Why can't I use fseek() function like this?

Question

I want to read content from file's end and print it to the screen. But fseek() function doesn't give permission me to reach line which is I need. I expressed in comment lines what I want to do. How can I solve this problem? Thanks in advance

 #include <stdio.h>

int main()
{
    FILE *ptrFILE;
    int i, k, value;   // Variables for loops and values inside of file

    if ((ptrFILE = fopen("Test.txt", "w"))==NULL)   // Open a file to just write knowledge in it
    {
        printf("File couldn't open..\n\n");
    }
    for (i = 1; i <= 20; i++)   // Write 20 lines to file (Multiplying numbers with 5 from 1 to 20) 
    {
        fprintf(ptrFILE, "%d\n", i * 5);
    }
    fclose(ptrFILE);   // Close the file

    ptrFILE = fopen("Test.txt", "r");   // Open file read mode
    printf("To display content from the beginning enter positive number or vice versa : ");   // Read user choice
    scanf("%d", &i);
    if (i > 0)
    {
        fseek(ptrFILE, 0L, SEEK_SET);
        for (k = 1; k <= i; k++)
        {
            fscanf(ptrFILE, "%d\n", &value);
            printf("%d\n", value);
        }
    }
    else
    {
        fseek(ptrFILE, -10L, SEEK_END);   // Why does C give permission to write instead of -10L like I expressed below? 
        fscanf(ptrFILE, "%d\n", &value);
        printf("%d\n", value);
    }

    /* Like this!!  
    else
    {
    int x;             <<<<<<<<<<<<<<<<<<<<<<<<<<<
    x= i*5;            <<<<<<<<<<<<<<<<<<<<<<<<<<< 
        fseek(ptrFILE, -xL, SEEK_END); <<<<<<<<<<<
        fscanf(ptrFILE, "%d\n", &value);
        printf("%d\n", value);
    }*/

    fclose(ptrFILE);
    return 0;
}

Answer 1

To convert your value x to a negative long value, you must use

fseek(ptrFILE, -((long) x), SEEK_END);

The issue with this code is, that you can not be sure that there actually is a number at the position you are seeking to. There could as well be a newline \n character, depending on what you wrote there before.