无法将 ftell 函数的返回值分配给 char 数组大小

Question

I am trying to print some values from a file with minimum allocated memory. I have used ftell() to find out the file, thus, to minimize used memory. I did 3 approaches and one of them was successful. I am clueless why 2 others do not print to the string since they seem to be analogical to the successful code.

The following string is located in the file that I attempt to output

123\n45 678

My attempts:

Successful

#include <stdio.h>
int main()
{
    int size = 15;
    char arr[size];

    FILE *pf = fopen(".txt", "r");

    fgets(arr, size, pf);

    puts(arr);

    fclose(pf);
    return 0;
}

Fail:

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

    int main()
    {
        FILE *pf = fopen(".txt", "r");
        int check = fseek(pf, 0, SEEK_END);
        if (check)
        {
         

   printf("could not fseek\n");
    }
    unsigned size = 0;
    size = ftell(pf);

    char *arr = NULL;
    arr = (char *)calloc(size, sizeof(char));

    if (arr == NULL)
    {
        puts("can't calloc");
        return -1;
    }

    fgets(arr, size, pf);
    puts(arr);

    free(arr);
    return 0;
}

output: nothing prints out

Fail #2:

#include <stdio.h>
int main()
{

    FILE *pf = fopen(".txt", "r");

    int check = fseek(pf, 0, SEEK_END);
    if (check)
    {
        printf("could not fseek\n");
    }
    int size = 0;
    size = ftell(pf);
    char arr[size];

    fgets(arr, size, pf);

    puts(arr);

    fclose(pf);

    return 0;
}

output: some garbage

0Y���

Answer 1

You forgot to move the file position back after seeking to the end of file, preventing from reading the contents of file.

size = ftell(pf);
fseek(pf, 0, SEEK_SET); /* add this */

Also you should allocate a few bytes more than the size of file for terminating null-character.