized];
    ...
    ad[i] = some value;
    ...
                FILE* pFile;
                pFile = fopen("obj_pattern.bin", "wb");
                fwrite(ad, 1, sized*sizeof(int), pFile);
                fclose(pFile);
    Now how can i again write given binary file obj_pattern.bin with some new values of ad[i] in another program. For example in first program i put ad[] values in the file as 1,2 now how can i again write that same file without deleting the previous saved values 1 and 2?
    Last edited by dinesh999lama; February 26th, 2017 at 09:36 PM.

  2. #2
    2kaud's Avatar
    2kaud is offline Super Moderator Power Poster
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    Location
    England
    Posts
    5,632

    Re: how to write binary file without deleting its previous contents

    Mode "wb" will delete any existing contents. If you open the file as mode "ab", this will append the data to the existing contents. There is also mode "a+b" which will allow reading from the file at a specified position but writing to the end of the file. If you use mode "r+b" then you can position the file pointer to the end of the file before you write but the file must exist.

    See https://msdn.microsoft.com/en-us/library/yeby3zcb.aspx
    Last edited by 2kaud; February 27th, 2017 at 02:32 AM.
    All advice is offered in good faith only. You are ultimately responsible for the effects of your programs and the integrity of the machines they run on. Anything I post, code snippets, advice, etc is licensed as Public Domain https://creativecommons.org/publicdomain/zero/1.0/

    C, C++ Compiler: Microsoft VS2017.2

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