Editing a big file in C++

[ArtigoOne]

I would like to know if there is a simple way to edit a binary file without rewriting it all or loading it in memory.

For example if I want to modify from byte 10 to byte 20 of a 4TB file.

Thank you for your help.

[seeplus]

Open the file for i/o in binary mode, seek to the starting byte, then write the required data to overwrite what is already there.

See
http://www.cplusplus.com/reference/ostream/ostream/seekp/
http://www.cplusplus.com/reference/ostream/ostream/write/

[salem c]

If you're not changing the length of the file at all, then you can just edit the bytes you want in situ.

Otherwise, you have to rewrite the whole file.

[ArtigoOne]

Thank you for replies. Why in my code it overwrites the file, which becomes of 3 bytes of length?

void change_byte()
{
    std::ofstream f{ _fname, std::ios_base::binary };

    if (f)
    {
        f.seekp(2);
            
        char* buffer = new char[1];
        buffer[0] = 0x88;

        f.write(buffer, 1);

        delete[] buffer;

        f.close();
    }
    else
    {
        throw std::runtime_error("File error.");
    }
}

[ArtigoOne]

I found the solution, opening file with:

std::ofstream f{ _fname, std::ios_base::binary | std::ios_base::out | std::ios_base::in };

Thank you for your help.