ANSI C - Text File: Modify Line?

问题

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)

回答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

回答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())

来源：https://stackoverflow.com/questions/23653962/ansi-c-text-file-modify-line

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