binary and out mode to open a file

When I use binary and out mode to open a exist file, and to modify the 4th and 8th byte data to 0x78, I found that the whole file will be rewrite, like below:

#include <fstream>
#include <iostream>
using namespace std;

int main(int argc, char **argv)
{
    fstream out;
    out.open("./test.txt", ios_base::binary | ios_base::out);

    uint8_t data = 0x78;

    out.seekp(4, ios_base::beg);
    out.write((char *)&data, 1);

    out.seekp(8, ios_base::beg);
    out.write((char *)&data, 1);

    out.close();
    return 0;
}

　　And the file original data is :

FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF FF

　　But the file modified is:

00 00 00 00 78 00 00 00 78

　　I have tried ios::app and ios::ate, the ios::app will out put my data at the end of the file and the skeep do not work.

　　Finally, I thought out that the file is just a out file, so the api will not see the original data of the file, so I should add the ios::in.

#include <fstream>
#include <iostream>
using namespace std;

int main(int argc, char **argv)
{
    fstream out;
    out.open("./test.txt", ios_base::binary | ios_base::out | ios_base::in);

    uint8_t data = 0x78;

    out.seekp(4, ios_base::beg);
    out.write((char *)&data, 1);

    out.seekp(8, ios_base::beg);
    out.write((char *)&data, 1);

    out.close();
    return 0;
}

　　

　　And finally, I got the right data.

FF FF FF FF 78 FF FF FF 78 FF FF FF FF FF FF FF