Writing to begging of file with fstream

Hello,

I am trying to stream data to a file, and then return to the file to add further data. When I add data the second time, I then want to update the value of the second byte in the whole file. I can't seem to do this!
Here is my sample code:

Code:

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int a = 1;
int b = 2;
int c = 3;
int d = 4;
int e = 5;
int f = 6;
int g = 7;
int x;

fstream out1("file.dat", ios::out | ios::binary | ios::trunc);
out1.write((char*)&a, sizeof(int));
out1.write((char*)&b, sizeof(int));
out1.write((char*)&c, sizeof(int));
out1.close();

fstream out2("file.dat", ios::out | ios::binary | ios::app);
out2.write((char*)&d, sizeof(int));
out2.write((char*)&e, sizeof(int));
out2.write((char*)&f, sizeof(int));
// Seek to the end of the first integar entry
out2.seekp(sizeof(int), ios::beg);
// Change the existing value to "7"
out2.write((char*)&g, sizeof(int));
out2.close();

fstream in("file.dat", ios::in | ios::binary);
in.seekg(0, ios::beg);
for (int i = 0; i < 10; i++)
{
        in.read((char*)&x, sizeof(int));
        cout << x;
}

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The output I get is "1, 2, 3, 4, 5, 6", but I want to be getting "1, 7, 3, 4, 5, 6", because in "out2", I seekp to the second integar entry, and change it to "7".

I have also tried using ios::ate in the constructor for "out2", but this gives me the out put "4, 7, 6, 6, 6, 6", which is suggesting that when I create my fstream object "in", any seekg commands are relative to the beginning of the "out2" stream, rather than the "out1" stream.

Thanks for any help!

You open the file with ios::app (append mode). In that mode you cannot seek for a write effectively since every write first moves the file pointer to the end of the file. Get rid of ios::app, add ios::in (as well as ios:: out), and seek to the end to append.

Code:

---

  fstream out2("file.dat", ios::in | ios::out | ios::binary);
  out2.seekp(0, ios::end);
  out2.write((char*)&d, sizeof(int));
  out2.write((char*)&e, sizeof(int));
  out2.write((char*)&f, sizeof(int)); // Seek to the end of the first integar entry
  out2.seekp(sizeof(int), ios::beg);  // Change the existing value to "7"
  out2.write((char*)&g, sizeof(int));
  out2.close();

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And you should probably only print 6 numbers in your for loop, since that's all you're writing

to the file. Or better yet, use more general loop control:

Code:

---

  while (in.read((char*)&x, sizeof(int)))
    cout << x;

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