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File I/O

Posted on 2003-10-24
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Last Modified: 2010-04-02
what is the easiest way to insert or remove something in the middle of a file?
is there a way to do it without rewriting the whole file? (aka the file is HUGE) and i dont want to overwrite exsisting data, I want to selectively insert and delete data
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Question by:GaryJoe
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3 Comments
 
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Accepted Solution

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brettmjohnson earned 375 total points
ID: 9617101
The quick answer is "no", except in very rare circumstances.  A file is a serial array
of bytes laid out on the disk.  To insert or delete data from the middle of the file
requires moving all data after the insertion/deletion point either backward or forward.

Here is code (from a previous question) that removes the first line of a file and shuffles
everything down.

#include <stdio.h>
#include <unistd.h>

int main(int argc, char **argv)
{
  FILE * fin = fopen(argv[1], "r");
  FILE * fout = fopen(argv[1], "r+");
  char buffer[4096];

  fgets(buffer, sizeof(buffer), fin);   // read first line & discard it

  while (fgets(buffer, sizeof(buffer), fin))
    fputs(buffer, fout);

  fclose(fin);
  ftruncate(fileno(fout), ftell(fout));
  fclose(fout);

  return 0;
}


Inserting is only slightly more difficult.  It involves rewriting the end of the file.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

int main(int argc, char **argv)
{
  FILE * fin = fopen(argv[1], "r");
  FILE * fout = fopen(argv[1], "r+");
  char *newline = "Insert this after line 3\n";
  off_t startgap, endgap, curpoint;
  int i, gaplen;
  static char buffer[1024*1024];

  /* skip passed the first 3 lines unmolested */
  for (i = 0; i < 3; i++)
    fgets(buffer, sizeof(buffer), fin);

  /* remember the start and endpoints of the gap we are making */
  startgap = ftello(fin);
  gaplen = strlen(newline);
  endgap = startgap + gaplen;

  /* move to the end of the file and start moving it out */
  fseeko(fin, 0, SEEK_END);
  curpoint = ftello(fin);

  while (curpoint > startgap) {
    off_t len = ((curpoint - startgap) > sizeof(buffer)) ? sizeof(buffer) : (curpoint - startgap);
    fseeko(fin, curpoint-len, SEEK_SET);
    curpoint = ftello(fin);
    fseeko(fout, curpoint+gaplen, SEEK_SET);
    fread(buffer, 1, len, fin);
    fwrite(buffer, 1, len, fout);
  }


  /* now write the new line into the gap we opened up */
  fseeko(fout, startgap, SEEK_SET);
  fputs(newline, fout);

  fclose(fin);
  fclose(fout);

  return 0;
}
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LVL 1

Expert Comment

by:Mustak_Shaikh
ID: 9617154
if you want to modiy one character then you can proceed with the following code:

#include<iostream.h>
#include<fstream.h>
int main()
{
      ofstream out_file;
      out_file.open("c:\\sample.txt");
      if(out_file.bad())
      {
            cout<<"Invalid file name"<<endl;
            return -1;
      }

      out_file.seekp(5);
      out_file.write("G",1);
      out_file.close();
      return 0;
}

Mustak_Shaikh
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