Read a file specified as an argument and return its' lines

Question

I have an exercise in which I have to read a file containing strings and I have to return the content using one/multiple arrays (this is because the second part of this exercise asks for these lines to be reversed, I'm having problems - and therefore ask for help - with the input). So far, I have this:

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

#define LENGTH 1024

int main(int argc, char *argv[]){

    char* input[LENGTH];

    if(argc==2){

        FILE *fp = fopen(argv[1], "rt");

        if(fp!=NULL){

            int i=0;
            while(fgets(input, sizeof(input), fp)!=NULL){
                input[i] = (char*)malloc(sizeof(char) * (LENGTH));
                fgets(input, sizeof(input), fp);
                i++;
            }
            printf("%s", *input);
            free(input);

        }
        else{
            printf("File opening unsuccessful!");
        }
    }
    else{
        printf("Enter an argument.");
    }

    return 0;

}

I also have to check whether or not memory allocation has failed. This program in its' current form returns nothing when run from the command line.

EDIT: I think it's important to mention that I get a number of warnings:

passing argument 1 of 'fgets' from incompatible pointer type [-Wincompatible-pointer-types]|

attempt to free a non-heap object 'input' [-Wfree-nonheap-object]|

EDIT 2: Example of input:

These
are
strings

... and the expected output:

esehT
era
sgnirts

In the exercise, it's specified that the maximum length of a line is 1024 characters.

You seem to be using the variable `input` for two different things: 1) As a buffer foir `fgets`; And 2) as an array of strings. You can't have it both ways, you can only use it for one thing. You need *two* variables: One for the buffer used for `fgets` and a separate variable used for the strings. — Some programmer dude, Nov 18 '21 at 10:19
Also, you seem to misunderstand how arrays and pointers in general work. You can't print all elements of an array using a single `printf` call, you must use a loop. And be careful not to pass the end of the initialized parts of the array. Similarly, you can't pass `input` to `free` and hope it will free all allocated pointers in it. Again you need a loop, and only loop over the pointers you actually allocated. — Some programmer dude, Nov 18 '21 at 10:22
Furthermore, you don't really need an array of strings *at all* in your code. All you need is the single buffer for `fgets`, which you print in the same loop as you read the input. So no need for pointers or dynamic allocation. — Some programmer dude, Nov 18 '21 at 10:23
Lastly, you read *two* lines per iteration of the loop. Skipping every other line, and ignoring its contents. — Some programmer dude, Nov 18 '21 at 10:23

Jabberwocky · Accepted Answer · 2021-11-18T10:55:18.183

You probably want something like this.

Comments are in the code

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

#define LENGTH 1024

int main(int argc, char* argv[]) {

  if (argc == 2) {

    FILE* fp = fopen(argv[1], "rt");

    if (fp != NULL) {
      char** lines = NULL;         // pointer to pointers to lines read
      int nboflines = 0;           // total number of lines read

      char input[LENGTH];          // temporary input buffer

      while (fgets(input, sizeof(input), fp) != NULL) {
        char* newline = malloc(strlen(input) + 1);        // allocate memory for line (+1 for null terminator)
        strcpy(newline, input);                           // copy line just read
        newline[strcspn(newline, "\n")] = 0;              // remove \n if any
        nboflines++;                                      // one more line
        lines = realloc(lines, nboflines * sizeof(char*)); // reallocate memory for one more line
        lines[nboflines - 1] = newline;                    // store the pointer to the line
      }

      fclose(fp);

      for (int i = 0; i < nboflines; i++)    // print the lins we've read
      {
        printf("%s\n", lines[i]);
      }
    }
    else {
      printf("File opening unsuccessful!");
    }
  }
  else {
    printf("Enter an argument.");
  }

  return 0;
}

Explanation about removing the \n left by fgets: Removing trailing newline character from fgets() input

Disclaimers:

there is no error checking for the memory allocation functions
memory is not freed. This is left as an exercise.
the way realloc is used here is not very efficient.
you still need to write the code that reverses each line and displays it.

You probably should decompose this into different functions:

a function that reads the file and returns the pointer to the lines and the number of lines read,
a function that displays the lines read
a function that reverses one line (to be written)
a function that reverses all lines (to be written)

This is left as an exercise.

Read a file specified as an argument and return its' lines

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