Write to the middle of an existing binary file c++

Question

I'm trying to open a binary file for writing without erasing the content. But I do not want to write to eof. I want to write to a specific position in file.

Here is a litte example:

ofstream out("test.txt", ios::binary | ios::app);
for(int i = 0; i < 100; i++)
    out.put('_');
out.write("Hallo", 5);
out.close();

ofstream out2("test.txt", ios::binary | ios::app);
out2.seekp(10);
out2.write("Welt", 4);
out2.close();

If using app, seek doesn't work. If not using app opening file erases data. Does anybody know an answer?

Nim · Accepted Answer · 2011-06-21T15:38:04.263

12

try the second overload of seekp, which allows you to provide an offset and a direction, this could be begining of file in your case (i.e. ios_base::beg). This of course assumes you know what you are doing and all you want to do is overwrite an existing number of characters.

EDIT: here is fully working example:

#include <iostream>
#include <fstream>

using namespace std;
int main()
{
  {
    ofstream out("test.txt", ios::binary);
    for(int i = 0; i < 100; i++)
      out.put('_');
    out.write("Hallo", 5);
  }

  {   
    fstream out2("test.txt", ios::binary | ios::out | ios::in);
    out2.seekp(10, ios::beg);
    out2.write("Welt", 4);
  }
}

edited Jun 21 '11 at 15:38

answered Jun 21 '11 at 15:21

Nim

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I just tried out2.seekp(10, ios_base::beg);. That doesn't change anything. – Jonny Schubert Jun 21 '11 at 15:26
It should not erase the file unless you specify ios::trunc – crashmstr Jun 21 '11 at 15:28
@Jonny Schubert, can you post your full example? – Nim Jun 21 '11 at 15:31
using namespace std; int main() { ifstream in("test.txt", ios::binary | ios::app); ofstream out("test.txt", ios::binary); for(int i = 0; i < 100; i++) out.put('_'); out.write("Hallo", 5); out.close(); ofstream out2("test.txt", ios::binary | ios::app); out2.seekp(10); out2.write("Welt", 4); out2.close(); int a; cin >> a; return 0; } – Jonny Schubert Jun 21 '11 at 15:32
1

actually, @stefaanv has a point in his answer, try `fstream` rather than `ofstream`. – Nim Jun 21 '11 at 15:33

score 6 · Answer 2 · answered Jun 21 '11 at 15:30

6

When opening with ios::app, it is as if you open a new file that just happened to be attached to an existing file: you can not access the existing file. I'm not sure, because I would do as in Kerrek's answer, but if you really want to try, you probably have to open with "ios::in | ios::out", similar to fopen("test.txt", "rw").

Or as crashmstr points out: ios::out might be enough.

answered Jun 21 '11 at 15:30

stefaanv

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you need both `ios::out` and `ios::in`, otherwise, it simply overwrites methinks.. – Nim Jun 21 '11 at 15:38
Yes because `ios::out` alone results in an open call of the following kind on linux : O_WRONLY|O_CREAT|O_TRUNC, while combining `ios::out | ios::in` results in an O_RDWR call, without the O_TRUNC. – daminetreg Oct 24 '12 at 12:22

score 5 · Answer 3 · answered Jun 21 '11 at 15:20

5

You cannot magically extend the file from the middle. Perhaps easiest to write to a new file: First copy the initial segment, then write your new data, then copy the remaining segment. When all is done, you can overwrite the original file.

answered Jun 21 '11 at 15:20

Kerrek SB

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I do not want to extend the file in the middle. I want to overwrite the inital values! – Jonny Schubert Jun 21 '11 at 15:25
@Jonny: That's not what you said in your question. You said "without erasing the content". Otherwise it's trivial, just open for write (without "append"), seek to the beginning and write. – Kerrek SB Jun 21 '11 at 15:36
I don't want to write to the beginning. I want to write somewhere in the file. The problem is opening the file again will erase the content. For example: File at the beginning: ______________________ File after manipulation: ______Bla_______ – Jonny Schubert Jun 21 '11 at 15:41
Doesn't this work: `std::ofstream myfile("test.txt", std::ios::binary); myfile.seekp(125, std::ios::beg); myfile.write(buf, n);`? This will seek to 125 and write from there, overwriting what was there originally. Make sure the file size is at least 125. – Kerrek SB Jun 21 '11 at 15:44

score 3 · Answer 4 · answered Jul 20 '11 at 18:56

According to the specification of fstream here

fstream::open

the ios::app "Sets the stream's position indicator to the end of the stream before EACH output operation." So ios::app doesn't work for replacing, seeks of any sort fail, at least for me.

Just using ios::out does wipe out the file contents preserving only the size, basically turning the file into trash.

ios::in|ios::out turned out as the only working thing for me.

score 0 · Answer 5 · answered Sep 13 '12 at 12:22

Working Code: This code searches for a string (OLD-STRING) in cout.exe and replaces with a new string (NEW-STRING).

`#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main(int argc, char *argv[])
{
  fstream ifs;
  ifs.open ("C:\\Users\\user\\Desktop\\cout.exe", fstream::binary | fstream::in | fstream::out);

  std::string str((std::istreambuf_iterator<char>(ifs)), std::istreambuf_iterator<char>());

  size_t pos = str.find("OLD-STRING");

  if (pos != string::npos)
  {
    cout << "string found at position: " << int(pos) << endl;

    ifs.seekp(pos);

    ifs.write("NEW-STRING", 10);

  }
  else
  {
    cout << "could not find string" << endl;
  }

  if (ifs.is_open())
    ifs.close();

  return 0;
}`

Write to the middle of an existing binary file c++

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