CODEWITHSUNDEEP
cfile
patoglu
patoglu

Reputation: 411

Replacing bytes at current offset in c

I'm currently developing a program that mimics UNIX file system. I've prepared my disk as file (1 MB) got all data blocks inside it. Now what I'm doing is implementing some simple commands like mkdir, ls etc. In order to work with those commands, I need to read specific offset(no problem with that) and write the modified blocks to specific location.

Simply my goal is:

SIIIDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD (Current Disk)

I wan't to change three blocks with AAA after 16.byte so it will be like:

SIIIDDDDDDDDDDDDAAADDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD (Modified Disk)

I'm not going to provide all of my implementation here I just want to have some ideas about it how can I implement it without buffering all the 1 MB data in my program. In short I know locations of my data blocks so I just want to replace that part of my file not whole file. Can't I simply do this with file stream functions ?

Another example:

fseek(from_disk,superblock.i_node_bit_map_starting_addr , SEEK_SET); //seek to known offset.

read_bit_map(&from_disk); // I can read at specific location without problem

... manipulate bit map ...

fseek(to_disk,superblock.i_node_bit_map_starting_addr , SEEK_SET); //seek to known offset.

write_bit_map(&to_disk); //Write back the data. 
                         //This will destroy the current data of file. (Tried with w+, a modes.)

Note: Not provided in example but I have two file pointers both writing and reading and I'm aware I need to close one before opening another.

Upvotes: 0

Views: 426

Answers (1)

Dave Ankin
Dave Ankin

Reputation: 1291

I think you are looking for the r+ (potentially rb+ mode). Here is a complete example, afterwards you can run grep -n hello data.txt to verify for yourself the result. You can run it with make prog && ./prog.

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

int main(int argc, char const *argv[])
{
  FILE *file;

  file = fopen("data.txt", "w+");
  char dummy_data[] = "This is stackoverflow.com\n";
  int dummy_data_length = strlen(dummy_data);

  for (int i = 0; i < 1000; ++i)
    fwrite(dummy_data, dummy_data_length, 1, file);

  fclose(file);

  file = fopen("data.txt", "r+");
  fseek(file, 500, SEEK_CUR);
  fwrite("hello", 5, 1, file);
  fclose(file);
  return 0;
}

Upvotes: 1

Related Questions