ANSI C - Text File: Modify Line?

Question

I have this text file:

Line 1. "house"
Line 2. "dog"
Line 3. "mouse"
Line 4. "car"
...

I want to change Line 2. "dog" in new Line 2."cards"

how can I do?

thanks!

(sorry for my bad English)

Answer 1

Your program could like this:

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

#define MAX_LINE_LENGTH 1000

int main()
{
  FILE * fp_src, *fp_dest;
  char line[MAX_LINE_LENGTH];

  fp_src = fopen("PATH_TO_FILE\\test.txt", "r"); // This is the file to change
  if (fp_src == NULL)
    exit(EXIT_FAILURE);

  fp_dest = fopen("PATH_TO_FILE\\test.txt_temp", "w"); // This file will be created
  if (fp_dest == NULL)
    exit(EXIT_FAILURE);


  while (fgets(line, 1000, fp_src) != NULL) {

    if (strncmp(line, "Line 2.", 7) == 0) {
      fputs("Line 2. \"cards\"\n", fp_dest);
      printf("Applied new content: %s", "Line 2. \"cards\"\n");
    }
    else {
      fputs(line, fp_dest);
      printf("Took original line: %s", line);
    }

  }

  fclose(fp_src);
  fclose(fp_dest);

  unlink("PATH_TO_FILE\\test.txt");
  rename("PATH_TO_FILE\\test.txt_temp", "PATH_TO_FILE\\test.txt");

  exit(EXIT_SUCCESS);
}

The following things you should consider when taking this solution into some production system:

• Does the maximum line length of 1000 staisfy your needs - maybe you want to come up with a solution that uses malloc() to dynamically allocate memory for one line
• You should take some random-filename-generator to generate the temporary file and make sure that it doesn't exist yet, so you don't overwrite an existing file
• With large files this approach is maybe not the best because you effectivly have your file-content twice in memory

Answer 2

You cannot edit disk files inline. You have to follow the process of:

• reading the file data to buffer, (fopen() -> fread()/fgets())

• then delete old file, (unlink()/remove())

• then modify the data in buffer,

• write back buffer to a new file, (fwrite)

• rename it to original file. (rename())