rewrite part of binary file

Question

I have image file with header and metainformation, image data stored after header, I want to rewrite image data, but other data around imagesection is corrupted(header) and file truncated.

How can I done it properly?

Here is the code:

FILE* f = _tfopen(fileName, _T("wb"));
if( f != NULL )
{
        uint64 headerSize = 8;
        char arr[2*3*3]; //w=3 h=2  RGB
        memset(arr,100,2*3*3);
        _fseeki64(f, headerSize, SEEK_SET);
        fwrite(arr,sizeof(char),sizeof(arr),f);
        fclose(f);
}

Answer 1

I think you need to change open mode from your file from "wb" to "r+b"

from cplusplus.com:

"w" write: Create an empty file for output operations. If a file with the same name already exists, its contents are discarded and the file is treated as a new empty file.