Read lines of text file into array, using double pointer and malloc

Question

I want to read lines of a text file, and the content of it is as below.

first
second
third

I've already written some code, but the result was different from what I expected. I hope you can help me a little. (code below)

/*
content of test.txt

first
second
third
*/

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

int main() {
  // double pointer for two string lines
  // and the two string lines will be pointed by (char *) pointer
  FILE *fp = fopen("test.txt", "r");
  char **lst = malloc(3 * sizeof(char *));  // corrected 2 -> 3

  // read lines until feof(fp) is not Null
  // Only 2 lines will be read by this loop
  int i = 0;
  while (!feof(fp)) {
    char line[10];
    fgets(line, 10, fp);
    *(lst + i) = line;
    printf("%d : %s", i, *(lst + i));
    i++;
  }

  /*
  Result:

  0 : first
  1 : second
  2 : third     // Why was this line printed here?

  There are 3 lines, not 2 lines!
  */

  printf("\n\n");

  int j;
  for (j = 0; j < i; j++) {
    printf("%d : %s\n", j, *(lst + j));
  }

  /*
  result:

  0 : third
  1 : third
  2 : third

  The whole lines were not printed correctly!
  */

  free(lst);
  return 0;
}

Expected output:

0 : first
1 : second
2 : third

Many thanks.

Do you use an IDE? Do you know how to use a debugger? If not, learn; set some breakpoints, step through your code, line by line, examinking variables & you will soon find your problem (and not need us :-) — Mawg
– Mawg, Commented Jun 6, 2019 at 8:41
Hint: instead of *(lst + i) write lst[i]. It's exactly the same thing, but latter is the more common way to do it. — Jabberwocky
– Jabberwocky, Commented Jun 6, 2019 at 8:49
Surprisingly similar to C read entire line of file {closed}. You would also benefit from How to read correctly certain strings from file in c? — David C. Rankin
– David C. Rankin, Commented Jun 6, 2019 at 8:57

Konrad Rudolph · Accepted Answer · 2019-06-06 08:48:44Z

1

First and foremost, you are allocating space for an array of two char*s and you have a single statically sized buffer for a string. But you’re attempting to read three strings. Where do you think the space for the strings is coming from? You’re not allocating it.

You need to make your various numbers match up: allocate an array of three strings, and then allocate three string buffers:

char **lst = malloc(3 * sizeof *lst);

for (int i = 0; i < 3; i++) {
    lst[i] = malloc(10);
    fgets(lst[i], 10, fp);
}

And don’t forget to free all allocated buffers subsequently:

for (int i = 0; i < 3; i++) {
    free(lst[i]);
}
free(lst);

… of course this code isn’t terribly great either since it hard-codes the number of lines you can read, and the maximum line length. But it should get you started.

answered Jun 6, 2019 at 8:48

Konrad Rudolph

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2 Comments

Larynx Over a year ago

Thanks for your answer. I corrected my code, but output is still incorrect.

Konrad Rudolph Over a year ago

@Larynx You only corrected one of several errors I mentioned.

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