how to write binary file without deleting its previous contents

[dinesh999lama]

I am writing a binary file like this:

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?